Difference between revisions of "cpp/language/main function"

Revision as of 16:56, 28 September 2022

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

The names of 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] (if non-null) 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:

Notes

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.

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

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

Example

#include <iostream>
#include <iomanip>
#include <cstdlib>
 
int main(int argc, char *argv[])
{
    std::cout << "argc == " << argc << '\n';
 
    for(int ndx{}; ndx != argc; ++ndx) {
        std::cout << "argv[" << ndx << "] == " << std::quoted(argv[ndx]) << '\n';
    }
    std::cout << "argv[" << argc << "] == "
              << static_cast<void*>(argv[argc]) << '\n';
    /*...*/
    return argc == 3 ? EXIT_SUCCESS : EXIT_FAILURE; // optional return value
}

argc == 3
argv[0] == "./convert"
argv[1] == "table_in.dat"
argv[2] == "table_out.dat"
argv[3] == 0

Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

See also