Compiling without libc

Question

I want to compile my C-code without the (g)libc. How can I deactivate it and which functions depend on it?

I tried -nostdlib but it doesn't help: The code is compilable and runs, but I can still find the name of the libc in the hexdump of my executable.

Answer 1

If you compile your code with -nostdlib, you won't be able to call any C library functions (of course), but you also don't get the regular C bootstrap code. In particular, the real entry point of a program on Linux is not main(), but rather a function called _start(). The standard libraries normally provide a version of this that runs some initialization code, then calls main().

Try compiling this with gcc -nostdlib -m32:

// Tell the compiler incoming stack alignment is not RSP%16==8 or ESP%16==12
__attribute__((force_align_arg_pointer))
void _start() {

    /* main body of program: call main(), etc */

    /* exit system call */
    asm("movl $1,%eax;"
        "xorl %ebx,%ebx;"
        "int  $0x80"
    );
    __builtin_unreachable();  // tell the compiler to make sure side effects are done before the asm statement
}

The _start() function should always end with a call to exit (or other non-returning system call such as exec). The above example invokes the system call directly with inline assembly since the usual exit() is not available.