Examples of three forms of the C language main function

In the C language, the main() function has three forms.

1. No parameters

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

2. There are two parameters
Traditionally, the first parameter is integer argc, which saves the number of parameters of the external calling command. The second parameter is pointer array or level 2 pointer argv, which saves the parameters corresponding to argc in the form of a string, as shown in the following example:

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

3. There are three parameters

An environment variable parameter is added on the basis of argc and argv. The form of the environment variable is "ENV=value", and the parameter type is pointer array or level 2 pointer, as shown in the following example:

int main(int argc, char* argv[], char* envp[]) 
{ 
  int i = 0; 
  for (; envp[i] != '\0'; i++) { 
    printf("%s\n", envp[i]); 
  } 
  printf("Hello World!\n"); 
  return 0; 
} 

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