How to use strtok in C properly so there is no memory leak?

Question

I am somewhat confused by what happens when you call strtok on a char pointer in C. I know that it modifies the contents of the string, so if I call strtok on a variable named 'line', its content will change. Assume I follow the bellow approach:

void function myFunc(char* line) {

    // get a pointer to the original memory block
    char* garbageLine = line;

    // Do some work
    // Call strtok on 'line' multiple times until it returns NULL
    // Do more work

    free(garbageLine);
}

Further assume that 'line' is malloced before it is passed to myFunc. Am I supposed to free the original string after using strtok or does it do the job for us? Also, what happens if 'line' is not malloced and I attempt to use the function above? Is it safer to do the following instead? (Assume the programmer won't call free if he knows the line is not malloced)

Invocation

char* garbageLine = line;
myFunc(line);
free(garbageLine);

Function definition

void function myFunc(char* line) {
    // Do some work
    // Call strtok on 'line' multiple times until it returns NULL
    // Do more work
}

Answer 1

strtok() will not free anything, as it has no knowledge of where the string is stored. It could be on the stack or the heap, it doesn't know or care! :)

Is it safer to do the following instead?

Your second example is much better, as it simplifies myFunc(), and makes it useful in more situations as the function does not need to know where the string is allocated. By removing the call to free() from myFunc() you are able to use the function to parse strings from the stack or the heap. The caller allocates the memory, the caller frees the memory!

Further reading: strtok()