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cargumentsstandardsprogram-entry-point

What is the purpose of "int main(void *framep)"?

In C, main() function takes only zero or two arguments.If we provide two arguments,then first argument must be int type. 

int main(int argc, char *argv[])

But, I saw following code, when browsing OpenBSD.

int main(void *framep){}

Is it valid in C?

GCC compiler gives following warnings:

prog.c:3:5: warning: first argument of 'main' should be 'int' [-Wmain]
 int main(void *p) {
     ^~~~
prog.c:3:5: warning: 'main' takes only zero or two arguments [-Wmain]

What is the purpose of it?

Solution

  • Oups, this is not a normal program but a kernel, so the normal rules for main do not really apply. When the program starts, no environment exists to pass argument values, and the return value of main will not be used either because when the kernel exits, nothing else exists. One comment says that the definition was modified to only cope with gcc requirements:

    return int, so gcc -Werror won't complain

    That is explicit in N1256 draft for C11 at 5.1.2.1 Freestanding environment :

    In a freestanding environment (in which C program execution may take place without any benefit of an operating system), the name and type of the function called at program startup are implementation-defined. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

    The effect of program termination in a freestanding environment is implementationdefined.

    As at kernel startup no OS still exists, so it actually runs in a freestanding environment. That probably means that is also needs to be compiled with special flags...