C语言不支持重载，多种main()如何实现的呢？

int main() {}
int main(int argc, char *argv[]) {}

int main(int argc = 1, char **argv = NULL)

int main() {
  return 0;
}

(gdb) bt
#0  main () at main.c:2
(gdb)

gcc -gdwarf-5 main.c  -o main

gdb ./main -q
Reading symbols from /mtad/main...done.
(gdb) set backtrace past-entry
(gdb) set backtrace past-main
(gdb) show backtrace past-entry
Whether backtraces should continue past the entry point of a program is on.
(gdb) show backtrace past-main
Whether backtraces should continue past "main" is on.

(gdb) bt
#0  main () at main.c:2
#1  0x00007ffff7a2f555 in __libc_start_main () from /lib64/libc.so.6
#2  0x0000000000400429 in _start ()
(gdb)

(gdb) r
Starting program: xxx
Missing separate debuginfos, use: debuginfo-install glibc-2.17-317.el7.x86_64

Breakpoint 1, 0x0000000000400400 in _start ()
(gdb) s
Single stepping until exit from function _start,
which has no line number information.
0x00007ffff7a2f460 in __libc_start_main () from /lib64/libc.so.6

#include "bp-sym.h"

    .text
    .globl _start
    .type _start,@function
_start:
    /* Clear the frame pointer.  The ABI suggests this be done, to mark
       the outermost frame obviously.  */
    xorl %ebp, %ebp

    /* Extract the arguments as encoded on the stack and set up
       the arguments for `main': argc, argv.  envp will be determined
       later in __libc_start_main.  */
    popl %esi  /* Pop the argument count.  */
    movl %esp, %ecx  /* argv starts just at the current stack top.*/

    /* Before pushing the arguments align the stack to a 16-byte
    (SSE needs 16-byte alignment) boundary to avoid penalties from
    misaligned accesses.  Thanks to Edward Seidl <seidl@janed.com>
    for pointing this out.  */
    andl $0xfffffff0, %esp
    pushl %eax  /* Push garbage because we allocate
                   28 more bytes.  */

    /* Provide the highest stack address to the user code (for stacks
       which grow downwards).  */
    pushl %esp

    pushl %edx  /* Push address of the shared library
                   termination function.  */

#ifdef SHARED
    /* Load PIC register.  */
    call 1f
    addl $_GLOBAL_OFFSET_TABLE_, %ebx

    /* Push address of our own entry points to .fini and .init.  */
    leal __libc_csu_fini@GOTOFF(%ebx), %eax
    pushl %eax
    leal __libc_csu_init@GOTOFF(%ebx), %eax
    pushl %eax

    pushl %ecx  /* Push second argument: argv.  */
    pushl %esi  /* Push first argument: argc.  */

    pushl BP_SYM (main)@GOT(%ebx)

    /* Call the user's main function, and exit with its value.
       But let the libc call main.    */
    call BP_SYM (__libc_start_main)@PLT
#else
    /* Push address of our own entry points to .fini and .init.  */
    pushl $__libc_csu_fini
    pushl $__libc_csu_init

    pushl %ecx  /* Push second argument: argv.  */
    pushl %esi  /* Push first argument: argc.  */

    pushl $BP_SYM (main)

    /* Call the user's main function, and exit with its value.
       But let the libc call main.    */
    call BP_SYM (__libc_start_main)
#endif

    hlt   /* Crash if somehow `exit' does return.  */

#ifdef SHARED
1: movl (%esp), %ebx
    ret
#endif

/* To fulfill the System V/i386 ABI we need this symbol.  Yuck, it's so
   meaningless since we don't support machines < 80386.  */
    .section .rodata
    .globl _fp_hw
_fp_hw: .long 3
    .size _fp_hw, 4
    .type _fp_hw,@object

/* Define a symbol for the first piece of initialized data.  */
    .data
    .globl __data_start
__data_start:
    .long 0
    .weak data_start
    data_start = __data_start

void _start() {
  %ebp = 0;
  int argc = pop from stack
  char ** argv = top of stack;
  __libc_start_main(main, argc, argv, __libc_csu_init, __linc_csu_fini,
  edx, top of stack);
}

STATIC int
LIBC_START_MAIN (int (*main) (int, char **, char ** MAIN_AUXVEC_DECL),
                 int argc, char *__unbounded *__unbounded ubp_av,
#ifdef LIBC_START_MAIN_AUXVEC_ARG
                 ElfW(auxv_t) *__unbounded auxvec,
#endif
                 __typeof (main) init,
                 void (*fini) (void),
                 void (*rtld_fini) (void), void *__unbounded stack_end)
{
#if __BOUNDED_POINTERS__
  char **argv;
#else
# define argv ubp_av
#endif

  /* Result of the 'main' function.  */
  int result;

  __libc_multiple_libcs = &_dl_starting_up && !_dl_starting_up;

...
...
  if (init)
    (*init) (argc, argv, __environ MAIN_AUXVEC_PARAM);
...

  result = main (argc, argv, __environ MAIN_AUXVEC_PARAM);

  exit (result);
}