diff --git a/include/Asm/format.h b/include/Asm/format.h
index da6dc06..2683a60 100644
--- a/include/Asm/format.h
+++ b/include/Asm/format.h
@@ -23,15 +23,31 @@
 
 /* AsmFormat */
 /* types */
-typedef struct _FormatPluginHelper
-{
-	char const * filename;
-	FILE * fp;
-	void * priv;
-} FormatPluginHelper;
+typedef struct _Format Format;
 
 typedef struct _FormatPlugin FormatPlugin;
 
+typedef struct _FormatPluginHelper
+{
+	Format * format;
+
+	/* callbacks */
+	/* accessors */
+	char const * (*get_filename)(Format * format);
+
+	/* useful */
+	ssize_t (*read)(Format * format, void * buf, size_t size);
+	off_t (*seek)(Format * format, off_t offset, int whence);
+
+	/* assembly */
+	ssize_t (*write)(Format * format, void const * buf, size_t size);
+
+	/* disassembly */
+	/* FIXME let a different architecture be specified in the callback */
+	int (*disas)(Format * format, char const * section,
+			off_t offset, size_t size, off_t base);
+} FormatPluginHelper;
+
 struct _FormatPlugin
 {
 	FormatPluginHelper * helper;
@@ -47,12 +63,7 @@ struct _FormatPlugin
 	int (*section)(FormatPlugin * format, char const * section);
 
 	char const * (*detect)(FormatPlugin * format);
-	/* FIXME:
-	 * - put the callback in the helper structure
-	 * - let a different architecture be specified in the callback */
-	int (*disas)(FormatPlugin * format, int (*callback)(
-				FormatPlugin * format, char const * section,
-				off_t offset, size_t size, off_t base));
+	int (*disas)(FormatPlugin * format);
 
 	void * priv;
 };
diff --git a/src/format.c b/src/format.c
index e90f5a6..6f9485b 100644
--- a/src/format.c
+++ b/src/format.c
@@ -34,9 +34,24 @@ struct _Format
 	FormatPluginHelper helper;
 	Plugin * handle;
 	FormatPlugin * plugin;
+
+	/* internal */
+	/* file */
+	char const * filename;
+	FILE * fp;
 };
 
 
+/* prototypes */
+/* callbacks */
+static char const * _format_get_filename(Format * format);
+
+static ssize_t _format_read(Format * format, void * buf, size_t size);
+static off_t _format_seek(Format * format, off_t offset, int whence);
+
+static ssize_t _format_write(Format * format, void const * buf, size_t size);
+
+
 /* public */
 /* functions */
 /* format_new */
@@ -100,10 +115,12 @@ int format_exit(Format * format)
 #endif
 	if(format->plugin->exit != NULL)
 		ret = format->plugin->exit(format->plugin);
+	format->helper.format = NULL;
+	format->helper.read = NULL;
+	format->helper.seek = NULL;
 	format->plugin->helper = NULL;
-	format->helper.fp = NULL;
-	format->helper.filename = NULL;
-	format->helper.priv = NULL;
+	format->fp = NULL;
+	format->filename = NULL;
 	return ret;
 }
 
@@ -124,9 +141,13 @@ int format_init(Format * format, char const * filename, FILE * fp)
 	fprintf(stderr, "DEBUG: %s(\"%s\", %p)\n", __func__, filename,
 			(void *)fp);
 #endif
-	format->helper.filename = filename;
-	format->helper.fp = fp;
-	format->helper.priv = NULL;
+	format->filename = filename;
+	format->fp = fp;
+	format->helper.format = format;
+	format->helper.get_filename = _format_get_filename;
+	format->helper.read = _format_read;
+	format->helper.seek = _format_seek;
+	format->helper.write = _format_write;
 	format->plugin->helper = &format->helper;
 	if(format->plugin->init != NULL)
 		return format->plugin->init(format->plugin, format->arch);
@@ -141,3 +162,62 @@ int format_section(Format * format, char const * section)
 		return 0;
 	return format->plugin->section(format->plugin, section);
 }
+
+
+/* private */
+/* functions */
+/* format_get_filename */
+static char const * _format_get_filename(Format * format)
+{
+	return format->filename;
+}
+
+
+/* format_read */
+static ssize_t _format_read(Format * format, void * buf, size_t size)
+{
+	if(fread(buf, size, 1, format->fp) == 1)
+		return size;
+	if(ferror(format->fp))
+		return -error_set_code(1, "%s: %s", format->filename,
+				strerror(errno));
+	if(feof(format->fp))
+		return -error_set_code(1, "%s: %s", format->filename,
+				"End of file reached");
+	return -error_set_code(1, "%s: %s", format->filename, "Read error");
+}
+
+
+/* format_seek */
+static off_t _format_seek(Format * format, off_t offset, int whence)
+{
+	if(whence == SEEK_SET)
+	{
+		if(fseek(format->fp, offset, whence) == 0)
+			return offset;
+	}
+	else if(whence == SEEK_CUR || whence == SEEK_END)
+	{
+		if(fseek(format->fp, offset, whence) == 0)
+			return ftello(format->fp);
+	}
+	else
+		return -error_set_code(1, "%s: %s", format->filename,
+				"Invalid argument for seeking");
+	return -error_set_code(1, "%s: %s", format->filename, strerror(errno));
+}
+
+
+/* format_write */
+static ssize_t _format_write(Format * format, void const * buf, size_t size)
+{
+	if(fwrite(buf, size, 1, format->fp) == 1)
+		return size;
+	if(ferror(format->fp))
+		return -error_set_code(1, "%s: %s", format->filename,
+				strerror(errno));
+	if(feof(format->fp))
+		return -error_set_code(1, "%s: %s", format->filename,
+				"End of file reached");
+	return -error_set_code(1, "%s: %s", format->filename, "Write error");
+}
diff --git a/src/format.h b/src/format.h
index a9232e8..71fb350 100644
--- a/src/format.h
+++ b/src/format.h
@@ -18,15 +18,11 @@
 #ifndef ASM_FORMAT_H
 # define ASM_FORMAT_H
 
-# include <stdio.h>
+# include "Asm/format.h"
 
 
 /* Format */
 /* public */
-/* types */
-typedef struct _Format Format;
-
-
 /* functions */
 Format * format_new(char const * format, char const * arch);
 void format_delete(Format * format);
diff --git a/src/format/dex.c b/src/format/dex.c
index 814445c..d61972d 100644
--- a/src/format/dex.c
+++ b/src/format/dex.c
@@ -117,12 +117,9 @@ static char _dex_signature[4] = "dex\n";
 static int _dex_init(FormatPlugin * format, char const * arch);
 static int _dex_destroy(FormatPlugin * format);
 static char const * _dex_detect(FormatPlugin * format);
-static int _dex_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _dex_disas(FormatPlugin * format);
 
 static int _dex_error(FormatPlugin * format);
-static int _dex_error_fread(FormatPlugin * format);
 
 
 /* public */
@@ -189,59 +186,52 @@ static char const * _dex_detect(FormatPlugin * format)
 
 
 /* dex_disas */
-static int _disas_map(FormatPlugin * format, DexHeader * dh, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
-static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size,
-		int (*callback)(FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _disas_map(FormatPlugin * format, DexHeader * dh);
+static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size);
 static int _disas_map_string_id(FormatPlugin * format, off_t offset,
 		size_t size);
 
-static int _dex_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _dex_disas(FormatPlugin * format)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	DexHeader dh;
 
-	if(fseek(fp, 0, SEEK_SET) != 0)
-		return -_dex_error(format);
-	if(fread(&dh, sizeof(dh), 1, fp) != 1)
-		return _dex_error_fread(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->read(helper->format, &dh, sizeof(dh)) != sizeof(dh))
+		return -1;
 	dh.map_off = _htol32(dh.map_off);
-	if(_disas_map(format, &dh, callback) != 0)
+	if(_disas_map(format, &dh) != 0)
 		return -1;
 	return 0;
 }
 
-static int _disas_map(FormatPlugin * format, DexHeader * dh, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _disas_map(FormatPlugin * format, DexHeader * dh)
 {
 	int ret = 0;
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	uint32_t size;
 	uint32_t i;
-	fpos_t pos;
+	off_t offset;
 	DexMapItem dmi;
 
 #ifdef DEBUG
 	fprintf(stderr, "DEBUG: %s()\n", __func__);
 #endif
-	if(fseek(fp, dh->map_off, SEEK_SET) != 0)
-		return -_dex_error(format);
-	if(fread(&size, sizeof(size), 1, fp) != 1)
-		return -_dex_error_fread(format);
+	if(helper->seek(helper->format, dh->map_off, SEEK_SET) != dh->map_off)
+		return -1;
+	if(helper->read(helper->format, &size, sizeof(size)) != sizeof(size))
+		return -1;
 	size = _htol32(size);
 #ifdef DEBUG
 	fprintf(stderr, "DEBUG: %s() %u items\n", __func__, size);
 #endif
 	for(i = 0; i < size; i++)
 	{
-		if(fread(&dmi, sizeof(dmi), 1, fp) != 1)
-			return -_dex_error_fread(format);
-		fgetpos(fp, &pos);
+		if(helper->read(helper->format, &dmi, sizeof(dmi))
+				!= sizeof(dmi))
+			return -1;
+		offset = helper->seek(helper->format, 0, SEEK_CUR);
 		dmi.type = _htol16(dmi.type);
 		dmi.size = _htol32(dmi.size);
 		dmi.offset = _htol32(dmi.offset);
@@ -253,44 +243,48 @@ static int _disas_map(FormatPlugin * format, DexHeader * dh, int (*callback)(
 		{
 			case TYPE_CODE_ITEM:
 				ret |= _disas_map_code(format, dmi.offset,
-						dmi.size, callback);
+						dmi.size);
 				break;
 			case TYPE_STRING_ID_ITEM:
 				ret |= _disas_map_string_id(format, dmi.offset,
 						dmi.size);
 		}
-		fsetpos(fp, &pos);
+		if(helper->seek(helper->format, offset, SEEK_SET) != offset)
+			return -1;
 		if(ret != 0)
 			break;
 	}
 	return ret;
 }
 
-static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size,
-		int (*callback)(FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	DexMapCodeItem dmci;
 	size_t i;
 	off_t seek;
 	size_t j;
 	DexMapTryItem dmti;
+	ssize_t s;
 
-	if(fseek(fp, offset, SEEK_SET) != 0)
-		return -_dex_error(format);
-	callback(format, ".text", ftello(fp), 0, 0);
+	if(helper->disas(helper->format, ".text", offset, 0, 0) != 0)
+		return -1;
 	for(i = 0; i < size; i++)
 	{
-		if(fread(&dmci, sizeof(dmci), 1, fp) != 1)
-			return -_dex_error_fread(format);
+		s = sizeof(dmci);
+		if(helper->read(helper->format, &dmci, s) != s)
+			return -1;
 		dmci.registers_size = _htol16(dmci.registers_size);
 		dmci.ins_size = _htol16(dmci.ins_size);
 		dmci.outs_size = _htol16(dmci.outs_size);
 		dmci.tries_size = _htol16(dmci.tries_size);
 		dmci.debug_info_off = _htol32(dmci.debug_info_off);
 		dmci.insns_size = _htol32(dmci.insns_size);
-		seek = (dmci.insns_size & 0x1) == 0x1 ? 2 : 0; /* padding */
+		seek = helper->seek(helper->format, 0, SEEK_CUR);
+		helper->disas(helper->format, NULL, seek, dmci.insns_size * 2,
+				0);
+		/* skip padding and try_items */
+		seek = (dmci.insns_size & 0x1) == 0x1 ? 2 : 0;
 #ifdef DEBUG
 		fprintf(stderr, "DEBUG: code item %lu, registers 0x%x"
 				", size 0x%x, debug @0x%x, tries 0x%x"
@@ -298,21 +292,22 @@ static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size,
 				dmci.insns_size * 2, dmci.debug_info_off,
 				dmci.tries_size, seek);
 #endif
-		callback(format, NULL, ftello(fp), dmci.insns_size * 2, 0);
-		/* skip padding and try_items */
-		if(seek != 0 && fseek(fp, seek, SEEK_CUR) != 0)
-			return -_dex_error(format);
+		if(seek != 0 && helper->seek(helper->format, seek, SEEK_CUR)
+				< 0)
+			return -1;
 		if(dmci.tries_size > 0)
 		{
 			for(j = 0; j < dmci.tries_size; j++)
 			{
-				if(fread(&dmti, sizeof(dmti), 1, fp) != 1)
-					return -_dex_error_fread(format);
+				s = sizeof(dmti);
+				if(helper->read(helper->format, &dmti, s) != s)
+					return -1;
 				dmti.start_addr = _htol32(dmti.start_addr);
 				dmti.insn_count = _htol16(dmti.insn_count);
 				dmti.handler_off = _htol16(dmti.handler_off);
 			}
-			callback(format, NULL, ftello(fp), 8, 0);
+			seek = helper->seek(helper->format, 0, SEEK_CUR);
+			helper->disas(helper->format, NULL, seek, 8, 0);
 		}
 	}
 	return 0;
@@ -321,21 +316,23 @@ static int _disas_map_code(FormatPlugin * format, off_t offset, size_t size,
 static int _disas_map_string_id(FormatPlugin * format, off_t offset,
 		size_t size)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	Dex * dex = format->priv;
 	size_t i;
 	DexStringIdItem dsii;
+	ssize_t s;
 
 	if(dex->strings_cnt != 0)
 		return -error_set_code(1, "%s: %s", "dex",
 				"String section already parsed");
-	if(fseek(fp, offset, SEEK_SET) != 0)
-		return -_dex_error(format);
+	if(helper->seek(helper->format, offset, SEEK_SET) != offset)
+		return -1;
 	if((dex->strings = malloc(sizeof(*dex->strings) * size)) == NULL)
 		return -_dex_error(format);
 	for(i = 0; i < size; i++)
 	{
-		if(fread(&dsii, sizeof(dsii), 1, fp) != 1)
+		s = sizeof(dsii);
+		if(helper->read(helper->format, &dsii, s) != s)
 			break;
 		dsii.string_data_off = _htol32(dsii.string_data_off);
 #ifdef DEBUG
@@ -349,7 +346,7 @@ static int _disas_map_string_id(FormatPlugin * format, off_t offset,
 	{
 		free(dex->strings);
 		dex->strings = NULL;
-		return -_dex_error_fread(format);
+		return -1;
 	}
 	dex->strings_cnt = size;
 	return 0;
@@ -359,16 +356,8 @@ static int _disas_map_string_id(FormatPlugin * format, off_t offset,
 /* dex_error */
 static int _dex_error(FormatPlugin * format)
 {
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			strerror(errno));
-}
+	FormatPluginHelper * helper = format->helper;
 
-
-/* dex_error_fread */
-static int _dex_error_fread(FormatPlugin * format)
-{
-	if(ferror(format->helper->fp))
-		return _dex_error(format);
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			"End of file reached");
+	return -error_set_code(1, "%s: %s",
+			helper->get_filename(helper->format), strerror(errno));
 }
diff --git a/src/format/elf.c b/src/format/elf.c
index c459717..133be52 100644
--- a/src/format/elf.c
+++ b/src/format/elf.c
@@ -56,18 +56,14 @@ typedef struct _ElfStrtab
 
 
 /* prototypes */
+static int _elf_error(FormatPlugin * format);
+
 /* plug-in */
 static int _elf_init(FormatPlugin * format, char const * arch);
 static char const * _elf_detect(FormatPlugin * format);
-static int _elf_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
-static int _elf_disas32(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
-static int _elf_disas64(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _elf_disas(FormatPlugin * format);
+static int _elf_disas32(FormatPlugin * format);
+static int _elf_disas64(FormatPlugin * format);
 
 /* ELF32 */
 static int _init_32(FormatPlugin * format);
@@ -152,8 +148,8 @@ FormatPlugin format_plugin =
 /* elf_error */
 static int _elf_error(FormatPlugin * format)
 {
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			strerror(errno));
+	return -error_set_code(1, "%s: %s", format->helper->get_filename(
+				format->helper->format), strerror(errno));
 }
 
 
@@ -196,22 +192,22 @@ static ElfArch * _init_arch(char const * arch)
 
 
 /* elf_detect */
-static char const * _detect_error(FormatPlugin * format);
 static char const * _detect_32(FormatPlugin * format, Elf32_Ehdr * ehdr);
 static char const * _detect_64(FormatPlugin * format, Elf64_Ehdr * ehdr);
 
 static char const * _elf_detect(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	union
 	{
 		Elf32_Ehdr ehdr32;
 		Elf64_Ehdr ehdr64;
 	} ehdr;
 
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _detect_error(format);
-	if(fread(&ehdr, sizeof(ehdr), 1, format->helper->fp) != 1)
-		return _detect_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return NULL;
+	if(helper->read(helper->format, &ehdr, sizeof(ehdr)) != sizeof(ehdr))
+		return NULL;
 	switch(ehdr.ehdr32.e_ident[EI_CLASS])
 	{
 		case ELFCLASS32:
@@ -219,17 +215,11 @@ static char const * _elf_detect(FormatPlugin * format)
 		case ELFCLASS64:
 			return _detect_64(format, &ehdr.ehdr64);
 	}
-	error_set_code(1, "%s: %s 0x%x\n", format->helper->filename,
+	error_set_code(1, "%s: %s 0x%x\n", helper->get_filename(helper->format),
 			"Unsupported ELF class", ehdr.ehdr32.e_ident[EI_CLASS]);
 	return NULL;
 }
 
-static char const * _detect_error(FormatPlugin * format)
-{
-	error_set_code(1, "%s: %s", format->helper->filename, strerror(errno));
-	return NULL;
-}
-
 static char const * _detect_32(FormatPlugin * format, Elf32_Ehdr * ehdr)
 {
 	format->disas = _elf_disas32;
@@ -272,13 +262,11 @@ static char const * _detect_64(FormatPlugin * format, Elf64_Ehdr * ehdr)
 
 
 /* elf_disas */
-static int _elf_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _elf_disas(FormatPlugin * format)
 {
 	if(_elf_detect(format) == NULL)
 		return -1;
-	return format->disas(format, callback);
+	return format->disas(format);
 }
 
 
@@ -291,10 +279,9 @@ static int _disas32_strtab(FormatPlugin * format, Elf32_Shdr * shdr,
 		size_t shdr_cnt, uint16_t ndx, char ** strtab,
 		size_t * strtab_cnt);
 
-static int _elf_disas32(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _elf_disas32(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf32_Ehdr ehdr;
 	Elf32_Shdr * shdr = NULL;
 	Elf32_Addr base = 0x0;
@@ -306,10 +293,10 @@ static int _elf_disas32(FormatPlugin * format, int (*callback)(
 	fprintf(stderr, "DEBUG: %s() \"%s\"\n", __func__,
 			format->helper->filename);
 #endif
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0
-			|| fread(&ehdr, sizeof(ehdr), 1, format->helper->fp)
-			!= 1)
-		return -_elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0
+			|| helper->read(helper->format, &ehdr, sizeof(ehdr))
+			!= sizeof(ehdr))
+		return -1;
 	if(_disas32_shdr(format, &ehdr, &shdr) != 0)
 		return -1;
 	if(_disas32_addr(format, &ehdr, &base) != 0
@@ -326,7 +313,8 @@ static int _elf_disas32(FormatPlugin * format, int (*callback)(
 			continue;
 		if(shdr[i].sh_type == SHT_PROGBITS
 				&& shdr[i].sh_flags & SHF_EXECINSTR)
-			callback(format, &shstrtab[shdr[i].sh_name],
+			helper->disas(helper->format,
+					&shstrtab[shdr[i].sh_name],
 					shdr[i].sh_offset, shdr[i].sh_size,
 					base);
 	}
@@ -338,24 +326,27 @@ static int _elf_disas32(FormatPlugin * format, int (*callback)(
 static int _disas32_shdr(FormatPlugin * format, Elf32_Ehdr * ehdr,
 		Elf32_Shdr ** shdr)
 {
+	FormatPluginHelper * helper = format->helper;
+	ssize_t size;
+
 	if(ehdr->e_shentsize == 0)
 	{
 		*shdr = NULL;
 		return 0;
 	}
 	if(ehdr->e_shentsize != sizeof(**shdr))
-		return -error_set_code(1, "%s: %s", format->helper->filename,
+		return -error_set_code(1, "%s: %s",
+				helper->get_filename(helper->format),
 				"Invalid section header size");
-	if(fseek(format->helper->fp, ehdr->e_shoff, SEEK_SET) != 0
-			|| (*shdr = malloc(ehdr->e_shentsize * ehdr->e_shnum))
-			== NULL)
+	if(helper->seek(helper->format, ehdr->e_shoff, SEEK_SET) < 0)
+		return -1;
+	size = sizeof(**shdr) * ehdr->e_shnum;
+	if((*shdr = malloc(size)) == NULL)
 		return -_elf_error(format);
-	if(fread(*shdr, sizeof(**shdr), ehdr->e_shnum, format->helper->fp)
-			!= ehdr->e_shnum)
+	if(helper->read(helper->format, *shdr, size) != size)
 	{
 		free(*shdr);
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				"Short read");
+		return -1;
 	}
 	return 0;
 }
@@ -363,14 +354,16 @@ static int _disas32_shdr(FormatPlugin * format, Elf32_Ehdr * ehdr,
 static int _disas32_addr(FormatPlugin * format, Elf32_Ehdr * ehdr,
 		Elf32_Addr * addr)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf32_Half i;
 	Elf32_Phdr phdr;
 
-	if(fseek(format->helper->fp, ehdr->e_phoff, SEEK_SET) != 0)
-		return -_elf_error(format);
+	if(helper->seek(helper->format, ehdr->e_phoff, SEEK_SET) < 0)
+		return -1;
 	for(i = 0; i < ehdr->e_phnum; i++)
-		if(fread(&phdr, sizeof(phdr), 1, format->helper->fp) != 1)
-			return -_elf_error(format);
+		if(helper->read(helper->format, &phdr, sizeof(phdr))
+				!= sizeof(phdr))
+			return -1;
 		else if(phdr.p_type == PT_LOAD && phdr.p_flags & (PF_R | PF_X))
 		{
 			*addr = phdr.p_vaddr;
@@ -384,19 +377,22 @@ static int _disas32_strtab(FormatPlugin * format, Elf32_Shdr * shdr,
 		size_t shdr_cnt, uint16_t ndx, char ** strtab,
 		size_t * strtab_cnt)
 {
+	FormatPluginHelper * helper = format->helper;
+
 	if(ndx >= shdr_cnt)
-		return -error_set_code(1, "%s: %s", format->helper->filename,
+		return -error_set_code(1, "%s: %s",
+				helper->get_filename(helper->format),
 				"Unable to read the string table");
 	shdr = &shdr[ndx];
-	if(fseek(format->helper->fp, shdr->sh_offset, SEEK_SET) != 0
-			|| (*strtab = malloc(shdr->sh_size)) == NULL)
+	if(helper->seek(helper->format, shdr->sh_offset, SEEK_SET) < 0)
+		return -1;
+	if((*strtab = malloc(shdr->sh_size)) == NULL)
 		return -_elf_error(format);
-	if(fread(*strtab, sizeof(**strtab), shdr->sh_size, format->helper->fp)
+	if(helper->read(helper->format, *strtab, shdr->sh_size)
 			!= shdr->sh_size)
 	{
 		free(*strtab);
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				"Short read");
+		return -1;
 	}
 	*strtab_cnt = shdr->sh_size;
 	return 0;
@@ -412,10 +408,9 @@ static int _disas64_strtab(FormatPlugin * format, Elf64_Shdr * shdr,
 		size_t shdr_cnt, uint16_t ndx, char ** strtab,
 		size_t * strtab_cnt);
 
-static int _elf_disas64(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _elf_disas64(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf64_Ehdr ehdr;
 	Elf64_Shdr * shdr = NULL;
 	Elf64_Addr base = 0x0;
@@ -425,12 +420,12 @@ static int _elf_disas64(FormatPlugin * format, int (*callback)(
 
 #ifdef DEBUG
 	fprintf(stderr, "DEBUG: %s() \"%s\"\n", __func__,
-			format->helper->filename);
+			helper->get_filename(helper->format));
 #endif
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0
-			|| fread(&ehdr, sizeof(ehdr), 1, format->helper->fp)
-			!= 1)
-		return -_elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0
+			|| helper->read(helper->format, &ehdr, sizeof(ehdr))
+			!= sizeof(ehdr))
+		return -1;
 	if(_disas64_shdr(format, &ehdr, &shdr) != 0)
 		return -1;
 	if(_disas64_addr(format, &ehdr, &base) != 0
@@ -447,7 +442,8 @@ static int _elf_disas64(FormatPlugin * format, int (*callback)(
 			continue;
 		if(shdr[i].sh_type == SHT_PROGBITS
 				&& shdr[i].sh_flags & SHF_EXECINSTR)
-			callback(format, &shstrtab[shdr[i].sh_name],
+			helper->disas(helper->format,
+					&shstrtab[shdr[i].sh_name],
 					shdr[i].sh_offset, shdr[i].sh_size,
 					base);
 	}
@@ -459,24 +455,27 @@ static int _elf_disas64(FormatPlugin * format, int (*callback)(
 static int _disas64_shdr(FormatPlugin * format, Elf64_Ehdr * ehdr,
 		Elf64_Shdr ** shdr)
 {
+	FormatPluginHelper * helper = format->helper;
+	ssize_t size;
+
 	if(ehdr->e_shentsize == 0)
 	{
 		*shdr = NULL;
 		return 0;
 	}
 	if(ehdr->e_shentsize != sizeof(**shdr))
-		return -error_set_code(1, "%s: %s", format->helper->filename,
+		return -error_set_code(1, "%s: %s",
+				helper->get_filename(helper->format),
 				"Invalid section header size");
-	if(fseek(format->helper->fp, ehdr->e_shoff, SEEK_SET) != 0
-			|| (*shdr = malloc(ehdr->e_shentsize * ehdr->e_shnum))
-			== NULL)
+	if(helper->seek(helper->format, ehdr->e_shoff, SEEK_SET) < 0)
+		return -1;
+	size = sizeof(**shdr) * ehdr->e_shnum;
+	if((*shdr = malloc(size)) == NULL)
 		return -_elf_error(format);
-	if(fread(*shdr, sizeof(**shdr), ehdr->e_shnum, format->helper->fp)
-			!= ehdr->e_shnum)
+	if(helper->read(helper->format, *shdr, size) != size)
 	{
 		free(*shdr);
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				"Short read");
+		return -1;
 	}
 	return 0;
 }
@@ -484,14 +483,16 @@ static int _disas64_shdr(FormatPlugin * format, Elf64_Ehdr * ehdr,
 static int _disas64_addr(FormatPlugin * format, Elf64_Ehdr * ehdr,
 		Elf64_Addr * addr)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf64_Quarter i;
 	Elf64_Phdr phdr;
 
-	if(fseek(format->helper->fp, ehdr->e_phoff, SEEK_SET) != 0)
-		return -_elf_error(format);
+	if(helper->seek(helper->format, ehdr->e_phoff, SEEK_SET) < 0)
+		return -1;
 	for(i = 0; i < ehdr->e_phnum; i++)
-		if(fread(&phdr, sizeof(phdr), 1, format->helper->fp) != 1)
-			return -_elf_error(format);
+		if(helper->read(helper->format, &phdr, sizeof(phdr))
+				!= sizeof(phdr))
+			return -1;
 		else if(phdr.p_type == PT_LOAD && phdr.p_flags & (PF_R | PF_X))
 		{
 			*addr = phdr.p_vaddr;
@@ -505,19 +506,23 @@ static int _disas64_strtab(FormatPlugin * format, Elf64_Shdr * shdr,
 		size_t shdr_cnt, uint16_t ndx, char ** strtab,
 		size_t * strtab_cnt)
 {
+	FormatPluginHelper * helper = format->helper;
+	ssize_t size;
+
 	if(ndx >= shdr_cnt)
-		return -error_set_code(1, "%s: %s", format->helper->filename,
+		return -error_set_code(1, "%s: %s",
+				helper->get_filename(helper->format),
 				"Unable to read the string table");
 	shdr = &shdr[ndx];
-	if(fseek(format->helper->fp, shdr->sh_offset, SEEK_SET) != 0
-			|| (*strtab = malloc(shdr->sh_size)) == NULL)
+	if(helper->seek(helper->format, shdr->sh_offset, SEEK_SET) < 0)
+		return -1;
+	size = sizeof(**strtab) * shdr->sh_size;
+	if((*strtab = malloc(size)) == NULL)
 		return -_elf_error(format);
-	if(fread(*strtab, sizeof(**strtab), shdr->sh_size, format->helper->fp)
-			!= shdr->sh_size)
+	if(helper->read(helper->format, *strtab, size) != size)
 	{
 		free(*strtab);
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				"Short read");
+		return -1;
 	}
 	*strtab_cnt = shdr->sh_size;
 	return 0;
@@ -549,6 +554,7 @@ static int es32_cnt = 0;
 /* init_32 */
 static int _init_32(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf32_Ehdr hdr;
 
 	memset(&hdr, 0, sizeof(hdr));
@@ -574,7 +580,7 @@ static int _init_32(FormatPlugin * format)
 		hdr.e_shentsize = _htol16(sizeof(Elf32_Shdr));
 		hdr.e_shstrndx = _htol16(SHN_UNDEF);
 	}
-	if(fwrite(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
+	if(helper->write(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
 		return _elf_error(format);
 	return 0;
 }
@@ -587,18 +593,19 @@ static int _exit_32_shdr(FormatPlugin * format, Elf32_Off offset);
 static int _exit_32(FormatPlugin * format)
 {
 	int ret = 0;
+	FormatPluginHelper * helper = format->helper;
 	long offset;
 
 	if(_section_32(format, ".shstrtab") != 0)
-		ret = 1;
-	else if(fwrite(shstrtab.buf, sizeof(char), shstrtab.cnt,
-				format->helper->fp) != shstrtab.cnt)
-		ret = _elf_error(format);
-	else if((offset = ftell(format->helper->fp)) == -1)
-		ret = _elf_error(format);
+		ret = -1;
+	else if(helper->write(helper->format, shstrtab.buf, shstrtab.cnt)
+			!= (ssize_t)shstrtab.cnt)
+		ret = -1;
+	else if((offset = helper->seek(helper->format, 0, SEEK_CUR)) < 0)
+		ret = -1;
 	else if(_exit_32_phdr(format, offset) != 0
 			|| _exit_32_shdr(format, offset) != 0)
-		ret = 1;
+		ret = -1;
 	free(es32);
 	es32 = NULL;
 	es32_cnt = 0;
@@ -610,13 +617,14 @@ static int _exit_32(FormatPlugin * format)
 
 static int _exit_32_phdr(FormatPlugin * format, Elf32_Off offset)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf32_Ehdr hdr;
 
 	if(es32_cnt == 0)
 		return 0;
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _elf_error(format);
-	if(fread(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->read(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
 		return _elf_error(format);
 	if(ea->endian == ELFDATA2MSB)
 	{
@@ -630,19 +638,20 @@ static int _exit_32_phdr(FormatPlugin * format, Elf32_Off offset)
 		hdr.e_shnum = _htol16(es32_cnt + 1);
 		hdr.e_shstrndx = _htol16(es32_cnt);
 	}
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _elf_error(format);
-	if(fwrite(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
-		return _elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->write(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
+		return -1;
 	return 0;
 }
 
 static int _exit_32_shdr(FormatPlugin * format, Elf32_Off offset)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf32_Shdr hdr;
 	int i;
 
-	if(fseek(format->helper->fp, 0, SEEK_END) != 0)
+	if(helper->seek(helper->format, 0, SEEK_END) < 0)
 		return _elf_error(format);
 	memset(&hdr, 0, sizeof(hdr));
 	if(ea->endian == ELFDATA2MSB)
@@ -655,20 +664,20 @@ static int _exit_32_shdr(FormatPlugin * format, Elf32_Off offset)
 		hdr.sh_type = _htol32(SHT_NULL);
 		hdr.sh_link = _htol32(SHN_UNDEF);
 	}
-	if(fwrite(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
-		return _elf_error(format);
+	if(helper->write(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
+		return -1;
 	for(i = 0; i < es32_cnt; i++)
 	{
-		if(i+1 == es32_cnt)
+		if(i + 1 == es32_cnt)
 			es32[i].sh_size = offset - es32[i].sh_offset;
 		else
 			es32[i].sh_size = es32[i + 1].sh_offset
 				- es32[i].sh_offset;
 		es32[i].sh_size = ea->endian == ELFDATA2MSB
 			? _htob32(es32[i].sh_size) : _htol32(es32[i].sh_size);
-		if(fwrite(&es32[i], sizeof(Elf32_Shdr), 1, format->helper->fp)
-				!= 1)
-			return _elf_error(format);
+		if(helper->write(helper->format, &es32[i], sizeof(Elf32_Shdr))
+				!= sizeof(Elf32_Shdr))
+			return -1;
 	}
 	return 0;
 }
@@ -677,13 +686,14 @@ static int _exit_32_shdr(FormatPlugin * format, Elf32_Off offset)
 /* section_32 */
 static int _section_32(FormatPlugin * format, char const * name)
 {
+	FormatPluginHelper * helper = format->helper;
 	int ss;
 	Elf32_Shdr * p;
 	ElfSectionValues * esv;
 	long offset;
 
 	if((ss = _elfstrtab_set(format, &shstrtab, name)) < 0)
-		return 1;
+		return -1;
 	if((p = realloc(es32, sizeof(*es32) * (es32_cnt + 1))) == NULL)
 		return _elf_error(format);
 	es32 = p;
@@ -693,8 +703,8 @@ static int _section_32(FormatPlugin * format, char const * name)
 	p->sh_name = ss;
 	p->sh_type = esv->type;
 	p->sh_flags = esv->flags;
-	if((offset = ftell(format->helper->fp)) == -1)
-		return _elf_error(format);
+	if((offset = helper->seek(helper->format, 0, SEEK_CUR)) < 0)
+		return -1;
 	p->sh_offset = offset;
 	p->sh_link = SHN_UNDEF; /* FIXME */
 	return 0;
@@ -710,6 +720,7 @@ static int es64_cnt = 0;
 /* init_64 */
 static int _init_64(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf64_Ehdr hdr;
 
 	memset(&hdr, 0, sizeof(hdr));
@@ -734,8 +745,8 @@ static int _init_64(FormatPlugin * format)
 		hdr.e_shentsize = _htol16(sizeof(Elf64_Shdr));
 	}
 	hdr.e_shstrndx = SHN_UNDEF;
-	if(fwrite(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
-		return _elf_error(format);
+	if(helper->write(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
+		return -1;
 	return 0;
 }
 
@@ -747,15 +758,16 @@ static int _exit_64_shdr(FormatPlugin * format, Elf64_Off offset);
 static int _exit_64(FormatPlugin * format)
 {
 	int ret = 0;
+	FormatPluginHelper * helper = format->helper;
 	long offset;
 
 	if(_section_64(format, ".shstrtab") != 0)
 		ret = 1;
-	else if(fwrite(shstrtab.buf, sizeof(char), shstrtab.cnt,
-				format->helper->fp) != shstrtab.cnt)
-		ret = _elf_error(format);
-	else if((offset = ftell(format->helper->fp)) == -1)
-		ret = _elf_error(format);
+	else if(helper->write(helper->format, shstrtab.buf, shstrtab.cnt)
+				!= (ssize_t)shstrtab.cnt)
+		ret = -1;
+	else if((offset = helper->seek(helper->format, 0, SEEK_CUR)) < 0)
+		ret = -1;
 	else if(_exit_64_phdr(format, offset) != 0
 			|| _exit_64_shdr(format, offset) != 0)
 		ret = 1;
@@ -770,14 +782,15 @@ static int _exit_64(FormatPlugin * format)
 
 static int _exit_64_phdr(FormatPlugin * format, Elf64_Off offset)
 {
+	FormatPluginHelper * helper = format->helper;
 	Elf64_Ehdr hdr;
 
 	if(es64_cnt == 0)
 		return 0;
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _elf_error(format);
-	if(fread(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
-		return _elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->read(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
+		return -1;
 	if(ea->endian == ELFDATA2MSB)
 	{
 		hdr.e_shoff = _htob64(offset);
@@ -790,19 +803,20 @@ static int _exit_64_phdr(FormatPlugin * format, Elf64_Off offset)
 		hdr.e_shnum = _htol16(es64_cnt);
 		hdr.e_shstrndx = _htol16(es64_cnt - 1);
 	}
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _elf_error(format);
-	if(fwrite(&hdr, sizeof(hdr), 1, format->helper->fp) != 1)
-		return _elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->write(helper->format, &hdr, sizeof(hdr)) != sizeof(hdr))
+		return -1;
 	return 0;
 }
 
 static int _exit_64_shdr(FormatPlugin * format, Elf64_Off offset)
 {
+	FormatPluginHelper * helper = format->helper;
 	int i;
 
-	if(fseek(format->helper->fp, 0, SEEK_END) != 0)
-		return _elf_error(format);
+	if(helper->seek(helper->format, 0, SEEK_END) < 0)
+		return -1;
 	for(i = 0; i < es64_cnt; i++)
 	{
 		if(i + 1 == es64_cnt)
@@ -812,9 +826,9 @@ static int _exit_64_shdr(FormatPlugin * format, Elf64_Off offset)
 				- es64[i].sh_offset;
 		es64[i].sh_size = ea->endian == ELFDATA2MSB
 			? _htob64(es64[i].sh_size) : _htol64(es64[i].sh_size);
-		if(fwrite(&es64[i], sizeof(Elf64_Shdr), 1, format->helper->fp)
-				!= 1)
-			return _elf_error(format);
+		if(helper->write(helper->format, &es64[i], sizeof(Elf64_Shdr))
+				!= sizeof(Elf64_Shdr))
+			return -1;
 	}
 	return 0;
 }
@@ -823,6 +837,7 @@ static int _exit_64_shdr(FormatPlugin * format, Elf64_Off offset)
 /* section_64 */
 static int _section_64(FormatPlugin * format, char const * name)
 {
+	FormatPluginHelper * helper = format->helper;
 	int ss;
 	Elf64_Shdr * p;
 	ElfSectionValues * esv;
@@ -839,8 +854,8 @@ static int _section_64(FormatPlugin * format, char const * name)
 	p->sh_name = ss;
 	p->sh_type = esv->type;
 	p->sh_flags = esv->flags;
-	if((offset = ftell(format->helper->fp)) == -1)
-		return _elf_error(format);
+	if((offset = helper->seek(helper->format, 0, SEEK_CUR)) < 0)
+		return -1;
 	p->sh_offset = offset;
 	p->sh_link = SHN_UNDEF; /* FIXME */
 	return 0;
diff --git a/src/format/flat.c b/src/format/flat.c
index 57402bf..235846f 100644
--- a/src/format/flat.c
+++ b/src/format/flat.c
@@ -16,9 +16,7 @@
 
 
 #include <System.h>
-#include <sys/stat.h>
 #include <string.h>
-#include <errno.h>
 #include "Asm.h"
 
 
@@ -26,9 +24,7 @@
 /* private */
 /* prototypes */
 /* plug-in */
-static int _flat_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _flat_disas(FormatPlugin * format);
 
 
 /* public */
@@ -53,14 +49,12 @@ FormatPlugin format_plugin =
 /* functions */
 /* plug-in */
 /* flat_disas */
-static int _flat_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _flat_disas(FormatPlugin * format)
 {
-	struct stat st;
+	FormatPluginHelper * helper = format->helper;
+	off_t offset;
 
-	if(fstat(fileno(format->helper->fp), &st) != 0)
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				strerror(errno));
-	return callback(format, ".data", 0, st.st_size, 0);
+	if((offset = helper->seek(helper->format, 0, SEEK_END)) < 0)
+		return -1;
+	return helper->disas(helper->format, ".data", 0, offset, 0);
 }
diff --git a/src/format/java.c b/src/format/java.c
index cc11635..ea50dcc 100644
--- a/src/format/java.c
+++ b/src/format/java.c
@@ -105,12 +105,9 @@ static char _java_signature[4] = "\xca\xfe\xba\xbe";
 static int _java_init(FormatPlugin * format, char const * arch);
 static int _java_exit(FormatPlugin * format);
 static char const * _java_detect(FormatPlugin * format);
-static int _java_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _java_disas(FormatPlugin * format);
 
 static int _java_error(FormatPlugin * format);
-static int _java_error_fread(FormatPlugin * format);
 
 
 /* public */
@@ -137,6 +134,7 @@ FormatPlugin format_plugin =
 /* java_init */
 static int _java_init(FormatPlugin * format, char const * arch)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaHeader jh;
 	JavaPlugin * java;
 
@@ -147,8 +145,8 @@ static int _java_init(FormatPlugin * format, char const * arch)
 	jh.minor = _htob16(0);
 	jh.major = _htob16(0x32); /* XXX choose a more appropriate version */
 	jh.cp_cnt = _htob16(0);
-	if(fwrite(&jh, sizeof(jh), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &jh, sizeof(jh)) != sizeof(jh))
+		return -1;
 	if((java = malloc(sizeof(*java))) == NULL)
 		return -_java_error(format);
 	memset(java, 0, sizeof(*java));
@@ -186,85 +184,96 @@ static int _java_exit(FormatPlugin * format)
 
 static int _exit_constant_pool(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t cnt = _htob16(java->constants_cnt + 1);
 
-	if(fwrite(&cnt, sizeof(cnt), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &cnt, sizeof(cnt)) != sizeof(cnt))
+		return -1;
 	/* XXX output the constants */
 	return 0;
 }
 
 static int _exit_access_flags(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t flags = _htob16(java->access_flags);
 
-	if(fwrite(&flags, sizeof(flags), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &flags, sizeof(flags))
+			!= sizeof(flags))
+		return -1;
 	return 0;
 }
 
 static int _exit_class_name(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	uint16_t index = _htob16(0);
 
 	/* FIXME really implement */
-	if(fwrite(&index, sizeof(index), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &index, sizeof(index))
+			!= sizeof(index))
+		return -1;
 	return 0;
 }
 
 static int _exit_super_name(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	uint16_t index = _htob16(0);
 
 	/* FIXME really implement */
-	if(fwrite(&index, sizeof(index), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &index, sizeof(index))
+			!= sizeof(index))
+		return -1;
 	return 0;
 }
 
 static int _exit_interface_table(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t cnt = _htob16(java->interfaces_cnt);
 
-	if(fwrite(&cnt, sizeof(cnt), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &cnt, sizeof(cnt)) != sizeof(cnt))
+		return -1;
 	/* XXX output the interfaces */
 	return 0;
 }
 
 static int _exit_field_table(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t cnt = _htob16(java->fields_cnt);
 
-	if(fwrite(&cnt, sizeof(cnt), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &cnt, sizeof(cnt)) != sizeof(cnt))
+		return -1;
 	/* XXX output the fields */
 	return 0;
 }
 
 static int _exit_method_table(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t cnt = _htob16(java->methods_cnt);
 
-	if(fwrite(&cnt, sizeof(cnt), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &cnt, sizeof(cnt)) != sizeof(cnt))
+		return -1;
 	/* XXX output the methods */
 	return 0;
 }
 
 static int _exit_attribute_table(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	JavaPlugin * java = format->priv;
 	uint16_t cnt = _htob16(java->attributes_cnt);
 
-	if(fwrite(&cnt, sizeof(cnt), 1, format->helper->fp) != 1)
-		return -_java_error(format);
+	if(helper->write(helper->format, &cnt, sizeof(cnt)) != sizeof(cnt))
+		return -1;
 	/* XXX output the attributes */
 	return 0;
 }
@@ -283,48 +292,47 @@ static int _disas_skip_constants(FormatPlugin * format, uint16_t cnt);
 static int _disas_skip_fields(FormatPlugin * format, uint16_t cnt);
 static int _disas_skip_interfaces(FormatPlugin * format, uint16_t cnt);
 
-static int _java_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _java_disas(FormatPlugin * format)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	JavaHeader jh;
 	JavaHeader2 jh2;
 	uint16_t u16;
 	off_t offset;
 	off_t end;
 
-	if(fseek(fp, sizeof(JavaHeader), SEEK_SET) != 0)
-		return -_java_error(format);
-	if(fread(&jh, sizeof(jh), 1, fp) != 1)
-		return -_java_error_fread(format);
+	/* FIXME can this be correct? */
+	if(helper->seek(helper->format, sizeof(jh), SEEK_SET) != sizeof(jh))
+		return -1;
+	if(helper->read(helper->format, &jh, sizeof(jh)) != sizeof(jh))
+		return -1;
 	/* skip constants */
 	jh.cp_cnt = _htob16(jh.cp_cnt);
 	if(jh.cp_cnt > 1 && _disas_skip_constants(format, jh.cp_cnt) != 0)
 		return -1;
 	/* skip interfaces */
-	if(fread(&jh2, sizeof(jh2), 1, fp) != 1)
-		return -_java_error_fread(format);
+	if(helper->read(helper->format, &jh2, sizeof(jh2)) != sizeof(jh2))
+		return -1;
 	jh2.interfaces_cnt = _htob16(jh2.interfaces_cnt);
 	if(_disas_skip_interfaces(format, jh2.interfaces_cnt) != 0)
 		return -1;
 	/* skip fields */
-	if(fread(&u16, sizeof(u16), 1, fp) != 1)
-		return -_java_error_fread(format);
+	if(helper->read(helper->format, &u16, sizeof(u16)) != sizeof(u16))
+		return -1;
 	u16 = _htob16(u16);
 	if(_disas_skip_fields(format, u16) != 0)
 		return -1;
 	/* disassemble the rest */
-	if((offset = ftello(fp)) == -1
-			|| fseek(fp, 0, SEEK_END) != 0
-			|| (end = ftello(fp)) == -1)
-		return -_java_error(format);
-	return callback(format, NULL, offset, end - offset, 0);
+	if((offset = helper->seek(helper->format, 0, SEEK_CUR)) < 0
+			|| (end = helper->seek(helper->format, 0, SEEK_END))
+			< 0)
+		return -1;
+	return helper->disas(helper->format, NULL, offset, end - offset, 0);
 }
 
 static int _disas_skip_attributes(FormatPlugin * format, uint16_t cnt)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	size_t i;
 	JavaAttributeInfo jai;
 
@@ -333,22 +341,23 @@ static int _disas_skip_attributes(FormatPlugin * format, uint16_t cnt)
 #endif
 	for(i = 0; i < cnt; i++)
 	{
-		if(fread(&jai, sizeof(jai), 1, fp) != 1)
-			return -_java_error_fread(format);
+		if(helper->read(helper->format, &jai, sizeof(jai))
+				!= sizeof(jai))
+			return -1;
 		jai.length = _htob32(jai.length);
 #ifdef DEBUG
 		fprintf(stderr, "DEBUG: %s() length=%u\n", __func__,
 				jai.length);
 #endif
-		if(fseek(fp, jai.length, SEEK_CUR) != 0)
-			return -_java_error(format);
+		if(helper->seek(helper->format, jai.length, SEEK_CUR) < 0)
+			return -1;
 	}
 	return 0;
 }
 
 static int _disas_skip_constants(FormatPlugin * format, uint16_t cnt)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	size_t i;
 	JavaCpInfo jci;
 	size_t size;
@@ -360,8 +369,9 @@ static int _disas_skip_constants(FormatPlugin * format, uint16_t cnt)
 #endif
 	for(i = 0; i < cnt; i++)
 	{
-		if(fread(&jci, sizeof(jci), 1, fp) != 1)
-			return -_java_error_fread(format);
+		if(helper->read(helper->format, &jci, sizeof(jci))
+				!= sizeof(jci))
+			return -1;
 		switch(jci.tag)
 		{
 			case CONSTANT_Double:
@@ -381,28 +391,33 @@ static int _disas_skip_constants(FormatPlugin * format, uint16_t cnt)
 				size = 2;
 				break;
 			case CONSTANT_Utf8:
-				if(fread(&u16, sizeof(u16), 1, fp) != 1)
-					return -_java_error_fread(format);
+				size = sizeof(u16);
+				if(helper->read(helper->format, &u16, size)
+						!= (ssize_t)size)
+					return -1;
 				u16 = _htob16(u16);
-				if(fseek(fp, u16, SEEK_CUR) != 0)
-					return -_java_error(format);
+				if(helper->seek(helper->format, u16, SEEK_CUR)
+						< 0)
+					return -1;
 				size = 0;
 				break;
 			default:
 				return -error_set_code(1, "%s: %s 0x%x",
-						format->helper->filename,
+						helper->get_filename(
+							helper->format),
 						"Unknown constant tag",
 						jci.tag);
 		}
-		if(size != 0 && fread(buf, sizeof(*buf), size, fp) != size)
-			return -_java_error_fread(format);
+		if(size != 0 && helper->read(helper->format, buf, size)
+				!= (ssize_t)size)
+			return -1;
 	}
 	return 0;
 }
 
 static int _disas_skip_fields(FormatPlugin * format, uint16_t cnt)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	size_t i;
 	JavaFieldInfo jfi;
 
@@ -411,8 +426,9 @@ static int _disas_skip_fields(FormatPlugin * format, uint16_t cnt)
 #endif
 	for(i = 0; i < cnt; i++)
 	{
-		if(fread(&jfi, sizeof(jfi), 1, fp) != 1)
-			return -_java_error_fread(format);
+		if(helper->read(helper->format, &jfi, sizeof(jfi))
+				!= sizeof(jfi))
+			return -1;
 		jfi.attributes_cnt = _htob16(jfi.attributes_cnt);
 		_disas_skip_attributes(format, jfi.attributes_cnt);
 	}
@@ -421,7 +437,7 @@ static int _disas_skip_fields(FormatPlugin * format, uint16_t cnt)
 
 static int _disas_skip_interfaces(FormatPlugin * format, uint16_t cnt)
 {
-	FILE * fp = format->helper->fp;
+	FormatPluginHelper * helper = format->helper;
 	size_t i;
 	uint16_t u16;
 
@@ -429,8 +445,9 @@ static int _disas_skip_interfaces(FormatPlugin * format, uint16_t cnt)
 	fprintf(stderr, "DEBUG: %s(%u)\n", __func__, cnt);
 #endif
 	for(i = 0; i < cnt; i++)
-		if(fread(&u16, sizeof(u16), 1, fp) != 1)
-			return -_java_error_fread(format);
+		if(helper->read(helper->format, &u16, sizeof(u16))
+				!= sizeof(u16))
+			return -1;
 	return 0;
 }
 
@@ -438,16 +455,6 @@ static int _disas_skip_interfaces(FormatPlugin * format, uint16_t cnt)
 /* java_error */
 static int _java_error(FormatPlugin * format)
 {
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			strerror(errno));
-}
-
-
-/* java_error_fread */
-static int _java_error_fread(FormatPlugin * format)
-{
-	if(ferror(format->helper->fp))
-		return _java_error(format);
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			"End of file reached");
+	return -error_set_code(1, "%s: %s", format->helper->get_filename(
+				format->helper->format), strerror(errno));
 }
diff --git a/src/format/pe.c b/src/format/pe.c
index e2b6286..73eb320 100644
--- a/src/format/pe.c
+++ b/src/format/pe.c
@@ -89,9 +89,7 @@ static char const _pe_header_signature[4] = "PE\0\0";
 /* plug-in */
 static int _pe_init(FormatPlugin * format, char const * arch);
 static char const * _pe_detect(FormatPlugin * format);
-static int _pe_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base));
+static int _pe_disas(FormatPlugin * format);
 
 /* useful */
 static char const * _pe_get_arch(uint16_t machine);
@@ -121,6 +119,7 @@ FormatPlugin format_plugin =
 /* pe_init */
 static int _pe_init(FormatPlugin * format, char const * arch)
 {
+	FormatPluginHelper * helper = format->helper;
 	int machine;
 	struct pe_msdos pm;
 	struct pe_header ph;
@@ -135,12 +134,10 @@ static int _pe_init(FormatPlugin * format, char const * arch)
 	ph.machine = _htol16(machine);
 	ph.timestamp = _htol32(time(NULL));
 	/* FIXME update the section and symbol lists */
-	if(fwrite(&pm, sizeof(pm), 1, format->helper->fp) != 1)
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				strerror(errno));
-	if(fwrite(&ph, sizeof(ph), 1, format->helper->fp) != 1)
-		return -error_set_code(1, "%s: %s", format->helper->filename,
-				strerror(errno));
+	if(helper->write(helper->format, &pm, sizeof(pm)) != sizeof(pm))
+		return -1;
+	if(helper->write(helper->format, &ph, sizeof(ph)) != sizeof(ph))
+		return -1;
 	return 0;
 }
 
@@ -150,25 +147,28 @@ static char const * _detect_error(FormatPlugin * format);
 
 static char const * _pe_detect(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	struct pe_msdos pm;
 	struct pe_header ph;
 
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _detect_error(format);
-	if(fread(&pm, sizeof(pm), 1, format->helper->fp) != 1)
-		return _detect_error(format);
-	if((pm.offset = _htol16(pm.offset)) != sizeof(pm)
-			&& fseek(format->helper->fp, pm.offset, SEEK_SET) != 0)
-		return _detect_error(format);
-	if(fread(&ph, sizeof(ph), 1, format->helper->fp) != 1)
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return NULL;
+	if(helper->read(helper->format, &pm, sizeof(pm)) != sizeof(pm))
+		return NULL;
+	if((pm.offset = _htol16(pm.offset)) != sizeof(pm))
 		return _detect_error(format);
+	if(helper->seek(helper->format, pm.offset, SEEK_SET) != pm.offset)
+		return NULL;
+	if(helper->read(helper->format, &ph, sizeof(ph)) != sizeof(ph))
+		return NULL;
 	ph.machine = _htol16(ph.machine);
 	return _pe_get_arch(ph.machine);
 }
 
 static char const * _detect_error(FormatPlugin * format)
 {
-	error_set_code(1, "%s: %s", format->helper->filename, strerror(errno));
+	error_set_code(1, "%s: %s", format->helper->get_filename(
+				format->helper->format), strerror(errno));
 	return NULL;
 }
 
@@ -176,47 +176,49 @@ static char const * _detect_error(FormatPlugin * format)
 /* pe_disas */
 static int _disas_error(FormatPlugin * format);
 
-static int _pe_disas(FormatPlugin * format, int (*callback)(
-			FormatPlugin * format, char const * section,
-			off_t offset, size_t size, off_t base))
+static int _pe_disas(FormatPlugin * format)
 {
+	FormatPluginHelper * helper = format->helper;
 	struct pe_msdos pm;
 	struct pe_header ph;
 	size_t i;
 	struct pe_section_header psh;
 
-	if(fseek(format->helper->fp, 0, SEEK_SET) != 0)
-		return _disas_error(format);
-	if(fread(&pm, sizeof(pm), 1, format->helper->fp) != 1)
-		return _disas_error(format);
+	if(helper->seek(helper->format, 0, SEEK_SET) != 0)
+		return -1;
+	if(helper->read(helper->format, &pm, sizeof(pm)) != sizeof(pm))
+		return -1;
 	if((pm.offset = _htol16(pm.offset)) != sizeof(pm)
-			&& fseek(format->helper->fp, pm.offset, SEEK_SET) != 0)
-		return _disas_error(format);
-	if(fread(&ph, sizeof(ph), 1, format->helper->fp) != 1)
+			&& helper->seek(helper->format, pm.offset, SEEK_SET)
+			!= pm.offset)
+		return -1;
+	if(helper->read(helper->format, &ph, sizeof(ph)) != sizeof(ph))
 		return _disas_error(format);
 	ph.section_cnt = _htol16(ph.section_cnt);
 	ph.opthdr_size = _htol16(ph.opthdr_size);
 	if(ph.section_cnt > 0 && ph.opthdr_size != 0
-			&& fseek(format->helper->fp, ph.opthdr_size, SEEK_CUR))
+			&& helper->seek(helper->format, ph.opthdr_size,
+				SEEK_CUR) < 0)
 		return _disas_error(format);
 	for(i = 0; i < ph.section_cnt; i++)
 	{
-		if(fread(&psh, sizeof(psh), 1, format->helper->fp) != 1)
-			return _disas_error(format);
+		if(helper->read(helper->format, &psh, sizeof(psh))
+				!= sizeof(psh))
+			return -1;
 		psh.name[sizeof(psh.name) - 1] = '\0';
 		psh.vaddr = _htol32(psh.vaddr);
 		psh.raw_size = _htol32(psh.raw_size);
 		psh.raw_offset = _htol32(psh.raw_offset);
-		callback(format, psh.name, psh.raw_offset, psh.raw_size,
-				psh.vaddr);
+		helper->disas(helper->format, psh.name, psh.raw_offset,
+				psh.raw_size, psh.vaddr);
 	}
 	return 0;
 }
 
 static int _disas_error(FormatPlugin * format)
 {
-	return -error_set_code(1, "%s: %s", format->helper->filename,
-			strerror(errno));
+	return -error_set_code(1, "%s: %s", format->helper->get_filename(
+				format->helper->format), strerror(errno));
 }