What should main() return in C and C++?
What is the correct (most efficient) way to define the main() function in C and C++ — int main() or void main() — and why? If int main() then return 1 or return 0 ?
There are numerous duplicates of this question, including:
main() function? main() function void main() and int main() ? main() 's signature in C++ main() ? — For C++, with a very good answer indeed. main() functions in C main() method in C int main() vs void main() in C Related:
int main(int argc, char **argv) int main(int argc, char *argv[]) char *envp[] as a third argument to main() portable? int main() function return a value in all compilers? main() function in C and C++ left to the user to define? int main(){} compile? main() in C++14? The return value for main should indicate how the program exited. Normal exit is generally represented by a 0 return value from main . Abnormal termination is usually signalled by a non-zero return but there is no standard for how non-zero codes are interpreted. Also as noted by others, void main() is explicitly prohibited by the C++ standard and shouldn't 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's also worth noting that in C++, int main() can be left without a return value 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.
Also, efficiency is not an issue with the main function. It can only be entered and left once (marking program start and termination) according to the C++ standard. For C, the case is different and re-entering main() is allowed, but should probably be avoided.
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.
ISO/IEC 9899:1989 (C90):
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. Further, the int return type can be omitted as it is a default.
If an implementation permits it, main() can be declared in other ways, 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. main() must have an explicit return statement at the end to avoid undefined behaviour.
Finally, there is nothing wrong from a standards point of view with calling main() from a program.
ISO/IEC 9899:1999 (C99):
For C99, everything is the same as above except:
int return type may not be omitted. main() . If you do, and main() finished, there is an implicit return 0 . 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:
argc shall be nonnegative. argv[argc] shall be a null pointer. 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. 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. 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; '. For a long time, that did not have a header that declared it, but the POSIX 2008 standard now requires it to be declared in <unistd.h> .
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;
}
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