If you are doing a lightning talk, please remember to either check that your laptop works with the projector in the hall perfectly before the talks start, or put your talk on a USB memory stick -- something portable such as PDF or S5 please -- so you can use the provided laptop (a ThinkPad T43, 1024x768).

First, a caveat. This probably requires GNU ld, which may or may not be a deal breaker for many people.

First, create a data file. Let's call it foo.txt, and put some text in it.

Hello, World!

Using ld this can be read in as a plain binary blob, and then written as a standard relocatable ELF object.

ld -r -b binary -o foo.o foo.txt

Now we have a standard ELF object with the data and some useful symbols defined. objdump will show you the contents.

$ objdump -x foo.o 
foo.o:     file format elf32-i386

Sections:
Idx Name          Size      VMA       LMA       File off  Algn
  0 .data         0000000d  00000000  00000000  00000034  2**0
                  CONTENTS, ALLOC, LOAD, DATA
SYMBOL TABLE:
00000000 l    d  .data  00000000 .data
0000000d g       .data  00000000 _binary_foo_txt_end
0000000d g       *ABS*  00000000 _binary_foo_txt_size
00000000 g       .data  00000000 _binary_foo_txt_start

Here we see 13 bytes of data, and a symbol which contains the address of the data. This is all we need to access it from a C program.

#include <stdio.h>
extern char _binary_foo_txt_start[];

int main (void) {
  puts (_binary_foo_txt_start);
  return 0;
}

Now if we compile this and link it against the generated object, we'll have a binary.

$ gcc -o test test.c foo.o
$ ./test
Hello, World!

Hooray! One small problem which alert people should have noticed: the string itself is in the .data section, which is read/write. For my use, I want it to be read-only data in the .rodata section so that it isn't copied for every instance of the application. As far as I know, this isn't possible with ld but objcopy will let us rename sections on the fly.

$ objcopy --rename-section .data=.rodata,alloc,load,readonly,data,contents foo.o foo.o
$ objdump  -h foo.o
...
  0 .rodata       0000000d  00000000  00000000  00000034  2**0

Excellent, problem solved. If you want to download this sample, I have a tarball. Many thanks to Daniel Jacobowitz for pointing out how to achieve this.

Update: note that any data embedded in the binary like this won't be terminated with a NULL. This is obvious in hindsight, but due to luck my example still worked. There might be a way of asking objcopy to append a 0 to the end of the data, but if not always remember to use the start and end pointers or size instead of just the start, or append a NULL yourself before converting to an ELF.

NP: (), Sigur Rós

Anybody know how to do this? Update: thanks to Daniel Jacobowitz for giving enough clues to a working, and clean, solution. I'll blog this shortly.

NP: The Sound Of A Handshake, cLOUDDEAD