What should main() return in C and C++?
What is the correct (most efficient) way to define the main() function in C and C++ — int main() or void main() — and why? And how about the arguments?
If int main() then return 1 or return 0?
There are numerous duplicates of this question, including:
- What are the valid signatures for C's
main()function? - The return type of
main()function - Difference between
void main()andint main()? main()'s signature in C++-
What is the proper declaration of
main()? — For C++, with a very good answer indeed. - Styles of
main()functions in C - Return type of
main()method in C int main()vsvoid main()in C
Related:
- C++ —
int main(int argc, char **argv) - C++ —
int main(int argc, char *argv[]) - Is
char *envp[]as a third argument tomain()portable? - Must the
int main()function return a value in all compilers? - Why is the type of the
main()function in C and C++ left to the user to define? - Why does
int main(){}compile? - Legal definitions of
main()in C++14?
Solution 1:
The return value for main indicates how the program exited. Normal exit is represented by a 0 return value from main. Abnormal exit is signaled by a non-zero return, but there is no standard for how non-zero codes are interpreted. As noted by others, void main() is prohibited by the C++ standard and should not be used. The valid C++ main signatures are:
int main()
and
int main(int argc, char* argv[])
which is equivalent to
int main(int argc, char** argv)
It is also worth noting that in C++, int main() can be left without a return-statement, at which point it defaults to returning 0. This is also true with a C99 program. Whether return 0; should be omitted or not is open to debate. The range of valid C program main signatures is much greater.
Efficiency is not an issue with the main function. It can only be entered and left once (marking the program's start and termination) according to the C++ standard. For C, re-entering main() is allowed, but should be avoided.
Solution 2:
The accepted answer appears to be targetted for C++, so I thought I'd add an answer that pertains to C, and this differs in a few ways. There were also some changes made between ISO/IEC 9899:1989 (C90) and ISO/IEC 9899:1999 (C99).
main() should be declared as either:
int main(void)
int main(int argc, char **argv)
Or equivalent. For example, int main(int argc, char *argv[]) is equivalent to the second one. In C90, the int return type can be omitted as it is a default, but in C99 and newer, the int return type may not be omitted.
If an implementation permits it, main() can be declared in other ways (e.g., int main(int argc, char *argv[], char *envp[])), but this makes the program implementation defined, and no longer strictly conforming.
The standard defines 3 values for returning that are strictly conforming (that is, does not rely on implementation defined behaviour): 0 and EXIT_SUCCESS for a successful termination, and EXIT_FAILURE for an unsuccessful termination. Any other values are non-standard and implementation defined. In C90, main() must have an explicit return statement at the end to avoid undefined behaviour. In C99 and newer, you may omit the return statement from main(). If you do, and main() finished, there is an implicit return 0.
Finally, there is nothing wrong from a standards point of view with calling main() recursively from a C program.