generating simplest binary code obj file

Question

I need to generate some simple obj/o file that would contain my binary procedure body (I need to write some very simple partial assembler)

I need to generate such obj binary that i could use with some linker to link and run this my procedure. I wonder 1) what would be the easiest obj format (I need something that i could link finaly with gcc but if somethink is easier there are as far as i know some tools to convert one obj format to another so i could use thic converter) 2) I wonder what this code generating this obj context would look like - assume that i have binary asm contents to flush into but i need only make the obj "envelope" around it

I even not necessarely need to flush more procedures than one (only one would suffice, in such case i could generate seperate obj for each procedure) but i would like also some code to embedd my exported procedure symbol name and also to embedd names of imported symbols (though it is maybe also not so critical, as i could pass them by arguments_ I m mainly interested for extremal simplicity that would allow me to run this

assume that i have say such data to flush

const int procedure_binary_body_max = 130;

char procedure_binary_body[procedure_binary_body_max]; //130 bytes of procedure body
char* procedure_symbol_name = "sse_dot";

and i need some code to make and save this as an obj file to disk that i could link it with gcc

could someone help with that? very much tnx

wouldn't be a way to write a translator instead of a partial assembler? The translator would simply translate your input to assembly source which is then conventionally assembled to linkable object file. — Deleted User
– Deleted User, Commented Jul 7, 2014 at 12:25
no, need obj generation, but could be as simple as possible - it shouldnt be too hard but some official documentations scares me, would need some more helpfull hint /snippet or something — user2214913
– user2214913, Commented Jul 7, 2014 at 12:42
the assembler can generate object from your translated source — Deleted User
– Deleted User, Commented Jul 15, 2014 at 21:06

score 0 · Accepted Answer · 2014-07-12 19:33:21Z

Note: the following is for linux on amd64, but the same principle would work on i386.

If you don't have a problem with calling external tools (in this case gcc and nasm), you can do it like this:

main.c:

#include <stdio.h>
#include <unistd.h>

void make_obj (const char *filename,
               const char *data,
               size_t length,
               const char *entry_point)
{
    const char *filename_s = "tmp.s";
    const char *filename_bin = "tmp.bin";

    FILE *f;
    char buf[1000];

    // create bin file
    f = fopen (filename_bin, "wb");
    fwrite (data, 1, length, f);
    fclose (f);

    // create assembly wrapper
    f = fopen (filename_s, "w");
    fprintf (f,
             "\t\tsection .text\n"
             "\t\tglobal %s\n"
             "%s:\n"
             "\t\tincbin \"%s\"\n",
             entry_point, entry_point, filename_bin);
    fclose (f);

    // call nasm to convert it to object file
    snprintf (buf, 1000, "nasm -f elf64 -o %s %s", filename, filename_s);
    system (buf);

    unlink (filename_bin);
    unlink (filename_s);
}

void test_add (const char *filename_o, const char *entry_point)
{
    const char *filename_c = "test.c";
    const char *filename_exe = "test";

    FILE *f;
    char buf[1000];

    f = fopen (filename_c, "w");
    fprintf (f,
             "#include <stdio.h>\n"
             "\n"
             "extern int %s (int a, int b);\n"
             "\n"
             "int main () {\n"
             "\tconst int a = 2;\n"
             "\tconst int b = 3;\n"
             "\n"
             "\tint c = %s (a, b);\n"
             "\n"
             "\tprintf (\"%%d + %%d = %%d\\n\", a, b, c);\n"
             "\n"
             "\treturn 0;\n"
             "}\n",
             entry_point, entry_point);
    fclose (f);

    snprintf (buf, 1000, "gcc -o %s %s %s",
              filename_exe, filename_c, filename_o);
    system (buf);

    snprintf (buf, 1000, "./%s", filename_exe);
    system (buf);

    unlink (filename_c);
    unlink (filename_exe);
}

int main ()
{
    const char code[] = {
        0x48, 0x89, 0xF8,            // mov rax,rdi
        0x48, 0x01, 0xF0,            // add rax,rsi
        0xC3                         // ret
    };

    const char *filename_o = "tmp.o";
    const char *entry_point = "add";

    make_obj (filename_o, code, sizeof(code), entry_point);
    test_add (filename_o, entry_point);
    unlink (filename_o);

    return 0;
}

Test it with:

cc -o main main.c
./main

If everything goes well main should generate an object file incorporating the machine code for an integer add function, a C file calling the machine code routine, compile the C file, and run it.

Result should be:

2 + 3 = 5

Comment out the unlink() calls to preserve the intermediate files.

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