I recently learnt about the __libc_start_main() function. I thought that __libc_start_main() calls the main() function as described in this answer, but when I checked the stack pointer $rsp after inserting a breakpoint in the main() function in the test program below, it's the address of __libc_start_call_main().
What's the difference between __libc_start_main() and __libc_start_call_main()?
#include <stdio.h>
int main(void)
{
puts("Sunghyeon Lee");
}
──(kali㉿kali)-[~]
└─$ gdb test
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Reading symbols from test...
(No debugging symbols found in test)
(gdb) b *main
Breakpoint 1 at 0x1139
(gdb) r
Starting program: /home/kali/test
[Thread debugging using libthread_db enabled]
Using host libthread_db library "/lib/x86_64-linux-gnu/libthread_db.so.1".
Breakpoint 1, 0x0000555555555139 in main ()
(gdb) x/a $rsp
0x7fffffffdec8: 0x7ffff7dd920a <__libc_start_call_main+122>
I searched for information about the difference between __libc_start_main() and __libc_start_call_main() but I haven't found an explanation.
I have never found the explanation about it.
Take a look at the commit which created __libc_start_call_main.
Effectively a chunk of __libc_start_main was split out into a separate routine.