undefined reference to `getline' in c

Question

I am learning to use getline in C programming and tried the codes from http://crasseux.com/books/ctutorial/getline.html

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

int main(int atgc, char *argv[])
{
    int bytes_read = 1;
    int nbytes = 10;
    char *my_string;

    my_string = (char *)malloc(nbytes+1);

    puts("Please enter a line of text");

    bytes_read = getline(&my_string, &nbytes, stdin);

    if (bytes_read == -1)
    {
        puts ("ERROR!");
    }
    else
    {
        puts ("You typed:");
        puts (my_string);
    }

    return 0;
 }

However, the problem is that the compiler keeps returning errors of this: undefined reference to 'getline'. Could you please tell me what the problem is? Thank you!

I am using Win7 64bit + Eclipse Indigo + MinGW

This is not a good C tutorial, or rather not a C tutorial at all. I would not recommend using it as your only or main source of information about C. — n. m. could be an AI, May 01 '23 at 09:47

score 21 · Answer 1 · edited Oct 28 '12 at 23:00

The other answers have covered most of this, but there are several problems. First, getline() is not in the C standard library, but is a POSIX 2008 extension. Normally, it will be available with a POSIX-compatible compiler, as the macros _POSIX_C_SOURCE will be defined with the appropriate values. You possibly have an older compiler from before getline() was standardized, in which case this is a GNU extension, and you must #define _GNU_SOURCE before #include <stdio.h> to enable it, and must be using a GNU-compatible compiler, such as gcc.

Additionally, nbytes should have type size_t, not int. On my system, at least, these are of different size, with size_t being longer, and using an int* instead of a size_t* can have grave consequences (and also doesn't compile with default gcc settings). See the getline manual page (http://linux.die.net/man/3/getline) for details.

With that change made, your program compiles and runs fine on my system.

@VimNing Nope. still not in the standard lib, but at least they're on the TR block now. Brick by brick... (and a whole lot of sitting on asses doing nada). — WhozCraig, Jun 28 '22 at 21:59

score 9 · Answer 2 · edited Apr 19 '16 at 17:05

9

I am also using MinGW. I checked MinGW headers and getline() does not appear in any C header, it appears only in C++ headers. This means the C function getline() does not exist in MinGW.

edited Apr 19 '16 at 17:05

answered Mar 17 '16 at 16:39

jdarthenay

3,062
1
15
20

score 8 · Answer 3 · answered Oct 28 '12 at 20:43

8

getline isn't a standard function, you need to set a feature test macro to use it, according to my man page,

_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700

for glibc 2.10 or later,

_GNU_SOURCE

before that.

answered Oct 28 '12 at 20:43

Daniel Fischer

181,706
17
308
431

score 2 · Answer 4 · edited Aug 02 '23 at 02:05

getline() is not in the C standard library. However, you may still use it with compiler flag -std=gnu99. This flag, telling the compiler to use the GNU dialect of ISO C99, is supported by both gcc and clang.

gcc main.c -std=gnu99

See gcc flags.

The manual (obtained by man getline in terminal) also says that

       getline(), getdelim():
           Since glibc 2.10:
               _POSIX_C_SOURCE >= 200809L
           Before glibc 2.10:
               _GNU_SOURCE

So you may add #define _POSIX_C_SOURCE 200809L or #define _GUN_SOURCE to your file to expose the declaration.

If you fancy, here is a basic implementation of getline()

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>

ssize_t getline(char **restrict buffer, size_t *restrict size,
                FILE *restrict fp) {
    register int c;
    register char *cs = NULL;

    if (cs == NULL) {
        register int length = 0;
        while ((c = getc(fp)) != EOF) {
            cs = (char *)realloc(cs, ++length+1);
            if ((*(cs + length - 1) = c) == '\n') {
                *(cs + length) = '\0';
                *buffer = cs;
                break;
            }
        }
        return (ssize_t)(*size = length);
    } else {
        while (--(*size) > 0 && (c = getc(fp)) != EOF) {
            if ((*cs++ = c) == '\n')
                break;
        }
        *cs = '\0';
    }
    return (ssize_t)(*size=strlen(*buffer));
}

undefined reference to `getline' in c

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