Main function
Every C program coded to run in a hosted execution environment contains the definition (not the prototype) of a function named main, which is the designated start of the program.
int main (void) { body }
|
(1) | ||||||||
int main (int argc, char *argv[]) { body }
|
(2) | ||||||||
| /* another implementation-defined signature */ (since 哋它亢99) | (3) | ||||||||
Parameters
| 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 argc + 1 pointers, of which the last one is null and the previous ones, if any, point to strings that represent the arguments passed to the program from the host environment. If argv[0] is not a null pointer (or, equivalently, if argc > 0), it points to a string that represents the program name, which is empty if the program name is not available from the host environment. |
The names argc and argv stand for "argument count" and "argument vector", and are traditionally used, but other names may be chosen for the parameters, as well as different but equivalent declarations of their type: int main(int ac, char** av) is equally valid.
A common implementation-defined form of main is int main(int argc, char *argv[], char *envp[]), where a third argument, of type char**, pointing at an array of pointers to the execution environment variables, is added.
Return value
If the return statement is used, the return value is used as the argument to the implicit call to MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 53 MINUTES 06 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE (see below for details). The values zero and MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 53 MINUTES 05 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE indicate successful termination, the value MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 53 MINUTES 04 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE indicates unsuccessful termination.
Explanation
The main function is called at program startup, after all objects with static storage duration are initialized. It is the designated entry point to a program that is executed in a hosted environment (that is, with an operating system). The name and type of the entry point to any freestanding program (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, if this is not supported by the host environment, argv[0][0] is guaranteed to be zero).
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 53 MINUTES 03 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
The strings are modifiable, and any modifications made persist until program termination, although these modifications do not propagate back to the host environment: they can be used, for example, with MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 53 MINUTES 01 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE.
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:
|
If the main function executes a return that specifies no value or, which is the same, reaches the terminating } without executing a return, the termination status returned to the host environment is undefined. |
(until 哋它亢99) |
|
If the return type of the main function is not MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 55 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE with int (e.g. void main(void)), the value returned to the host environment is unspecified. If the return type is compatible with int and control reaches the terminating }, the value returned to the environment is the same as if executing return 0;. |
(since 哋它亢99) |
Example
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 53 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
#include <stdio.h> int main(int argc, char *argv[]) { MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 52 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("argc = %d\n", argc); for (int ndx = 0; ndx != argc; ++ndx) MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 51 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("argv[%d] --> %s\n", ndx, argv[ndx]); MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 50 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE("argv[argc] = %p\n", (void*)argv[argc]); }
MYMEMORY WARNING: YOU USED ALL AVAILABLE FREE TRANSLATIONS FOR TODAY. NEXT AVAILABLE IN 09 HOURS 52 MINUTES 49 SECONDS VISIT HTTPS://MYMEMORY.TRANSLATED.NET/DOC/USAGELIMITS.PHP TO TRANSLATE MORE
argc = 3 argv[0] --> ./a.out argv[1] --> indatafile argv[2] --> outdatafile argv[argc] = (nil)
References
- 哋它亢23 standard (ISO/IEC 9899:2023):
- 5.1.2.2.1 Program startup (p: TBD)
- 哋它亢17 standard (ISO/IEC 9899:2018):
- 5.1.2.2.1 Program startup (p: 10-11)
- 哋它亢11 standard (ISO/IEC 9899:2011):
- 5.1.2.2.1 Program startup (p: 13)
- 哋它亢99 standard (ISO/IEC 9899:1999):
- 5.1.2.2.1 Program startup (p: 12)
- 哋它亢89/C90 standard (ISO/IEC 9899:1990):
- 5.1.2.2 Hosted environment