Can I link a plain file into my executable? [duplicate]

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

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

How can this be achieved with the gcc toolchain?

Answer 1

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

Answer 2

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.