Command parsing UNIX shell C

Question

I was trying to create a program where I could parse the command and parameters enters by the user into specific arrays (these commands and params would execute commands like "ls", "ls -l", "ls -l | wc" However, I am having problem with parsing:

        //Split the command and store each string in parameter[]
    cp = (strtok(command, hash));                      //Get the initial string (the command)
    parameter[0] = (char*) malloc(strlen(cp)+ 1);                     //Allocate some space to the first element in the array
    strncpy(parameter[0], cp, strlen(cp)+ 1);
    for(i = 1; i < MAX_ARG; i++)
    {
    cp = strtok(NULL, hash);                 //Check for each string in the array
    parameter[i] = (char*) malloc(strlen(cp)+ 1);
    strncpy(parameter[i], cp, strlen(cp)+ 1);                      //Store the result string in an indexed off array
        if(parameter[i]  == NULL)
        {
            break;
        }
    if(strcmp(parameter[i], "|") == 0)
    {
        cp = strtok(NULL, hash);
        parameter2[0] = (char*) malloc(strlen(cp)+ 1);
        strncpy(parameter2[0], cp, strlen(cp)+ 1);
        //Find the second set of commands and parameters
        for (j = 1; j < MAX_ARG; j++)
        {
            cp = strtok(NULL, hash);
            if (cp == NULL)
            {
                leave = 1;
                break;
            }
            else
            {
                parameter2[j] = (char*) malloc(strlen(cp)+ 1);
                strncpy(parameter2[j], cp, strlen(cp)+ 1);
            }

        }
    }
    if (leave == 1)
    {
        break;
    }
}

I run into a problem when I do if (strlen(cp) == NULL), there is a segmentation fault. I am trying to break out of the larger for loop once all the inputs have been entered into arrays. I can succesfully enter the correct string elements into the arrays but I just can't exit the loop once I have done so.

Tio Pepe · Accepted Answer · 2011-10-20 08:58:12Z

3

strtokmay return NULL pointers if can't found more tokens. So, you must check cp value before using it:

cp = strtok(NULL, hash);
if (cp != NULL)
{
    ........
}

answered Oct 20, 2011 at 8:58

Tio Pepe

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5 Comments

Josh Over a year ago

I still got a segmentation fault. The problem is that after the last element has been stored for example, the cp is pointing to a garbage element. for example ("my name is josh | i am in college), the program can correctly store all the token in the correct place, but once it comes at the end of college, it gets a grabage value and thats where it should leave the loop...thats what I was trying to do

Tio Pepe Over a year ago

But, if cp == NULL you must break the loop. Are you doing this?

Josh Over a year ago

yeah, whenever I do it I get a segmentation error...its is SIGSEGV segmentation fault

Tio Pepe Over a year ago

which is the value of hash and MAX_ARG? Give me an example of your input

Josh Over a year ago

I am sorry I was another part of the program was causing the error (the break statement didnt break out of the outter most loop)

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