Question

What really are the valid signatures for main function in C? I know:

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

Are there other valid ones?

Answer 1

The C11 standard explicitly mentions these two:

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

although it does mention the phrase "or equivalent" with the following footnote:

Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as char ** argv, and so on.

In addition, it also provides for more (implementation-defined) possibilities.

The relevant text (section 5.1.2.2.1, but this particular aspect is unchanged from C99) states:

The function called at program startup is named main. The implementation declares no prototype for this function. It shall be defined with a return type of int and with no parameters:

int main(void) { /* ... */ }

or with two parameters (referred to here as argc and argv, though any names may be used, as they are local to the function in which they are declared):

int main(int argc, char *argv[]) { /* ... */ }

or equivalent; or in some other implementation-defined manner.

If they are declared, the parameters to the main function shall obey the following constraints:

• The value of argc shall be nonnegative.

• argv[argc] shall be a null pointer.

• If the value of argc is greater than zero, the array members argv[0] through argv[argc-1] inclusive shall contain pointers to strings, which are given implementation-defined values by the host environment prior to program startup. The intent is to supply to the program information determined prior to program startup from elsewhere in the hosted environment. If the host environment is not capable of supplying strings with letters in both uppercase and lowercase, the implementation shall ensure that the strings are received in lowercase.

• If the value of argc is greater than zero, the string pointed to by argv[0] represents the program name; argv[0][0] shall be the null character if the program name is not available from the host environment. If the value of argc is greater than one, the strings pointed to by argv[1] through argv[argc-1] represent the program parameters.

• The parameters argc and argv and the strings pointed to by the argv array shall be modifiable by the program, and retain their last-stored values between program startup and program termination.

Note that this is for a hosted environment, the ones you normally see in C programs. A free-standing environment (such as an embedded system) is far less constrained, as stated in 5.1.2.1 of that same standard:

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.