The `main` Function in C

In C programming, the main function is the entry point of every program. When a C program starts running, execution always begins from main, regardless of how many other functions the program contains.

1. Function Signature

C allows two standard and portable forms of main:

Recommended (with arguments)

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

or equivalently:

int main(int argc, char **argv);

Simplest form

int main(void);

Non-standard

Some compilers allow:

void main();

but this is not valid in standard C and should be avoided.

2. Meaning of Parameters

`argc`

Stands for argument count.
Represents the number of command-line arguments.
Always ≥ 1 (the first argument is the program name).

`argv`

Stands for argument vector.
An array of C strings containing command-line arguments.
argv[0] = program name
argv[1] ~ argv[argc-1] = user-provided arguments

3. Return Type (`int`)

The return type of main must be:

int

Returning a value from main reports the program’s execution status to the operating system.

Common return values:

return 0; — program finished successfully
return 1; — program ended with an error (custom meaning)

If you omit return 0; in C99 or later, the compiler automatically inserts it.

4. Example 1: Basic Program

#include <stdio.h>
int main(void) {
     printf("Hello, world!\n");
     return 0; 
}

5. Example 2: Program with Arguments

#include <stdio.h>  
int main(int argc, char *argv[]) {
     printf("Number of arguments: %d\n", argc);
     for (int i = 0; i < argc; i++) {
        printf("argv[%d] = %s\n", i, argv[i]);     
    }     
    return 0; 
}

6. Key Characteristics of `main`

It is required in every C program.
There can be only one main function.
Program execution always begins at the first statement inside main.
Returning an integer communicates status to the operating system.

7. Special Notes

The C standard only guarantees two valid signatures:
- int main(void)
- int main(int argc, char *argv[])
main can also be declared with additional implementation-defined arguments:

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

but this is compiler-specific and not portable.

[[Command Line Arguments 命令行参数 | Learn more about Command Line Arguments]] [[Main Function Return Values]]

The main Function in C