Passing a double pointer to modify string

Question

So I have a program that works sometimes, but other times it doesn't. Sometimes putting in a printf statement will make the program magically work (and sometimes a printf will make it break). This makes me think I messed up as far as pointers/allocation goes, as I still have trouble wrapping my head around some things. The program is too long to post all the code, so I'll just show where I think the problem is.

I was given a function that takes a FILE* and a few char** as parameters, returning an int. The function is parsing a file, storing necessary strings to where the double pointers point to.

Here is how I used the function (alot of code is omitted/simplified):

char **s1;
char **s2;
char **s3;

int main(int argc, char ** argv){
    s1 = (char**)malloc(20);
    s2 = (char**)malloc(20);
    s3 = (char**)malloc(20);

    /* Unnecessary code omitted */
    read(inFile);
}

int read(FILE* in){
    /* omitted code */
    parse(in,s1,s2,s3);
    printf("%s",*s1); /* This is to show how I access the strings */
}

Im pretty sure that somewhere in my program, those strings are getting overwritten because I didn't allocate the memory properly. Hopefully my mistake is visible in the code snippet I gave, because I don't have many other theories for why my code doesn't work

Answer 1

Since the API to parse() is specified with char ** I believe it is safe to assume that you really do need the double-indirection in the call, but not in the declaration.

So probably what you need is to skip the malloc() calls and say:

char *s1, *s2, *s3;
...
parse(in, &s1, &s2, &s3);

This would allow parse() to allocate its own space and return the three pointers to its caller by modifying the pointers themselves. I appreciate your efforts to distill the question to its core but it might be interesting to see the prototype for parse().