In a C/C++ program how does the system (windows, linux, mac OS X) call the main() function

Question

I am looking for a more technical explanation then the OS calls the function. Can anyone help me out or point me to a website or book?

Answer 1

It's OS dependent. In OS X, there's a frame in the mach header that contains the start address for the EIP (instruction pointer) register.

Once the binary is loaded, the OS launches execution from this address:

cristi:test diciu$ otool -l ./a.out | grep -A 10 LC_UNIXTHREAD
        cmd LC_UNIXTHREAD
    cmdsize 80
     flavor i386_THREAD_STATE
      count i386_THREAD_STATE_COUNT
[..]
        ss  0x00000000 eflags 0x00000000 eip 0x00001f8c cs  0x00000000
[..]

The address is the address of the "start" function from the binary:

cristi:test diciu$ nm ./a.out
0000200c D _NXArgc
00002008 D _NXArgv
00002000 D ___progname
00001fe0 t __dyld_func_lookup
00001000 A __mh_execute_header
[..]
00001f8c T start

In Mac OS X, it's the "start" function that gets called first, even before the "main" function:

(gdb) b start
Breakpoint 1 at 0x1f90
(gdb) b main
Breakpoint 2 at 0x1ff4
(gdb) r
Starting program: /Users/diciu/Programming/test/a.out 
Reading symbols for shared libraries ++. done

Breakpoint 1, 0x00001f90 in start ()