Question

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Pointer to local variable
Can a local variable's memory be accessed outside its scope?

gcc 4.4.4 c89

In main I call a function to pass a line of text to a function. I want to perform some operation on it. However, that would mean that line is of no use. So in my get_string function I copy the contents and return the result. The only problem, is that the memory to that result would be lost and pointing to something unexpected.

I am just wondering how can I pass the result back, without and still keep the ordinal line of data?

Many thanks for any advice,

code snippet from main:

    if(fgets(line_data, (size_t)STRING_SIZE, fp) == NULL) {
        fprintf(stderr, "WARNING: Text error reading file line number [ %d ]\n", i);
    }

    if(get_string(line_data) != NULL) {
        if(strcmp(get_string(line_data), "END") == 0)
            break;
    }
    else {
        fprintf(stderr, "WARNING: Cannot get name of student at line [ %d ]\n", i);
    }

    /* Fill student info */
    strncpy(stud[i].name, line_data, (size_t)STRING_SIZE);

Call this function

char* get_string(char *line_data)
{
    char *quote = NULL;
    char result[STRING_SIZE] = {0};

    strncpy(result, line_data, (size_t)STRING_SIZE);

    /* Find last occurance */
    if((quote = strrchr(result, '"')) == NULL) {
        fprintf(stderr, "Text file incorrectly formatted for this student\n");
        return NULL;
    }
    /* Insert nul in place of the quote */
    *quote = '\0';

    /* Overwite the first quote by shifting 1 place */
    memmove(result - 1, result, strlen(result) + 1);

    return result;
}

Answer 1

For your direct question – either use malloc(3) and tell the user of the function to de-allocate the return pointer (this is sort of prone to memory leaks since it’s so easy to ignore return value in C), or provide the second parameter as a receive buffer:

char* get_string( const char* line_data, char* receive_buf, size_t buf_size );

The third parameter is for the function to know how large the receive buffer is.

Now to your code – the line memmove(result - 1, result, strlen(result) + 1); corrupts your stack.