Is a main() required for a C program?

Question

Well the title says it all. Is a main() function absolutely essential for a C program?

I am asking this because I was looking at the Linux kernel code,

Answer 1

Well, no, but ...

C99 specifies that main() is called in the hosted environment "at program startup", however, you don't have to use the C runtime support. Your operating system executes image files and starts a program at an address provided by the linker.

If you are willing to write your program to conform to the operating system's requirements rather than C99's, you can do it without main(). The more modern (and complex) the system, though, the more trouble you will have with the C library making assumptions that the standard runtime startup is used.

Here is an example for Linux...

$ cat > nomain.S
.text
_start:
    call    iamnotmain
    movl    $0xfc, %eax
    xorl    %ebx, %ebx
    int     $0x80
.globl _start
$ cat > demo.c

void iamnotmain(void) {
    static char s[] = "hello, world\n";
    write(1, s, sizeof s);
}
$ as -o nomain.o nomain.S
$ cc -c demo.c
$ ld -static nomain.o demo.o -lc
$ ./a.out
hello, world

It's arguably not "a C99 program" now, though, just a "Linux program" with a object module written in C.