C read file line by line

Question

I wrote this function to read a line from a file:

const char *readLine(FILE *file) {

    if (file == NULL) {
        printf(\"Error: file pointer is null.\"         


        

Answer 1

You make the mistake of returning a pointer to an automatic variable. The variable line is allocated in the stack and only lives as long as the function lives. You are not allowed to return a pointer to it, because as soon as it returns the memory will be given elsewhere.

const char* func x(){
    char line[100];
    return (const char*) line; //illegal
}

To avoid this, you either return a pointer to memory which resides on the heap eg. lineBuffer and it should be the user's responsibility to call free() when he is done with it. Alternatively you can ask the user to pass you as an argument a memory address on which to write the line contents at.