C Returning char[] Warning “returns address of local variable”

ccompiler-warningspointersreturn

This is a portion of a homework assignment.

I am trying to read and return a single line of a file in my method getLine.

char *getLine(FILE *input) {
    char line[30];
    if(fgets(line, sizeof(line), input)!=NULL)
    {
        return line;
    }else{
        return NULL;
    }
}

This would seem to work from what I have been taught regarding pointers, however I am unable to remove the warning message warning: function returns address of local variable [enabled by default]. This warning is referring to the line return line;. My assignment requires that I have no warnings or errors when I compile. I don't see what I am doing wrong.

Most of the help I found suggested malloc-ing space for the line of text, but we haven't covered that in class yet even though I have done some in another class. Is that really the best way to do this? If so, would I be able to free anywhere in the program?

Best Answer

char line[30]; is an array with automatic storage duration. Memory where it resides is deallocated once the execution goes out of the scope of your function, thus pointer to this memory that you return becomes invalid (dangling pointer).

Trying to access memory, that has already been deallocated, results in undefined behaviour.

You can allocate your array dynamically and let the caller explicitly deallocate it:

char *getLine() {
    char* line = malloc(30);
    ...
    return line;
}

// somewhere:
char* line = getLine();
...
free(line);

Related Solutions

R – C: problem with char*

I see a few problems. First of all, char* sOutput[STRMAX] is an array of char*'s - presumably you meant char sOutput[STRMAX]?

Secondly, and more importantly, when you declare an array in a function in that way (char sOutput[STRMAX]), it gets allocated on the stack and deallocated when the function returns. Thus, if you try to return it, you'll get undefined results, because it's not technically supposed to exist anymore!

The solution is to pass a buffer into the function for it to use:

char* reverse(char const sPhrase[], char sOutput[])

(I added the const so you don't accidentally overwrite sPhrase). Then call reverse like this:

reverse(sPhrase, sReverse);

Now as for whether your algorithm works...

Length of line in getline() function – c

Using 120 as an initial allocation is a sensible choice made by those who wrote the library; it is longer than most lines, so there's a decent chance it will only have to do one memory allocation. (FWIW: I wrote an implementation of getline() and getdelim() to use on machines without a version in the standard library. It used 256 as the minimum allocation (rather than 120) — except when compiled for testing, when it used 8 as the starting point to make sure the reallocation code gets more exercise. The allocated size doubles when there isn't enough room. It was a similar thought process to what the library designers did in the library you use; get the memory allocation big enough first time around that the code mostly won't need to reallocate.)
You can find the code in the library, change the default, and rebuild the library. OTOH, that's hard work.
There's a function called getline() that tells you how many chars were in the line it just read via its return value. Note that it returns -1 (not EOF) on error. Although it seldom happens, it is theoretically possible for EOF to be a negative value other than -1.

You can also rig the system to use shorter lines (until you read a long line) by:

FILE *file_in;
size_t len = 16;
char *line = malloc(len);

And (for the ultimate in sleazeball programming) you don't even have to error check the allocation since char *line = NULL; is also a valid input to getline():

ssize_t nbytes;

while ((nbytes = getline(&line, &len, file_in)) > 0)
    ...process a line of nbytes including newline...

Best Answer

Related Solutions

R – C: problem with char*

Length of line in getline() function – c

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