linux - ELF files and additional symbols -

i'm reading elf file format , i've noticed small hello world test program written in c++ contains additional initialization in _start symbol:

0000000000400770 <_start>: ...       40077f:       49 c7 c0 60 09 40 00    mov    $0x400960,%r8       400786:       48 c7 c1 f0 08 40 00    mov    $0x4008f0,%rcx       40078d:       48 c7 c7 5d 08 40 00    mov    $0x40085d,%rdi ... 

40077f __libc_csu_fini.

4008f0 __libc_csu_init.

40085d main.

shouldn't _start main? why not? happen if removed both of calls 40077f , 40008f0 , replaced nop? basically, significance of requiring libc?

looking @ glibc source code:

/* these functions passed __libc_start_main startup code.    these statically linked each program.  dynamically linked    programs, module come libc_nonshared.a , differs    libc.a module in doesn't call preinit array.  */   void __libc_csu_init (int argc, char **argv, char **envp) {   /* dynamically linked executables preinit array executed      dynamic linker (before initializing shared object).  */  #ifndef libc_nonshared   /* static executables, preinit happens right before init.  */   {     const size_t size = __preinit_array_end - __preinit_array_start;     size_t i;     (i = 0; < size; i++)       (*__preinit_array_start [i]) (argc, argv, envp);   } #endif  #ifndef no_initfini   _init (); #endif    const size_t size = __init_array_end - __init_array_start;   (size_t = 0; < size; i++)       (*__init_array_start [i]) (argc, argv, envp); }  /* function should not used anymore.  run executable's    destructor other.  cannot remove function,    though.  */ void __libc_csu_fini (void) { #ifndef libc_nonshared   size_t = __fini_array_end - __fini_array_start;   while (i-- > 0)     (*__fini_array_start [i]) ();  # ifndef no_initfini   _fini (); # endif #endif } 

this allow library initialization code run. libraries linked in program can tag functions __attribute__((constructor)) in gcc, , mechanism run functions before main, allowing libraries initialize before program start.