In C, how is the main() method initially called?
The operating system calls the main() function. Eventually.
The Executable and Linkable Format (ELF) which many Unix OS's use defines an entry point address and an INIT address. That is where the program begins to run after the OS finishes its exec() call. On a Linux system this is _init in the .init section. After that returns it jumps to the entry point address which is _start in the .text section.
The C compiler links a standard library to every application which provides these operating system defined initialization and entry points. That library then calls main().
Here is my C source code for the example:
#include <stdio.h>int main() { puts("Hello world!"); return 0;}From objdump -d:
Disassembly of section .init:0000000000001000 <_init>: 1000: f3 0f 1e fa endbr64 1004: 48 83 ec 08 sub $0x8,%rsp 1008: 48 8b 05 d9 2f 00 00 mov 0x2fd9(%rip),%rax # 3fe8 <__gmon_start__> 100f: 48 85 c0 test %rax,%rax 1012: 74 02 je 1016 <_init+0x16> 1014: ff d0 callq *%rax 1016: 48 83 c4 08 add $0x8,%rsp 101a: c3 retq Disassembly of section .text:0000000000001060 <_start>: 1060: f3 0f 1e fa endbr64 1064: 31 ed xor %ebp,%ebp 1066: 49 89 d1 mov %rdx,%r9 1069: 5e pop %rsi 106a: 48 89 e2 mov %rsp,%rdx 106d: 48 83 e4 f0 and $0xfffffffffffffff0,%rsp 1071: 50 push %rax 1072: 54 push %rsp 1073: 4c 8d 05 66 01 00 00 lea 0x166(%rip),%r8 # 11e0 <__libc_csu_fini> 107a: 48 8d 0d ef 00 00 00 lea 0xef(%rip),%rcx # 1170 <__libc_csu_init> 1081: 48 8d 3d c1 00 00 00 lea 0xc1(%rip),%rdi # 1149 <main> 1088: ff 15 52 2f 00 00 callq *0x2f52(%rip) # 3fe0 <__libc_start_main@GLIBC_2.2.5> 108e: f4 hlt 108f: 90 nop0000000000001140 <frame_dummy>: 1140: f3 0f 1e fa endbr64 1144: e9 77 ff ff ff jmpq 10c0 <register_tm_clones>From readelf -h you can see the Entry point address that matches _start:
ELF Header: Magic: 7f 45 4c 46 02 01 01 00 00 00 00 00 00 00 00 00 Class: ELF64 Data: 2's complement, little endian Version: 1 (current) OS/ABI: UNIX - System V ABI Version: 0 Type: DYN (Shared object file) Machine: Advanced Micro Devices X86-64 Version: 0x1 Entry point address: 0x1060 Start of program headers: 64 (bytes into file) Start of section headers: 17416 (bytes into file) Flags: 0x0 Size of this header: 64 (bytes) Size of program headers: 56 (bytes) Number of program headers: 13 Size of section headers: 64 (bytes) Number of section headers: 36 Section header string table index: 35From readelf -d:
Dynamic section at offset 0x2dc8 contains 27 entries: Tag Type Name/Value 0x0000000000000001 (NEEDED) Shared library: [libc.so.6] 0x000000000000000c (INIT) 0x1000 0x000000000000000d (FINI) 0x11e8 0x0000000000000019 (INIT_ARRAY) 0x3db8 0x000000000000001b (INIT_ARRAYSZ) 8 (bytes) 0x000000000000001a (FINI_ARRAY) 0x3dc0 0x000000000000001c (FINI_ARRAYSZ) 8 (bytes) 0x000000006ffffef5 (GNU_HASH) 0x3a0 0x0000000000000005 (STRTAB) 0x470 0x0000000000000006 (SYMTAB) 0x3c8 0x000000000000000a (STRSZ) 130 (bytes) 0x000000000000000b (SYMENT) 24 (bytes) 0x0000000000000015 (DEBUG) 0x0 0x0000000000000003 (PLTGOT) 0x3fb8 0x0000000000000002 (PLTRELSZ) 24 (bytes) 0x0000000000000014 (PLTREL) RELA 0x0000000000000017 (JMPREL) 0x5e0 0x0000000000000007 (RELA) 0x520 0x0000000000000008 (RELASZ) 192 (bytes) 0x0000000000000009 (RELAENT) 24 (bytes) 0x000000000000001e (FLAGS) BIND_NOW 0x000000006ffffffb (FLAGS_1) Flags: NOW PIE 0x000000006ffffffe (VERNEED) 0x500 0x000000006fffffff (VERNEEDNUM) 1 0x000000006ffffff0 (VERSYM) 0x4f2 0x000000006ffffff9 (RELACOUNT) 3 0x0000000000000000 (NULL) 0x0You can see that INIT is equal to the address of _init.
There is a whole array of function pointers in INIT_ARRAY also. See objdump -s -j .init_array c-test:
c-test: file format elf64-x86-64Contents of section .init_array: 3db8 40110000 00000000 @....... You can see that address 0x3db8 is the same as INIT_ARRAY in the ELF header.
The address 0x1140 (remember little-endian byte layout from 40110000) is the function frame_dummy you can see in the disassembly. Which then calls register_tm_clones and who knows what else.
The code for initialization is in a set of files named crtbegin.o and crtend.o (and variants of those names). The __libc_start_main function is defined in libc.so.6. These libraries are part of GCC. That code does various things necessary for a C program like setting up stdin, stdout, global and static variables and other things.
The following article describes quite well what it does in Linux (taken from an answer below with less votes): http://dbp-consulting.com/tutorials/debugging/linuxProgramStartup.html
I believe someone else's answer already described what Windows does.
Eventually it is operating system. Usually there is some medium between real entry point and main function, this is inserted by compiler linker.
Some details (related to Windows): There is header in PE file called IMAGE_OPTIONAL_HEADER which has the field AddressOfEntryPoint, which is in turn address of the first code byte in the file that will be executed.