Execute a Program: the execvp() System Call

The created child process does not have to run the same program as the parent process does. The exec type system calls allow a process to run any program files, which include a binary executable or a shell script. On this page, we only discuss one such system call: execvp(). The execvp() system call requires two arguments:

1. The first argument is a character string that contains the name of a file to be executed.
2. The second argument is a pointer to an array of character strings. More precisely, its type is char **, which is exactly identical to the argv array used in the main program:

   int  main(int argc, char **argv)
   

   Note that this argument must be terminated by a zero.

When execvp() is executed, the program file given by the first argument will be loaded into the caller's address space and over-write the program there. Then, the second argument will be provided to the program and starts the execution. As a result, once the specified program file starts its execution, the original program in the caller's address space is gone and is replaced by the new program.

execvp() returns a negative value if the execution fails (e.g., the request file does not exist).

The following is an example (in file shell.c). Click here to download a copy.

#include  <stdio.h>
#include  <sys/types.h>

void  parse(char *line, char **argv)
{
     while (*line != '\0') {       /* if not the end of line ....... */ 
          while (*line == ' ' || *line == '\t' || *line == '\n')
               *line++ = '\0';     /* replace white spaces with 0    */
          *argv++ = line;          /* save the argument position     */
          while (*line != '\0' && *line != ' ' && 
                 *line != '\t' && *line != '\n') 
               line++;             /* skip the argument until ...    */
     }
     *argv = '\0';                 /* mark the end of argument list  */
}

void  execute(char **argv)
{
     pid_t  pid;
     int    status;

     if ((pid = fork()) < 0) {     /* fork a child process           */
          printf("*** ERROR: forking child process failed\n");
          exit(1);
     }
     else if (pid == 0) {          /* for the child process:         */
          if (execvp(*argv, argv) < 0) {     /* execute the command  */
               printf("*** ERROR: exec failed\n");
               exit(1);
          }
     }
     else {                                  /* for the parent:      */
          while (wait(&status) != pid)       /* wait for completion  */
               ;
     }
}

void  main(void)
{
     char  line[1024];             /* the input line                 */
     char  *argv[64];              /* the command line argument      */

     while (1) {                   /* repeat until done ....         */
          printf("Shell -> ");     /*   display a prompt             */
          gets(line);              /*   read in the command line     */
          printf("\n");
          parse(line, argv);       /*   parse the line               */
          if (strcmp(argv[0], "exit") == 0)  /* is it an "exit"?     */
               exit(0);            /*   exit if it is                */
          execute(argv);           /* otherwise, execute the command */
     }
}

Function parse() takes an input line and returns a zero-terminated array of char pointers, each of which points to a zero-terminated character string. This function loops until a binary zero is found, which means the end of the input line line is reached. If the current character of line is not a binary zero, parse() skips all white spaces and replaces them with binary zeros so that a string is effectively terminated. Once parse() finds a non-white space, the address of that location is saved to the current position of argv and the index is advanced. Then, parse() skips all non-whtitespace characters. This process repeats until the end of string line is reached and at that moment argv is terminated with a zero.

For example, if the input line is a string as follows:

"cp  abc.CC   xyz.TT"

Function execute() takes array argv[], treats it as a command line arguments with the program name in argv[0], forks a child process, and executes the indicated program in that child process. While the child process is executing the command, the parent executes a wait(), waiting for the completion of the child. In this special case, the parent knows the child's process ID and therefore is able to wait a specific child to complete.

The main program is very simple. It prints out a command prompt, reads in a line, parses it using function parse(), and determines if the name is "exit". If it is "exit", use exit() to terminate the execution of this program; otherwise, the main uses execute() to execute the command.