implicit declaration of function ‘getline’ warning thrown in one code, but not in another

Question

THIS QUESTION IS NOT HOW TO REMOVE THE WARNING

I am writing a shell. I referred this source . I used the same headers (in the same order), as he did, in my code.

When compiling his code, I do not get any warnings for implicit declaration of getline . But when I compile mine, it does get thrown.

The man page suggests to use #define _GNU_SOURCE , and adding that removed the warning from my code.

So why was no warning thrown for the code in the blog, as he did not use #define _GNU_SOURCE ?

---

Here is the minimal code (I copied all the headers as I mentioned above)

// #define _GNU_SOURCE
#include <sys/wait.h>
#include <sys/types.h>
#include <unistd.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main()
{
  ssize_t bytes_read;
  size_t input_buffer_size = 1024;
  char *user_input = (char *)malloc(input_buffer_size * sizeof(char));

  while (1)
  {
    printf("> ");
    bytes_read = getline(&user_input, &input_buffer_size, stdin);

    printf("%s\n", user_input);
  }

  return 0;
}

And here is the compilation process I used...

gcc -std=c11 -o bin/shell src/shell.c

Here is the error that I get if I leave the first line commented.

src/shell.c: In function ‘main’:
src/shell.c:18:18: warning: implicit declaration of function ‘getline’ [-Wimplicit-function-declaration]
   18 |     bytes_read = getline(&user_input, &input_buffer_size, stdin);
      |                  ^~~~~~~

Answer 1

It appears that the person who wrote the tutorial you're referring to, did not supply any special compilation options when they were testing their code. I see only one compilation command anywhere on that page, and it is gcc -o main main.c . Thus, they got GCC's defaults, which, typically, make getline available on computers that have it.

You, however, used the compiler flag -std=c11 when you compiled your code. One of the effects of this flag is that GCC directs the C library's headers to declare only the functions, constants, variables, etc. that are specified by ISO C2011. (Depending on which C library you're using, this directive may or may not have any effect — but Ubuntu uses the GNU C library, which implements it thoroughly.) getline is not part of ISO C2011, so it is not declared and you get an "implicit declaration" diagnostic when you try to use it.

Use of the hyperconformant -std=cXX modes is almost always a mistake. There are exactly three differences between -std=cXX and -std=gnuXX and none of them is desirable in practice:

1. As discussed above, it directs the headers not to declare anything that's not part of the specified revision of ISO C. As you saw for yourself, this is almost never what you want when writing a nontrivial C program. It also has a nasty tendency to break library headers — both third-party headers and the C library's own headers — because they are rarely, if ever, tested in this mode.

2. It disables " system-specific predefined macros " that pollute the user namespace (eg linux , unix , arm ). This is abstractly desirable but, like #1, has a nasty tendency to break library headers that are rarely, if ever, tested in this mode.

3. It enables trigraphs , which are a kludge to make C work with "national variants" of ASCII that are missing some punctuation. These are so rarely used and cause so much practical confusion that they were actually stripped out of C++ 2017 (not C 2017, though).

To compile your own code with a reasonably picky level of conformance diagnostics, but not risk breaking library headers, there is a better combination of options:

cc -std=gnuXX -g -Og -Wall -Wextra -Wpedantic -Wold-style-definition -Wwrite-strings

(Pick a suitable XX; if you have no reason to choose anything else, I'd go with 11.) You may or may not want to add a -D switch for one of the _xxx_SOURCE feature selection macros ; explaining how those work and how to choose one is a whole question in itself.