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What should main() return in C and C++?

804

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:

Related:

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  • 36

    I still think it’s fairly vague too. Define “most efficient” for me. Efficient in what sense? In the sense of taking up less memory? In the sense of running faster? I can see the useful answers but I still think the question is phrased pretty poorly.

    Oct 16, 2008 at 12:51

  • 11

    Pish posh, the context of efficient is obvious here, especially with the examples (which are likely there to clarify the definition of ‘efficient’). Hopefully the poor buffer didn’t crawl into a hole and regret the question entirely. One could say, regardless of void or int, a value is returned, so it has no impact on file size, operations executed, nor memory allocated. And people, across most OSs, tend to return 0 on success, and something else on -other- success, or failure – but there is no standard. Ultimately, no difference in efficiency in any obvious way.

    – Kit10

    Dec 11, 2012 at 17:02


  • “correct (most efficient)” doesn’t make sense. Efficient is one thing, correct is another. main is called once (and in C++ can only be called once: no recursion). If you don’t want execution to spend a lot of time in main, then don’t invoke the program a large number of times: make the program implement the repetition.

    – Kaz

    Oct 17, 2013 at 16:14

  • 3

    I find it interesting that none of the answers, as far as I can tell, provide a fully working example, including the #include statements

    – puk

    Nov 11, 2013 at 21:42

  • 6

    Return values make no sense on a platform with no OS. You’re not returning to anything. If you hit a return in main(...) on an embedded device, your system goes into an unpredictable state and your washing machine will become self-aware and try to kill you. So, we use void main() in that case. This is industry standard practice in bare-metal embedded.

    – 3Dave

    Jun 26, 2019 at 0:53


646

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.

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  • 83

    main CAN be entered/left multiple times, but that program probably wouldn’t win any design awards 😉

    – korona

    Oct 15, 2008 at 12:38

  • 19

    C99 also has the C++ mis-feature that reaching the end of the main() function is equivalent to returning 0 — if main() is defined to return a type compatible with int (section 5.1.2.2.3).

    Oct 15, 2008 at 12:43

  • 71

    reentering main is not valid C++. Explicitly in the standard, 3.6.1.3 states ‘main shall not be used within a program’

    – workmad3

    Oct 15, 2008 at 12:59

  • 134

    stdlib.h provides EXIT_SUCCESS and EXIT_FAILURE for this purpose

    – Clay

    Oct 15, 2008 at 13:13

  • 29

    0 and non-zero are correct but entirely meaningless to someone reading your code. This question is proof that people don’t know what valid/invalid codes are. EXIT_SUCCESS/EXIT_FAILURE are much more clear.

    – JaredPar

    Oct 15, 2008 at 16:32

206

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.

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  • 1

    @Lundin I don’t think you need a quote to say that someone is allowed to make a compiler that accepts non-standard-conforming programs, or to have a non-stardard-conforming compiler. That’s common knowledge and common sense

    Jul 7, 2015 at 14:44

  • 4

    @KABoissonneault Implementation-defined behavior is a term from the standard, as opposed to completely undocumented behavior. If you implement something that is listed as implementation-defined behavior, you still follow the standard. In this case C89 which was quoted, lists no such implementation-defined behavior, hence the need of quote, to prove that he is not just making things up out of the blue.

    – Lundin

    Jul 8, 2015 at 7:00


  • 1

    @Lundin You’re seeing this the wrong way. What we’re talking about is not implementation-defined behavior, we’re talking about an implementation deviating from the standard if they choose so. It’s more like a child disobeying their parents: you don’t need a quote from the parents to tell you in what way a child can go against what the parents said. You just know that the moment the child chooses to do so, they’re no longer compliant with their parents’ guildelines

    Jul 8, 2015 at 12:07

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    @KABoissonneault The part I quoted in my comment is definitely about implementation-defined behavior (as opposed to non-standard compiler extensions.) Thus I am talking about implementation-defined behavior. If you are having a monologue about something else, best of luck with that.

    – Lundin

    Jul 8, 2015 at 12:33

  • 2

    @Lundin I guess the wording in the quote is confusing (the part where they say “but this makes the program implementation defined”) but I’m pretty sure the person was talking about non-standard behavior (as said in “If an implementation permits it” and “and no longer strictly conforming [to the standard]”) as opposed to actual implementation defined behavior. The person should definitely reword their answer, but I still don’t think a quote from the standard is necessary on that

    Jul 8, 2015 at 12:39

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Standard C — Hosted Environment

For a hosted environment (that’s the normal one), the C11 standard (ISO/IEC 9899:2011) says:

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

or equivalent;10) or in some other implementation-defined manner.

If they are declared, the parameters to the main function shall obey the following
constraints:

  • The value of argc shall be nonnegative.
  • argv[argc] shall be a null pointer.
  • If the value of argc is greater than zero, the array members argv[0] through
    argv[argc-1] inclusive shall contain pointers to strings, which are given
    implementation-defined values by the host environment prior to program startup. The
    intent is to supply to the program information determined prior to program startup
    from elsewhere in the hosted environment. If the host environment is not capable of
    supplying strings with letters in both uppercase and lowercase, the implementation
    shall ensure that the strings are received in lowercase.
  • If the value of argc is greater than zero, the string pointed to by argv[0]
    represents the program name; argv[0][0] shall be the null character if the
    program name is not available from the host environment. If the value of argc is
    greater than one, the strings pointed to by argv[1] through argv[argc-1]
    represent the program parameters.
  • The parameters argc and argv and the strings pointed to by the argv array shall
    be modifiable by the program, and retain their last-stored values between program
    startup and program termination.

10) Thus, int can be replaced by a typedef name defined as int, or the type of argv can be written as
char **argv, and so on.

Program termination in C99 or C11

The value returned from main() is transmitted to the ‘environment’ in an implementation-defined way.

5.1.2.2.3 Program termination

1 If the return type of the main function is a type compatible with int, a return from the
initial call to the main function is equivalent to calling the exit function with the value
returned by the main function as its argument;11) reaching the } that terminates the
main function returns a value of 0. If the return type is not compatible with int, the
termination status returned to the host environment is unspecified.

11) In accordance with 6.2.4, the lifetimes of objects with automatic storage duration declared in main
will have ended in the former case, even where they would not have in the latter.

Note that 0 is mandated as ‘success’. You can use EXIT_FAILURE and EXIT_SUCCESS from <stdlib.h> if you prefer, but 0 is well established, and so is 1. See also Exit codes greater than 255 — possible?.

In C89 (and hence in Microsoft C), there is no statement about what happens if the main() function returns but does not specify a return value; it therefore leads to undefined behaviour.

7.22.4.4 The exit function

¶5 Finally, control is returned to the host environment. If the value of status is zero or EXIT_SUCCESS, an implementation-defined form of the status successful termination is returned. If the value of status is EXIT_FAILURE, an implementation-defined form of the status unsuccessful termination is returned. Otherwise the status returned is implementation-defined.

Standard C++ — Hosted Environment

The C++11 standard (ISO/IEC 14882:2011) says:

3.6.1 Main function [basic.start.main]

¶1 A program shall contain a global function called main, which is the designated start of the program. […]

¶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.
All implementations
shall allow both of the following definitions of main:

int main() { /* ... */ }

and

int main(int argc, char* argv[]) { /* ... */ }

In the latter form argc shall be the number of arguments passed to the program from the environment
in which the program is run. If argc is nonzero these arguments shall be supplied in argv[0]
through argv[argc-1] as pointers to the initial characters of null-terminated multibyte strings (NTMBSs) (17.5.2.1.4.2) and argv[0] shall be the pointer to the initial character of a NTMBS that represents the
name used to invoke the program or "". The value of argc shall be non-negative. The value of argv[argc]
shall be 0. [ Note: It is recommended that any further (optional) parameters be added after argv. —end
note ]

¶3 The function main shall not be used within a program. The linkage (3.5) of main is implementation-defined. […]

¶5 A return statement in main has the effect of leaving the main function (destroying any objects with automatic
storage duration) and calling std::exit with the return value as the argument. If control reaches the end
of main without encountering a return statement, the effect is that of executing

return 0;

The C++ standard explicitly says “It [the main function] shall have a return type of type int, but otherwise its type is implementation defined”, and requires the same two signatures as the C standard to be supported as options. So a ‘void main()’ is directly not allowed by the C++ standard, though there’s nothing it can do to stop a non-standard implementation allowing alternatives. Note that C++ forbids the user from calling main (but the C standard does not).

There’s a paragraph of §18.5 Start and termination in the C++11 standard that is identical to the paragraph from §7.22.4.4 The exit function in the C11 standard (quoted above), apart from a footnote (which simply documents that EXIT_SUCCESS and EXIT_FAILURE are defined in <cstdlib>).

Standard C — Common Extension

Classically, Unix systems support a third variant:

int main(int argc, char **argv, char **envp) { ... }

The third argument is a null-terminated list of pointers to strings, each of which is an environment variable which has a name, an equals sign, and a value (possibly empty). If you do not use this, you can still get at the environment via ‘extern char **environ;‘. This global variable is unique among those in POSIX in that it does not have a header that declares it.

This is recognized by the C standard as a common extension, documented in Annex J:

J.5.1 Environment arguments

¶1 In a hosted environment, the main function receives a third argument, char *envp[],
that points to a null-terminated array of pointers to char, each of which points to a string
that provides information about the environment for this execution of the program (5.1.2.2.1).

Microsoft C

The Microsoft VS 2010 compiler is interesting. The web site says:

The declaration syntax for main is

 int main();

or, optionally,

int main(int argc, char *argv[], char *envp[]);

Alternatively, the main and wmain functions can be declared as returning void (no return value). If you declare main or wmain as returning void, you cannot return an exit code to the parent process or operating system by using a return statement. To return an exit code when main or wmain is declared as void, you must use the exit function.

It is not clear to me what happens (what exit code is returned to the parent or OS) when a program with void main() does exit — and the MS web site is silent too.

Interestingly, MS does not prescribe the two-argument version of main() that the C and C++ standards require. It only prescribes a three argument form where the third argument is char **envp, a pointer to a list of environment variables.

The Microsoft page also lists some other alternatives — wmain() which takes wide character strings, and some more.

The Microsoft Visual Studio 2005 version of this page does not list void main() as an alternative. The versions from Microsoft Visual Studio 2008 onwards do.

Standard C — Freestanding Environment

As noted early on, the requirements above apply to hosted environments. If you are working with a freestanding environment (which is the alternative to a hosted environment), then the standard has much less to say. For a freestanding environment, the function called at program startup need not be called main and there are no constraints on its return type. The standard says:

5.1.2 Execution environments

Two execution environments are defined: freestanding and hosted. In both cases,
program startup occurs when a designated C function is called by the execution
environment. All objects with static storage duration shall be initialized (set to their initial values) before program startup. The manner and timing of such initialization are otherwise unspecified. Program termination returns control to the execution environment.

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. Any library facilities available to a freestanding program, other than the minimal set required by clause 4, are implementation-defined.

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

The cross-reference to clause 4 Conformance refers to this:

¶5 A strictly conforming program shall use only those features of the language and library specified in this International Standard.3) It shall not produce output dependent on any unspecified, undefined, or implementation-defined behavior, and shall not exceed any minimum implementation limit.

¶6 The two forms of conforming implementation are hosted and freestanding. A conforming hosted implementation shall accept any strictly conforming program. A conforming freestanding implementation shall accept any strictly conforming program in which the use of the features specified in the library clause (clause 7) is confined to the contents of the standard headers <float.h>, <iso646.h>, <limits.h>, <stdalign.h>,
<stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>, and
<stdnoreturn.h>. A conforming implementation may have extensions (including
additional library functions), provided they do not alter the behavior of any strictly conforming program.4)

¶7 A conforming program is one that is acceptable to a conforming implementation.5)

3) A strictly conforming program can use conditional features (see 6.10.8.3) provided the use is guarded by an appropriate conditional inclusion preprocessing directive using the related macro. For example:

#ifdef __STDC_IEC_559__ /* FE_UPWARD defined */
    /* ... */
    fesetround(FE_UPWARD);
    /* ... */
#endif

4) This implies that a conforming implementation reserves no identifiers other than those explicitly reserved in this International Standard.

5) Strictly conforming programs are intended to be maximally portable among conforming implementations. Conforming programs may depend upon non-portable features of a conforming implementation.

It is noticeable that the only header required of a freestanding environment that actually defines any functions is <stdarg.h> (and even those may be — and often are — just macros).

Standard C++ — Freestanding Environment

Just as the C standard recognizes both hosted and freestanding environment, so too does the C++ standard. (Quotes from ISO/IEC 14882:2011.)

1.4 Implementation compliance [intro.compliance]

¶7 Two kinds of implementations are defined: a hosted implementation and a freestanding implementation. For a hosted implementation, this International Standard defines the set of available libraries. A freestanding
implementation is one in which execution may take place without the benefit of an operating system, and has an implementation-defined set of libraries that includes certain language-support libraries (17.6.1.3).

¶8 A conforming implementation may have extensions (including additional library functions), provided they do not alter the behavior of any well-formed program. Implementations are required to diagnose programs that
use such extensions that are ill-formed according to this International Standard. Having done so, however, they can compile and execute such programs.

¶9 Each implementation shall include documentation that identifies all conditionally-supported constructs that it does not support and defines all locale-specific characteristics.3

3) This documentation also defines implementation-defined behavior; see 1.9.

17.6.1.3 Freestanding implementations [compliance]

Two kinds of implementations are defined: hosted and freestanding (1.4). For a hosted implementation, this International Standard describes the set of available headers.

A freestanding implementation has an implementation-defined set of headers. This set shall include at least the headers shown in Table 16.

The supplied version of the header <cstdlib> shall declare at least the functions abort, atexit, at_quick_exit, exit, and quick_exit (18.5). The other headers listed in this table shall meet the same requirements as for a hosted implementation.

Table 16 — C++ headers for freestanding implementations

Subclause                           Header(s)
                                    <ciso646>
18.2  Types                         <cstddef>
18.3  Implementation properties     <cfloat> <limits> <climits>
18.4  Integer types                 <cstdint>
18.5  Start and termination         <cstdlib>
18.6  Dynamic memory management     <new>
18.7  Type identification           <typeinfo>
18.8  Exception handling            <exception>
18.9  Initializer lists             <initializer_list>
18.10 Other runtime support         <cstdalign> <cstdarg> <cstdbool>
20.9  Type traits                   <type_traits>
29    Atomics                       <atomic>

What about using int main() in C?

The standard §5.1.2.2.1 of the C11 standard shows the preferred notation — int main(void) — but there are also two examples in the standard which show int main(): §6.5.3.4 ¶8 and §6.7.6.3 ¶20. Now, it is important to note that examples are not ‘normative’; they are only illustrative. If there are bugs in the examples, they do not directly affect the main text of the standard. That said, they are strongly indicative of expected behaviour, so if the standard includes int main() in an example, it suggests that int main() is not forbidden, even if it is not the preferred notation.

6.5.3.4 The sizeof and _Alignof operators

¶8 EXAMPLE 3 In this example, the size of a variable length array is computed and returned from a function:

#include <stddef.h>

size_t fsize3(int n)
{
    char b[n+3]; // variable length array
    return sizeof b; // execution time sizeof
}
int main()
{
    size_t size;
    size = fsize3(10); // fsize3 returns 13
    return 0;
}

7

  • @DavidBowling: A function definition like int main(){ … } does specify that the function takes no arguments, but does not provide a function prototype, AFAICT. For main() that is seldom a problem; it means that if you have recursive calls to main(), the arguments won’t be checked. For other functions, it is more of a problem — you really need a prototype in scope when the function is called to ensure that the arguments are correct.

    Dec 13, 2017 at 15:58


  • 1

    @DavidBowling: You don’t normally call main() recursively, outside of places like IOCCC. I do have a test program that does it — mainly for novelty. If you have int i = 0; int main() { if (i++ < 10) main(i, i * i); return 0; } and compile with GCC and don’t include -Wstrict-prototypes, it compiles cleanly under stringent warnings. If it’s main(void), it fails to compile.

    Dec 13, 2017 at 16:42

  • I’m reading “The C Programming Language” by Dennis Ritchie, and although his main( ) functions have return values, he never precedes main( ) with int. Do you know why? It seems like everyone here is saying that it should be written int main( ), but the creator of C didn’t write it that way in his book on ANSI C.

    – Joe

    May 26, 2021 at 18:36

  • Because even the 2nd Edn of “The C Programming Language” pre-dates the first standard C (I have a copy with “Based on Draft Proposed ANSI C” blazoned across the top-right corner of the cover). And in C90, you did not have to include the return type for a function if the return type was int. And if a function was used without a prior declaration, it was assumed to return int. BUT: the C90 standard is not the current standard. The current standard is C18, superseding C11 and C99. ——— […continued…]

    May 26, 2021 at 18:41


  • 1

    I’d recommend either King “C Programming: A Modern Approach” or Gustedt “Modern C” — see The Definitive C Book Guide and List, a title which is more grandiose than the Q&A content.

    May 26, 2021 at 19:04