How to add NULL to an array of string in C

Question

I'm trying to add NULL to a string to make execvp work. Here is my code :

void execCmd(char *cmd[]){
    int count = 0;
    while(strcmp(cmd[count],NULL) != 0) count++;
    cmd[count] = NULL;
    execvp(cmd[0], cmd);
    perror("execvp");
}

here is my body code. It works with pipe. Tell me if you need my client code too (even if I'm pretty sure it won't be useful because it works)

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <fcntl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <ctype.h>
#define TAILLE_CHAINE 15500
int isPalindrome(char *t);
void execCmd(char *cmd[]);
void RemoveSpaces(char* source);
char *replace(const char *s, char ch, const char *repl);
int main()
{
    char tube_serveur[TAILLE_CHAINE]="tube_serveur.fifo";
    int sortieTube; /* pointera sur la sortie du tube serveur */
    int entree_tube;/* pointera sur le tube client */
    char chaineALire[TAILLE_CHAINE];/*chaine à lire dans le tube serveur*/
    char chaineAEcrire[TAILLE_CHAINE]; /*chaine à écrire dans le tube du client*/
    char **chaineCmd = malloc(sizeof(char*) * (250));
    char *tube_client;/*un pointeur pour la fonction strtok (chaineALire, ";") qui permet de segmenter chaineALire */
    char *mot;/* mot que le client veut tester si c'est un palindrome ou non */
    int i =0;
    do
    {   /***************** création tube serveur **********************/
        if(mkfifo(tube_serveur, 0644) != 0)
        {   fprintf(stderr, "Impossible de créer le tube nommé.\n");
            exit(EXIT_FAILURE);
        }
        /**************** Attente d'un appel client ******************/
        if((sortieTube = open (tube_serveur, O_RDONLY)) == -1)
        {
            fprintf(stderr, "Impossible d'ouvrir la sortie du tube nommé.\n");
            exit(EXIT_FAILURE);
        }
        read(sortieTube, chaineALire, TAILLE_CHAINE);
        unlink(tube_serveur);
        printf("%s\n", chaineALire);
        tube_client = strtok (chaineALire, ";");/* segmente la chaine par le séparateur ; donc tube_client vaut le nom du tube du client (1ère partie de la chaine)*/
        mot = strtok (NULL, ";");
        if ((strcmp(mot,",") == 0) || (strcmp(mot, "stop") == 0))/*Si le mot vaut FIN on le serveur inutile d'utiliser la fonction de palindrome */
        {
            execCmd(chaineCmd);
            int k =0;
            chaineCmd = NULL;
            chaineCmd = malloc(sizeof(char*) * (250));

        }
        if (strcmp(mot,"stop") == 0)/*Si le mot vaut FIN on le serveur inutile d'utiliser la fonction de palindrome */
        {
            memset( chaineAEcrire, '\0', sizeof(chaineAEcrire) );
            sprintf(chaineAEcrire,"Fin du serveur");
            //break;
        }
        chaineCmd[i] = malloc((strlen(mot) + 1) * sizeof(char));
        strcat(chaineCmd[i], mot);
        printf ("client : %s\n", tube_client);
        printf ("message : %s\n", mot);
        /************* Ecriture dans le tube client ********/
        unlink(tube_client);/*on détruit le dernier tube client*/
        if(mkfifo(tube_client, 0644) != 0)
        {   fprintf(stderr, "Impossible de créer le tube nommé.\n");
            exit(EXIT_FAILURE);
        }
        if((entree_tube = open(tube_client, O_WRONLY)) == -1)
        {   fprintf(stderr, "Impossible d'ouvrir l'entrée du tube nommé.\n");
            exit(EXIT_FAILURE);
        }

        else
        {
            if(isPalindrome(mot))/* Le mot est-il un palindrome? */
            {   memset(chaineAEcrire, '\0', sizeof(chaineAEcrire) );/*remplace toutes les cases de la chaine par \0 : vide la chaine*/
                sprintf(chaineAEcrire,mot);/*on écrit le mot dans la chaine*/
                strcat(chaineAEcrire, " est un palaindrome."); /* on indique que c'est un palindrome */
            }
            else
            {   memset( chaineAEcrire, '\0', sizeof(chaineAEcrire) );
                sprintf(chaineAEcrire,mot);
                strcat(chaineAEcrire, " n'est pas un palaindrome.");
            }
        }
        write(entree_tube,chaineAEcrire, TAILLE_CHAINE);/* réponse au client */
        i++;
    }
    while((strcmp(mot,"stop") != 0) );
    /************** Programme terminé *****************************/
    unlink(tube_serveur);
    printf("fin du serveur");
    return EXIT_SUCCESS;
}
int isPalindrome(char *t)
{   int i;/*emplacement de la lettre en commencant par le début (ex : t[0] puis t[1]...*/
    int j;/*emplacement de la lettre en commencant par le début (ex : t[n] puis t[n-1]...*/
    int n=strlen(t); /* taille de la chaine  */
    for(i=0,j=n-1;i<j;i++,j--)
    {   if(tolower(t[i])!=tolower(t[j])) /*on met en minuscule car A!=a or Anna est un palindrome */
        {     return 0;/*ce n'est pas un palindrome*/
        }
    }
    return 1;
}
void execCmd(char *cmd[]){
    execvp(cmd[0], cmd);
    perror("execvp");
}
char *replace(const char *s, char ch, const char *repl) {
    int count = 0;
    const char *t;
    for(t=s; *t; t++)
        count += (*t == ch);

    size_t rlen = strlen(repl);
    char *res = malloc(strlen(s) + (rlen-1)*count + 1);
    char *ptr = res;
    for(t=s; *t; t++) {
        if(*t == ch) {
            memcpy(ptr, repl, rlen);
            ptr += rlen;
        } else {
            *ptr++ = *t;
        }
    }
    *ptr = 0;
    return res;
}

How can I do this?

Thanks for helping

strcmp compares two character arrays. NULL is not a valid character array. Besides, if it already ends in NULL that's the same as '\0'. — lurker
– lurker, Commented Nov 18, 2016 at 22:20
You don't need to use strcmp to test for null. Just test the pointer against NULL. — Govind Parmar
– Govind Parmar, Commented Nov 18, 2016 at 22:23
OK, here comes the confusion between the null-terminator and the NULL pointer.. — Eugene Sh.
– Eugene Sh., Commented Nov 18, 2016 at 22:27

user268396 · Accepted Answer · 2016-11-18 22:28:25Z

1

execvp() expects the last element to be a NULL pointer, not a \0 char to signal the end.

You call it like this:

static int example(const char * binary, int num_argv, char * argv[])
{
    argv[num_argv - 1] = NULL; /* ensure argv is NULL terminated. */
    int r = execvp(binary, argv);
    if(r < 0) // execvp() failed
    {
        perror("failed to execvp()");
        /* 
         * terminate the current process regardless
         * otherwise you could end up with a process stuck in limbo
         * which is particularly troublesome if you rely on 
         * a simple fork() + some kind of wait() in a parent process
         * to cleanup child processes
         */
        exit(255);
    }
    return r;
}

edited Nov 18, 2016 at 22:28

answered Nov 18, 2016 at 22:20

user268396

12.1k2 gold badges33 silver badges26 bronze badges

Sign up to request clarification or add additional context in comments.

8 Comments

Vincent Normand Over a year ago

Ok, I'll update this. But how can I add NULL to my string?

Ed Heal Over a year ago

You do not need to add it to the string.What is calling execCmd

Vincent Normand Over a year ago

so can I do this? void execCmd(char *cmd[]){ execvp(cmd[0], cmd); perror("execvp"); }

David Schwartz Over a year ago

You just want cmd[count] = NULL;.

Vincent Normand Over a year ago

Exactly, to be able to run my execvp

|

Collectives™ on Stack Overflow

How to add NULL to an array of string in C

1 Answer 1

8 Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

1 Answer 1

8 Comments

Your Answer

Sign up or log in

Post as a guest

Related