Main function

C++

Language

Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

Utilities library

Strings library

Containers library

Algorithms library

Iterators library

Numerics library

Input/output library

Localizations library

Regular expressions library (C++11)

Atomic operations library (C++11)

Thread support library (C++11)

Filesystem library (C++17)

Technical Specifications

C++ language

Basic Concepts

language keywords

phases of translation

comments

the main() function

names and identifiers

types

fundamental types

objects

scope

object lifetime

storage duration and linkage

definitions and ODR

name lookup

qualified name lookup

unqualified name lookup

the as-if rule

undefined behavior

memory model

A program shall contain a global function named main, which is the designated start of the program. It shall have one of the following forms:

int main () { body }	(1)
int main (int argc, char *argv[]) { body }	(2)
/* another implementation-defined form, with int as return type */	(3)

argc	-	Non-negative value representing the number of arguments passed to the program from the environment in which the program is run.
argv	-	Pointer to the first element of an array of pointers to null-terminated multibyte strings that represent the arguments passed to the program from the execution environment (argv[0] through argv[argc-1]). The value of argv[argc] is guaranteed to be a null pointer.
body	-	The body of the main function

The names argc and argv are arbitrary, as well as the representation of the types of the parameters: int main(int ac, char** av) is equally valid.

A very common implementation-defined form of main() has a third argument (in addition to argc and argv), of type char*[], pointing at an array of pointers to the execution environment variables.

Explanation

The main function is called at program startup after initialization of the non-local objects with static storage duration. It is the designated entry point to a program that is executed in hosted environment (that is, with an operating system). The entry points to freestanding programs (boot loaders, OS kernels, etc) are implementation-defined.

The parameters of the two-parameter form of the main function allow arbitrary multibyte character strings to be passed from the execution environment (these are typically known as command line arguments), the pointers argv[1] .. argv[argc-1] point at the first characters in each of these strings. argv[0] is the pointer to the initial character of a null-terminated multibyte string that represents the name used to invoke the program itself (or an empty string "" if this is not supported by the execution environment). The strings are modifiable, although these modifications do not propagate back to the execution environment: they can be used, for example, with std::strtok. The size of the array pointed to by argv is at least argc+1, and the last element, argv[argc], is guaranteed to be a null pointer.

The main function has several special properties:

1) It cannot be used anywhere in the program

a) in particular, it cannot be called recursively

b) its address cannot be taken

2) It cannot be predefined and cannot be overloaded: effectively, the name main in the global namespace is reserved for functions (although it can be used to name classes, namespaces, enumerations, and any entity in a non-global namespace, except that a function called "main" cannot be declared with C language linkage in any namespace (since C++17))

3) It cannot be defined as deleted or declared with C language linkage (since C++17), inline, static, or constexpr

4) The body of the main function does not need to contain the return statement: if control reaches the end of main without encountering a return statement, the effect is that of executing return 0;.

5) Execution of the return (or the implicit return upon reaching the end of main) is equivalent to first leaving the function normally (which destroys the objects with automatic storage duration) and then calling std::exit with the same argument as the argument of the return. (std::exit then destroys static objects and terminates the program)

6) If the main function is defined with a function-try-block, the exceptions thrown by the destructors of static objects (which are destroyed by the implied std::exit) are not caught by it.

7) (since C++14) The return type of the main function cannot be deduced (auto main() {... is not allowed)

Notes

The manner in which the arguments given at the OS command line are converted into the multibyte arrays referenced by argv may involve implementation-defined processing:

• Parsing C++ Command-Line Arguments MSDN
• Shell Introduction POSIX