execvp()

culin · 01-22-2007, 07:13 AM

Hi friends,
this is a code written by me, here based on the value of "i" the corresponding case is executed and it lists the file in the specified directory... This is fine..
But how can i achive that all the 3 cases must be executed ( need not be using switch case ) that is i want all the 3 listings in the specified directories..( HOW CAN I DO THIS USING FORK() AND EXEC()and is there any other method to spawn all the 3 processess(i.e listings))

Code:

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

int spawn3(void)
{
        pid_t child_pid1,child_pid2,child_pid3;
        int i=2;
        char* arg_list1[] =
         {
                "ls", /* argv[0], the name of the program. */
                "-l",
                "/home/hello/include",
                NULL /* The argument list must end with a NULL. */
        };

        char* arg_list2[] =
         {
                "ls", // argv[0], the name of the program.
                "-l",
                "/home/hello/scripting",
                NULL // The argument list must end with a NULL.
        };

        char* arg_list3[] =
         {
                "ls", // argv[0], the name of the program.
                "-l",
                "/home/hello/Proj_conf",
                NULL // The argument list must end with a NULL.
        };


        switch(i)
        {
                case 3: child_pid3=fork();
                        if (child_pid3 != 0)
                        return child_pid3;
                        else
                        {
                                execvp ("ls", arg_list3);
                                fprintf (stderr, "\nan error occurred in execvp\n\n\n");
                        }
                        break;
                case 2:  child_pid2=fork();
                        if (child_pid2 != 0)
                        return child_pid2;
                        else
                        {
                                execvp ("ls", arg_list2);
                                fprintf (stderr, "\nan error occurred in execvp\n\n\n");
                        }
                        break;

                case 1: child_pid1=fork();
                        if (child_pid1 != 0)
                        return child_pid1;
                        else
                        {
                                execvp ("ls", arg_list1);
                                fprintf (stderr, "\nan error occurred in execvp\n\n\n");
                        }
                        break;

                default: exit(0);
        }

}


int main (int argc , char *argv[])
{
        int child_pid;
        child_pid=spawn3();
        printf("returned to main() and the child pid is %d\n\n",child_pid);
        return 0;
}

please someone help me
thanks....

tronayne · 01-22-2007, 07:51 AM

Here's an example from Stephen G. Kochan & Patrick H. Wood, Topics in C Programming, Revised Edition (New York: John Wiley & Sons, Inc., 1991) that will give you an idea to build on. The program is a simple command-line interpreter (a "shell") and uses a clever bit of parsing (the breakup() function) to get arguments in the right places (in the argc[] array) before calling execvp().

Code:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <wait.h>
#include <sys/types.h>
#include <sys/stat.h>

/*
 *      simple command interpreter
 *      supports < and > redirection and command line arguments
*/

int     breakup (char *, char **);

void    main    (void)
{
        char    line [BUFSIZ], *args [15];
        int     process, nargs;

        process = 0;
        for ( ; ; ) {           /* loop forever                 */
                (void) fprintf (stderr, "cmd: ");
                if (gets (line) == (char *) NULL) {
                        exit (EXIT_SUCCESS);
                }
                if ((process = fork ()) > 0)    /* parent       */
                        (void) wait ((int *) NULL);
                else if (process == 0) {        /* child        */
                        /*      parse command line              */
                        nargs = breakup (line, args);
                        /*      make sure there's something     */
                        if (nargs == 0)
                                exit (EXIT_SUCCESS);
                        /*      execute program                 */
                        (void) execvp (args [0], args);
                        /*      some problem if execvp returns  */
                        (void) fprintf (stderr, "cannot execute %s\n", line);
                        exit (errno);
                } else if (process == -1) {     /* can't create */
                        (void) fprintf (stderr, "can't fork\n");
                        exit (errno);
                }
        }
}

/*
 *      break up command line and return in "args"
 *      recognize < file and > file constructs and redirect
 *      standard input and output as appropriate
*/

int     breakup (char *line, char *args [])
{
        char    *strptr = line, *file;
        int     nargs = 0;

        while ((args [nargs] = strtok (strptr, " \t")) != (char *) NULL) {
                strptr = (char *) NULL;
                /*      output redirection      */
                if (args [nargs] [0] == '>') {
                        if (args [nargs][1] != '\0')
                                file = &args [nargs] [1];
                        else {
                                file = strtok (strptr, " \t");
                                if (file == (char *) NULL) {
                                        (void) fprintf (stderr, "no file after >\n");
                                        return (0);
                                }
                        }
                        (void) close (1);
                        if (open (file, O_WRONLY | O_TRUNC | O_CREAT, 0666) == -1) {
                                (void) fprintf (stderr, "can't open %s for output\n", file);
                                return (0);
                        }
                        --nargs;
                /*      input redirection       */
                } else if (args [nargs] [0] == '<') {
                        if (args [nargs] [1] != '\0')
                                file = &args [nargs] [1];
                        else {
                                file = strtok (strptr, " \t");
                                if (file == (char *) NULL) {
                                        (void) fprintf (stderr, "no file after <\n");
                                        return (0);
                                }
                        }
                        (void) close (0);
                        if (open (file, O_RDONLY) == -1) {
                                (void) fprintf (stderr, "can't open %s for input\n", file);
                                return (0);
                        }
                        --nargs;
                }
                ++nargs;
        }
        args [nargs] = (char *) NULL;
        return (nargs);
}

As an aside, Kochan and Wood's books are excellent, full of working examples, and a great help to both beginners and those of us long in the tooth -- I understand there is a newly revised edition of Topics either just or just about to be published; might be worth a look-see.

culin · 01-24-2007, 02:27 AM

Thanks for the code tronayne

varun_shrivastava · 01-24-2007, 06:00 AM

here is one more code i developed for "myshell"

Code:

# include <stdio.h>
# include <sys/types.h>
# include <unistd.h>
int main()
{
	int i = 0, j;
	char buff[100], *cptr[10], *bptr, cmd[10];
	while(1)
	{
		i=0;
		printf("\nMY SHELL: VARUN::-> ");
		fgets((char *)buff,100,stdin);
		bptr = buff;
		for(j=0;j<10;j++)
			*(cptr+j) = (char *)malloc(10);
		while(*bptr!='\0')
		{
			j = 0;
			while(*bptr != ' ' && *bptr != '\n')
			{
				*(*(cptr+i)+j) = *bptr++;
				j++;
			}
			if(*bptr==' '||*bptr=='\n')
			{
				*(*(cptr+i)+j) = '\0';
				bptr++;
			}
			i++;
		}	
		*(cptr+i) = NULL;
		strcpy(cmd,*cptr);
		switch(fork())
		{
			case 0:
					execvp(cmd,cptr);
					exit(1);	
			default:

					wait(NULL);
					break;
		}
	}
}