How to create a child process with C library call system()?

System() function in c

Do you want to execute a shell command in your program and check the status of the command? Do you want to create a child process and wait for that process to end? Do you want to check if the program has the access to the shell execution? You often have these requirements for your program and we have the solution.

C runtime library has an interesting function called system() to invoke commands in the shell. It takes commands and arguments in a string separated by spaces. We generally call fork() and then execxx() to spawn a new process. It does same and hopefully wraps those calls inside it. It also makes calling interface simpler and easy to use. We have next example to elaborate this.

System() function Description

The C library function int system(const char *command) is used to create a child process. A string containing the command name or program name followed by the arguments are passed to the function. This simple but effective function can be used to execute any command like pause, cls, echo etc.

System() - Declaration

Following is the declaration for system() function.

 
 int system(const char *command);

System() - Parameters

command − This is the C string containing the command.

System() - Return Value

This function can return these following values:

User can pass command string as NULL. This function will return a nonzero value if shell is available else a value of zero will be returned.

If a child process could not be created, or its status could not be retrieved, the return value is -1 and errno is set to indicate the error.

If a shell could not be executed in the child process, then the return value is as though the child shell terminated by calling _exit(2) with the status 127.

If all system calls succeed, then the return value is the termination status of the child shell used to execute command. (The termination status of a shell is the termination status of the last command it executes.)

System function - check if shell exection available or not.

 
 #include <stdio.h>

 #include <stdlib.h>

 int main()

 {

     int status;

     status = system(NULL);

     if (status)

       printf("Shell execution is supported.");

     else

       printf("Shell execution is not supported.");

     return 0;

 }

Output:

Shell execution is supported.

System function in c example

 
 #include <stdio.h>

 #include <stdlib.h>

 #include <malloc.h>

 #include <string.h>

 int main (int argc, char * argv[])

 {

   char * cmd_line;

   char * arg;

   int index;

   int len;

   int ret = -1;

   if (argc > 1)

   {

     index = 1;

     while (arg = argv [index])

     {

       len += strlen (arg);

       len ++;

       index ++;

     }

     cmd_line = (char *) calloc (1, len);

     if (cmd_line == NULL)

     {

         perror ("memory allocation error!\n");

         return -1;

     }

     index = 1;

     while (arg = argv [index])

     {

       index ++;

       strcat (cmd_line, arg);

       if (argv [index])

       {

         strcat (cmd_line," ");

       }

     }

     ret = system (cmd_line);

     free (cmd_line);

     return ret;

   }

   else

   {

     printf ("At least one argument needed.\n");

     printf ("Syntax: redo \n");

     return -1;

   }

 }