Why can't I link my C code with my library on Ubuntu?

Question

gcc -Wall -Werror -Wextra -Llibft -lft -Ilibft/include main.c

When I use the command above, the linker throws the following error message:

/tmp/ccwCUgov.o: In function `main':
main.c:(.text+0xc): undefined reference to `ft_putendl'
collect2: error: ld returned 1 exit status

I have to mention that the library libft.a in use does exist in the libft/ directory.

I'm coding on Ubuntu 18.04LTS right now(2019), but I have never encountered such problem on macOS.

The code in main.c:

#include "libft.h"

int main()
{
    ft_putendl("Hello world!");
    return (0);
}

The statement `#include "libft.h"` tells the compiler to look in the same directory as the source code first, and then in the default directory. — Weather Vane, Aug 12 '19 at 20:21
@WeatherVane, how is `#include` directive related to the linking? — r3mus n0x, Aug 12 '19 at 20:25
@r3musn0x the question says compiler error, but now you mention it *ld* is one system's linker. MSVC would say: *unresolved external symbol ft_putendl referenced in function _main*, for a linker error. — Weather Vane, Aug 12 '19 at 20:28
@WeatherVane I said that I compiled this code on my mac with no error popping-up. I don't understand why this happens on ubuntu — Glitch, Aug 12 '19 at 20:33
But why putting at the end? compiling on mac gives no problem — Glitch, Aug 12 '19 at 20:49

S.S. Anne · Answer 1 · 2019-08-12T20:53:08.770

0

Put -lft last in your linking command, like this:

gcc -Wall -Werror -Wextra -Llibft -Ilibft/include main.c -lft

This causes libft to be linked last, which should cause the needed functions to be included (the linker only includes objects that are needed, and at the time of processing libft.a, the objects aren't needed yet).

This probably works on Mac either because Mac includes all objects from the archive or because it links libraries last.

edited Aug 12 '19 at 20:53

answered Aug 12 '19 at 20:49

S.S. Anne

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does this means that all libraries should be added last? – Glitch Aug 12 '19 at 20:52
@aihya Yes. Except possibly shared libraries (not sure if they're linked anyways or not). – S.S. Anne Aug 12 '19 at 20:54
What I understood is that when the compiler runs, it compile objects in sequence, which means that if some object file a.o is written before another b.o, and b.o depends on a.o, it may not work. and vise versa. Right? – Glitch Aug 12 '19 at 21:03
@aihya Correct. – S.S. Anne Aug 12 '19 at 21:11

Why can't I link my C code with my library on Ubuntu?

1 Answers1