Gnu c routines' entry point

Gnu c routines' entry point

 

I have validated that “_start” is the entry point of Gnu c routines.

The following are the verified materials:

 

/* test.c */

int main(void)

{

}

 

root@ubuntu:/work/learning/c/before_main# gcc -g -o test test.c

root@ubuntu:/work/learning/c/before_main# readelf -h test

ELF Header:

  Magic:   7f 45 4c 46 01 01 01 00 00 00 00 00 00 00 00 00

  Class:                             ELF32

  Data:                              2's complement, little endian

  Version:                           1 (current)

  OS/ABI:                            UNIX - System V

  ABI Version:                       0

  Type:                              EXEC (Executable file)

  Machine:                           Intel 80386

  Version:                           0x1

  Entry point address:   0x80482c0     <---From here ,we sure its entry point is ox80482c0

  Start of program headers:          52 (bytes into file)

  Start of section headers:          3716 (bytes into file)

  Flags:                             0x0

  Size of this header:               52 (bytes)

  Size of program headers:           32 (bytes)

  Number of program headers:         7

  Size of section headers:           40 (bytes)

  Number of section headers:         38

  Section header string table index: 35

 

 

root@ubuntu:/work/learning/c/before_main# gdb -q  test

Using host libthread_db library "/lib/tls/i686/cmov/libthread_db.so.1".

(gdb) disassemble 0x80482c0                    //disassemble the entry point

Dump of assembler code for function _start: <----And from here ,we know the entry point is "_start", not "__main"

0x080482c0 <__libc_start_main@plt+28>:  xor    %ebp,%ebp

0x080482c2 <__libc_start_main@plt+30>:  pop    %esi

0x080482c3 <__libc_start_main@plt+31>:  mov    %esp,%ecx

0x080482c5 <__libc_start_main@plt+33>:  and    $0xfffffff0,%esp

……

End of assembler dump.

(gdb)

 

root@ubuntu:/work/learning/c/before_main# objdump -t test | grep __main

 

root@ubuntu:/work/learning/c/before_main# objdump -t test | grep _start

08049420 l       .ctors 00000000              .hidden __init_array_start

0804951c  w      .data  00000000              data_start

080482c0 g     F .text  00000000              _start

00000000  w      *UND*  00000000              __gmon_start__

00000000       F *UND*  000001b2              __libc_start_main@@GLIBC_2.0

0804951c g       .data  00000000              __data_start

08049528 g       *ABS*  00000000              __bss_start

 

So the conclusion is that use “__main” as default entry point is arm’s implementation mode , while gnu c uses “_start” as default entry point.

 

Reference: << Before main() >>