How to create a function to that returns next line of input in C

Question

I'm pretty new to C, so as part of my learning I want to make a function that simply returns a string which is the next line in the input without any arguments needed.

This is what I have so far:

#define BUFFER 256
char *str;
char *getline()
{
    if (str == 0)
        str = (char*)calloc(BUFFER, sizeof(char));
    if (fgets(str, BUFFER, stdin) != 0)
    {
        size_t len = strlen(str);
        if (len > 0 && str[len-1] == '\n')
            str[len-1] = '\0';
        return str;
    }
    return 0;
}

Now, is this the best way to do such thing? Is there a better way you can thing of? If it is the best way, where should I free what I've allocated with calloc?

Answer 1

This works, but here are some notes to keep in mind:

• Don't cast the return value of malloc().

• You don't even need calloc() - fgets() NUL-terminates the string.

• Even better, use a static array.

• Don't use global variables. Declare str with static storage duration instead.

• Instead of 256, use LINE_MAX - how come a line can only be shorter than 256 characters?

• getline() is the name of a function in the POSIX C library. Rename your function to avoid name collision.

• Check if malloc() failed!

• Don't forget to free() the return value of the function after having read the last line if you use the approach with dynamic memory management.

• The check for the newline ending can be simplified like this:

---

char *p = strchr(str, '\n');
if (p)
    *p = 0;

All in all, the function I would write is something like this:

#include <string.h>
#include <stdlib.h>
#include <stdio.h>
#include <limits.h>

char *my_getline_stdin()
{
    static char buf[LINE_MAX];

    if (fgets(buf, sizeof(buf), stdin)) {
        char *p = strchr(buf, '\n');
        if (p)
            *p = 0;

        return buf;
    }

    return NULL;
}