C void function modify pointer array

Question

I have a function tokenize and I want to modify a char pointer array through the function. But I don't get the result after many tries. Could someone help me?

Below is the code:

void **tokenize(char **argv, char *line, int *numTokens, char *delimiter)
{
    int argc = 0;
    char *token = strtok(line, delimiter);
    while (token != NULL)
    {
        argv[argc++] = token;
        token = strtok(NULL, delimiter);
    }
    argv[argc++] = NULL;
    *numTokens = argc - 1;
}

void process_cmd(char *cmdline){
    char *temp[20];
    int *num2;

    tokenize(temp, cmdline, num2, "|"); 

    printf("%s\n", temp[0]);
    printf("%s\n", temp[1]);
    printf("%d\n", *num2);

}

Where is num2 pointing? Perhaps you should take more time to study how emulating pass by reference in C really works, and how to use the address-of operator &. — Some programmer dude
– Some programmer dude, Commented Oct 19, 2019 at 2:11

Antti Haapala · Accepted Answer · 2019-10-19 05:58:10Z

1

In your process_cmd function, the local variable num2 is not initialized when it is defined, and you use it directly at the call to tokenize. And you write to the location pointed to by numTokens in tokenize, which results in an undefined behavior. In most cases your program will crash when trying to write through numTokens; but theoretically anything could happen.

To fix this, you need to change your definition of num2 to:

int num2;

and call tokenize as follows:

tokenize(temp, cmdline, &num2, "|");

edited Oct 19, 2019 at 5:58

Antti Haapala

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answered Oct 19, 2019 at 2:14

Lancern

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1 Comment

hang99 Over a year ago

Thanks for your help.

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