Every C program coded to run in a hosted execution environment contains the definition (not the prototype) of a function namedmain, which is the designated start of the program.
intmain(void){body} | (1) | ||||||||
intmain(intargc,char*argv[]){body} | (2) | ||||||||
| /* another implementation-defined signature */(since C99) | (3) | ||||||||
Contents |
| 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 ofargc+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. Ifargv[0] is not a null pointer (or, equivalently, ifargc > 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 namesargc andargv 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 isint main(int argc,char*argv[],char*envp[]), where a third argument, of typechar**, pointing atan array of pointers to theexecution environment variables, is added.
If the return statement is used, the return value is used as the argument to the implicit call toexit() (see below for details). The values zero andEXIT_SUCCESS indicate successful termination, the valueEXIT_FAILURE indicates unsuccessful termination.
Themain 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 ahosted environment (that is, with an operating system). The name and type of the entry point to anyfreestanding 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 ascommand line arguments). The pointersargv[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).
If the host environment cannot supply both lowercase and uppercase letters, the command line arguments are converted to lowercase.
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, withstrtok.
The size of the array pointed to byargv is at leastargc+1, and the last element,argv[argc], is guaranteed to be a null pointer.
Themain function has several special properties:
If the main function executes areturn that specifies no value or, which is the same, reaches the terminating} without executing areturn, the termination status returned to the host environment is undefined. | (until C99) |
If the return type of the main function is notcompatible withint (e.g.void main(void)), the value returned to the host environment is unspecified. If the return type is compatible withint and control reaches the terminating}, the value returned to the environment is the same as if executingreturn0;. | (since C99) |
Demonstrates how to inform a program about where to find its input and where to write its results.Invocation: ./a.out indatafile outdatafile
Possible output:
argc = 3argv[0] --> ./a.outargv[1] --> indatafileargv[2] --> outdatafileargv[argc] = (nil)
C++ documentation for main function |
