Can I link a plain file into my executable?

Question

Some frameworks (Qt, Windows, Gtk...) offer functionality to add resources to your binaries. I wonder if it would be possible to achieve this without the framework, since all that is really needed is

a symbol to contain the resource's address within the binary (data segment)
a symbol to represent the length of the resource
the resource itself

How can this be achieved with the gcc toolchain?

Well, you can implement all those things, and then what you'll have is a resource framework. — Lightness Races in Orbit, Dec 13 '12 at 09:40
@LightnessRacesinOrbit: indeed, but _without_ the Gui, Serialization, XML, ... that I don't currently need :) — xtofl, Dec 13 '12 at 09:50
Yeah you'll have a resource framework, not a behemothic does-everything framework. — Lightness Races in Orbit, Dec 13 '12 at 09:51
See also http://stackoverflow.com/questions/4864866/c-c-with-gcc-statically-add-resource-files-to-executable-library — nos, Dec 13 '12 at 09:59
@nos: ... and that question contains the answer to my question! http://stackoverflow.com/a/4865249/6610 — xtofl, Dec 13 '12 at 10:09
Precisely the same has been covered here as well: [Embedding a File in an Executable, aka Hello World](http://www.linuxjournal.com/content/embedding-file-executable-aka-hello-world-version-5967) — P.P, Aug 16 '15 at 10:27

score 48 · Accepted Answer · answered Dec 13 '12 at 09:56

48

You could do this:

objcopy --input binary \
        --output elf32-i386 \
        --binary-architecture i386 my_file.xml myfile.o

This produces an object file that you can link into your executable. This file will contain these symbols that you'll have to declare in your C code to be able to use them

00000550 D _binary_my_file_xml_end
00000550 A _binary_my_file_xml_size 
00000000 D _binary_my_file_xml_start

answered Dec 13 '12 at 09:56

nos

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exactly what I needed. Do you happen to know whether `objcopy` exists on windows too? – xtofl Dec 13 '12 at 10:10
2

@xtofl It's available on windows if you use mingw or cygwin at least, but converting the file to a C array would be the most portable thing to do across toolchains. – nos Dec 13 '12 at 10:24

score 31 · Answer 2 · edited May 23 '17 at 12:16

At its most basic, the equivalent is a char array full of bytes.

On Linux you can use xxd -i <file> to "compile" files into char arrays, then link the arrays into your binary and use the constituent bytes however you please.

Here's an example from my own code's makefile, that creates a "resource file" called templates.h containing a bunch of char arrays representing HTML templates:

templates.h:
    @echo "#ifndef REDACTED_TEMPLATES_H" > templates.h
    @echo "#define REDACTED_TEMPLATES_H" >> templates.h
    @echo "// Auto-generated file! Do not modify!" >> templates.h
    @echo "// NB: arrays are not null-terminated" >> templates.h
    @echo "// (anonymous namespace used to force internal linkage)" >> templates.h
    @echo "namespace {" >> templates.h
    @echo "namespace templates {" >> templates.h
    @cd templates;\
    for i in * ;\
    do \
        echo "Compiling $$i...";\
        xxd -i $$i | sed -e 's/ =/ __attribute__((unused)) =/' >> ../templates.h;\
    done;\
    cd ..
    @echo "}" >> templates.h
    @echo "}" >> templates.h
    @echo "#endif" >> templates.h

^{(see also: How best can I programmatically apply `__attribute__ ((unused))` to these auto-generated objects?)}

The result looks a little like:

#ifndef REDACTED_TEMPLATES_H
#define REDACTED_TEMPLATES_H
// Auto-generated file! Do not modify!
// NB: arrays are not null-terminated
// (anonymous namespace used to force internal linkage)
namespace {
namespace templates {
unsigned char alert_email_finished_events_html[] __attribute__((unused)) = {
  0x3c, 0x74, 0x61, 0x62, 0x6c, 0x65, 0x20, 0x63, 0x6c, 0x61, 0x73, 0x73,
  0x3d, 0x22, 0x6e, 0x6f, 0x64, 0x65, 0x2d, 0x69, 0x6e, 0x66, 0x6f, 0x2d,
[..]
  0x7d, 0x7d, 0x0d, 0x0a, 0x3c, 0x2f, 0x74, 0x61, 0x62, 0x6c, 0x65, 0x3e,
  0x0d, 0x0a
};
unsigned int alert_email_finished_events_html_len __attribute__((unused)) = 290;
unsigned char alert_email_finished_events_list_html[] __attribute__((unused)) = {
  0x3c, 0x74, 0x72, 0x20, 0x63, 0x6c, 0x61, 0x73, 0x73, 0x3d, 0x22, 0x73,
  0x65, 0x70, 0x61, 0x72, 0x61, 0x74, 0x65, 0x2d, 0x70, 0x72, 0x65, 0x76,
[..]
  0x73, 0x74, 0x7d, 0x7d, 0x0d, 0x0a
};
unsigned int alert_email_finished_events_list_html_len __attribute__((unused)) = 42;
}
}
#endif

Note that this particular example is optimal when using the resource in only one Translation Unit, but the general approach can be adapted to suit your needs.

Can I link a plain file into my executable?

2 Answers2

Linked

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