what is difference between int32_t main() and int main()

Question

I've seen some people use int main() for the main function while some use int32_t main(). What is the difference between both and where to use which.

Answer 1

(This answer is for C.)

For a hosted C implementation (in contrast to a “freestanding” or “embedded” implementation, the 2018 standard says, in 5.1.2.2.1 1 that main shall be defined with a return type of int, except that an implementation may define other acceptable definitions. Specifically, it just says the return type shall be int, not a type that is just compatible with or the same width as int.

What is int32_t? It is specified by 7.20, which discusses <stdint.h>. 7.20 1 says this header declares various integer types. 7.20.1.1 says “The typedef name intN_t designates a signed integer type with width N, no padding bits, and a two’s complement representation…” It does not say these types are necessarily the same as the types char, short, int, long, and so on, even if their properties match. A “typedef name” is an alias for another type, not a type by itself, but the int32_t typedef name could be a name for some type other than int, even a type that is otherwise indistinguishable from int.

Thus:

• Your C implementation could have two types int and int32_t that have the same properties except for the fact they are different types, where the latter is a typedef name for some built-in type. If so, these are different types, and int32_t main(void) does not satisfy the requirement of 5.1.2.2.1 1 unless your C implementation specifically documents that main may be defined this way.
• Your C implementation could have int being a 16-bit type while int32_t is a typedef name for long. Again, int32_t main(void) would not satisfy 5.1.2.2.1 1.
• Your C implementation could have int32_t being a typedef name for int. In this case, int32_t main(void) would satisfy 5.1.2.2.1 1.

There is really no point to using int32_t, though. int accomplishes all that is necessary.

Addendum

There is some discussion in the comments that int main() is wrong for C. C 2018 5.1.2.2.2.1 1 says that main may be defined in a way “equivalent” to the forms it shows, int main(void) and int main(int argc, char *argv[]). When a function is defined using () for the parameter list, the function is defined to have no parameters, and this is equivalent to using (void). (Note that using () in a declaration that is not a definition leaves it unspecified whether there are any parameters, but a definition settles it.)

Answer 2

The C Standard has a pretty clear cut answer to this.

5.1.2.2.1 Program startup

1. 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

Using int32_t would be by definition "non-standard". In practice, almost all architectures are simply going to declare int32_t as typedef int int32_t, so it's unlikely to be a problem, but it's always a bad idea.