Segmentation Fault in C programming when using strtok()

Question

I am writing a simple shell and the command (ls & whoami;) works however, when I only type the command ls the code breaks due to a segmentation fault on line 51. I am wonder how to go about this I have used Valgrind and a ton of printf statements but nothing is seeming to work. Let me know what you think.

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <wait.h>

#define MAX_LINE 80
#define MAX_CHAR 100

int main()
{
    int should_run = 1;
    char line[MAX_CHAR];
    char *args[MAX_LINE/2 +1];
    int ampersandFlag = 0;


    /*---------------------------------------------------*/
    /*                Start of The Program               */

    while(should_run){
        printf("osh>");
        fgets(line,MAX_CHAR,stdin);

        /* get rid of newline char */
        int k = 0;
        while(line[k] != '\n'){
            k++;
        }
        line[k] = '\0';

        /* get rid of semicolon char */
        int l = 0;
        while(line[l] != ';'){
            l++;
        }
        line[l] = '\0';


        int i = 0;
        args[i] = strtok(line, " ");

        if(args[i] == NULL){
            return 1;
        }

        while(args[i] != NULL){
            printf("%s\n", args[i]);
            i++;
            args[i] = strtok(NULL, " ");
        }

        /* Handles the exit command.  */
        int a = 0;
        if(strcmp(args[a], "exit") == 0){
            exit(0);
        }
        if(strcmp(args[1], "&") == 0){
            args[1] = NULL;
            ampersandFlag = 1;
        }

        /*---------------------------------------------------*/
        /*                     Start Forking                 */

        int returnForkValue = fork();
        int status;

        if(returnForkValue < 0){
            perror("Fork Failed");
        }

        else if(returnForkValue == 0){
            printf("This is the child\n");
            if(ampersandFlag == 1){
                int stat;
                int retFork = fork();

                if(retFork == 0){
                    printf("This is second child");
                    execvp(args[2],args);
                }
                else{
                    printf("This is the second parent");
                    waitpid(retFork, &stat, 0);
                }
            }
            execvp(args[0],args);
            printf("This won't be printed if execvp is successul\n");
        }

        else{
            printf("This is the parent\n");
            waitpid(returnForkValue, &status, 0);
        }
    }
    return 0;
}

Answer 1

If you have exactly one token, only args[0] will point to a valid string. The next args[1] will be a NULL pointer. All other elements of args will be uninitialized.

In this case the line

        if(strcmp(args[1], "&") == 0){

will try to use a NULL pointer as a string.

After the loop

while(args[i] != NULL){

the variable i will contain the number of tokens.

You should only use index values < i to access args .

The code to execute the second command is wrong. Instead of

                    execvp(args[2],args);

you would have to use

                    execvp(args[2],&args[2]);

but this is only valid if the number of tokens is at least 3.

BTW: Your code is a bit inconsistent. To check for "exit" you use variable a as index for args , for other comparisons you use hard-coded numbers.

When I run the program with valgrind and enter ls I get an error message showing the NULL pointer access.

==30002== Invalid read of size 1
==30002==    at 0x483EED4: strcmp (in /usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==30002==    by 0x10943A: main (test.c:58)
==30002==  Address 0x0 is not stack'd, malloc'd or (recently) free'd

I get an additional error message

==30002== Conditional jump or move depends on uninitialised value(s)
==30002==    at 0x10933C: main (test.c:34)

This happens because your loop

while(line[l] != ';'){

does not check for the terminating NUL ( '\0' ) character and will read past the end of the string if it doesn't contain a semicolon.