Why is the type of the main function in C and c++ left to the user to define? [duplicate]_问答_开发者

This question already has answers here: What should main() return in C and C++? (19 answers) Closed 9 years ago.

Why is main() a user defined function ?

When will I use void main() and i开发者_Python百科nt main()?

EDIT This answer is not as complete as it could be since it doesn't really address the strange sentence "or otherwise in some implementation-defined manner". I have now written a more complete answer which also addresses C90, C11 and C++. END OF EDIT

Here is what the C standard says (ISO C 9899:1999):

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. / .. / The effect of program termination in a freestanding environment is implementation-defined.

5.1.2.2 Hosted environment

A hosted environment need not be provided, but shall conform to the following specifications if present.

5.1.2.2.1 Program startup

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[]) { /* ... */ }

The text in the C++ standard is more or less identical. Please note that "Program startup" in the text is a subclause to hosted environment.

This means:

If your program is running in a hostless environment (your program is an embedded system or an operative system), it may have any return type. void main() is most common.
If your program is running in a hosted environment (on top of an OS), main() must return int, and may have additional parameters.

Lundin is correct about C, but in C++ the wording is sufficiently distinct to make a difference:

[C++11: 3.6.1/1]: A program shall contain a global function called main, which is the designated start of the program. It is implementation-defined whether a program in a freestanding environment is required to define a main function.

[C++11: 3.6.1/2]: An implementation shall not predefine the main function. This function shall not be overloaded. It shall have a return type of type int, but otherwise its type is implementation-defined [..]

The first bolded passage does not override or cancel out the second.

main returns int in C++, always.

The return type for main is determined by the implementation, not the programmer. Check your compiler documentation to see what the legal signatures are for main. Don't assume that void main() is one of them. In a hosted environment, main normally returns int. In a freestandaing environment, the entry point may not even be named main, but its return type will still be determined by the implementation, not the programmer.

There are 3 situations:

free standing implementation
conforming hosted implementation with no extensions
hosted implementation with extensions

In 1. there need not be a function named main at all. The implementation defines how a program starts.

In 2. a program starts executing at a function named main, defined with one of the following 2 'signatures': int main(void) or int main(int argc, char **argv)

In 3. a program starts executing at a function named main, defined as allowed by the implementation. This function must return int to be Standard conformant. For example: int main(int argc, char **argv, char **envp) or int main(wchar_t**). Note that programs which use these forms are not necessarily valid in all hosted implementations (and may become invalid for the original author if the implementation changes).

Originally, in the C language, there was no such type as void and therefore the function had to return int.

In practice, returning int allows you to run another process from your process (using fork and exec) and if you can get the return result from that process you will know whether it worked or not.

Many compilers don't support void main(), therefore you should always use int main().