Editing Entry point (section)

===C and C++===
In [[C (programming language)|C]] and [[C++]], the [[function prototype]] of the main function must be equivalent to one of the following:

<syntaxhighlight lang="c">
int main();
int main(void);
int main(int argc, char **argv);
</syntaxhighlight>

The main function is the entry point for application programs written in ISO-standard C or C++. Low-level system programming (such as for a [[bare-metal]] [[embedded system]]) might specify a different entry point (for example via a reset [[interrupt vector]]) using functionality not defined by the language standard.

The [[parameter (computer science)|parameters]] <code>argc</code>, ''argument count'', and <code>argv</code>, ''argument vector'',{{cn|date=January 2025}} respectively give the number and values of the program's [[command-line argument]]s. The names of <code>argc</code> and <code>argv</code> may be any valid identifier, but it is common convention to use these names. Other platform-dependent formats are also allowed by the C and C++ standards, except that in C++ the return type must always be <code>int</code>;<ref>Section 3.6.1.2, Standard C++ 2011 edition.</ref> for example, [[Unix]] (though not [[POSIX.1]]) and [[Windows]] have a third argument giving the program's [[environment variable|environment]], otherwise accessible through <code>getenv</code> in <code>[[stdlib.h]]</code>:

<syntaxhighlight lang="c">
int main(int argc, char **argv, char **envp);
</syntaxhighlight>

[[Darwin (operating system)|Darwin]]-based operating systems, such as [[macOS]], have a fourth parameter containing arbitrary OS-supplied information, such as the path to the executing binary:<ref>{{Cite web |url=http://unixjunkie.blogspot.com/2006/02/char-apple-argument-vector.html |title=The <code>char *apple</code> Argument Vector |access-date=2014-05-12 |archive-url=https://web.archive.org/web/20151222173356/http://unixjunkie.blogspot.com/2006/02/char-apple-argument-vector.html |archive-date=2015-12-22 |url-status=dead }}</ref>

<syntaxhighlight lang="c">
int main(int argc, char **argv, char **envp, char **apple);
</syntaxhighlight>

The value returned from the main function becomes the [[exit status]] of the process, though the C standard only ascribes specific meaning to two values: <code>EXIT_SUCCESS</code> (traditionally 0) and <code>EXIT_FAILURE</code>. The meaning of other possible return values is implementation-defined. In case a return value is not defined by the programmer, an implicit <code>return 0;</code> at the end of the <code>main()</code> function is inserted by the compiler; this behavior is required by the C++ standard.

It is guaranteed that <code>argc</code> is non-negative and that <code>argv[argc]</code> is a [[null pointer]]. By convention, the command-line arguments specified by <code>argc</code> and <code>argv</code> include the name of the program as the first element if <code>argc</code> is greater than 0; if a user types a command of "<code>rm file</code>", the [[shell (computing)|shell]] will initialise the <code>[[rm (Unix)|rm]]</code> process with <code>argc = 2</code> and <code>argv = {"rm", "file", NULL}</code>.  As <code>argv[0]</code> is the name that processes appear under in <code>[[ps (Unix)|ps]]</code>, <code>[[top (software)|top]]</code> etc., some programs, such as [[daemon (computing)|daemons]] or those running within an [[interpreter (computing)|interpreter]] or [[virtual machine]] (where <code>argv[0]</code> would be the name of the host executable), may choose to alter their argv to give a more descriptive <code>argv[0]</code>, usually by means of the <code>[[exec (system call)|exec]]</code> system call.

The <code>main()</code> function is special; normally every C and C++ program must define it exactly once.

If declared, <code>main()</code> must be declared as if it has external linkage; it cannot be declared <code>static</code> or <code>inline</code>.

In C++, <code>main()</code> must be in the global [[namespace]] (i.e. <code>::main</code>), cannot be overloaded, and cannot be a [[member function]], although the name is not otherwise reserved, and may be used for member functions, classes, enumerations, or non-member functions in other namespaces. In C++ (unlike C) <code>main()</code> cannot be called [[recursion (computer science)|recursively]] and cannot have its address taken.