Error implicit declaration of function getline

Hi I’m almost done with a project I have for class. I need to sort the priority of people in an airline based on a few factors. This is my project description:

«An airline company uses the formula shown below to determine the priority of passengers on the
waiting list for overbooked flights.
Priority Number = (A / 1000) + B — C
where
A is the customer’s total mileage in the past year
B is the number of years in his or her frequent flier program
C is a sequence number representing the customer’s arrival position when he or she booked the
flight.
Given a file of overbooked customers as shown in the table below, write a program that reads the
file and determines each customer’s priority number. The program then builds a priority queue
using the priority number and prints a list of waiting customers in priority sequence.»

I feel my code encompasses the main idea, but I am having issues with getline. When I compile the code (shown below) it gives me an error:
«|24|warning: implicit declaration of function ‘getline’ [-Wimplicit-function-declaration]|»

Please help me fix this so it will compile. I’ve tried a lot of things and nothing works.

Here is the code:

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

int main()
{
    FILE * fp; //create file pointer to be read later
    char * line = NULL;
    int temp,count=0,len=0,mileage[100],read,years[100],sequence[100],priority[100];
    char name[100][100],tempst[100];
    fp = fopen("customers.txt", "r"); //opening the file

    if (fp == NULL)
        exit(EXIT_FAILURE);
    int i;



    while ((read = getline(&line, &len, fp)) != -1) { //start reading file and recording data
        i=0;

        while(line[i]!=' ' || (line[i+1]>='A' && line[i+1]<='z'))
        {
            name[count][i]=line[i];
            i++;
        }
        name[count][i++]='';
        mileage[count]=0;

        while(line[i]!=' ')
        {
            mileage[count]=mileage[count]*10+(line[i]-'0');
            i++;
        }
        i++;
        years[count]=0;

        while(line[i]!=' ')
        {
            years[count]=years[count]*10+(line[i]-'0');
            i++;
        }
        i++;
        sequence[count]=0;

        while(line[i]!='')
        {
            sequence[count]=sequence[count]*10+(line[i]-'0');
            i++;
        }
        priority[count]=(mileage[count]/1000)+years[count]-sequence[count];
        count++;

    }

    for( i=0;i<count-1;i++) // calculate priority
    {
        for(int j=i+1;j<count;j++)
        {
            if(priority[i]<priority[j])
            {
                temp=priority[i];
                priority[i]=priority[j];
                priority[j]=temp;
                strcpy(tempst,name[i]);
                strcpy(name[i],name[j]);
                strcpy(name[j],tempst);
            }
        }
    }

    for( i=0;i<count;i++) //print priority
    {
        printf("%s %dn",name[i],priority[i]);
    }
    return 0;
}

1. 01-26-2006

   
   

   #1

   

   
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   Implicit declaration of function …???
   

   Hi,
   I keep getting the warning of:

   Code:

    Implicit declaration of funciton getline()

   whenever i use getline() function

   What does this warning mean actually ?
   Is this a serious warning, or just ignorable.

   Cheer!!

   ---

2. 01-26-2006

   
   

   #2

   

   
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   getline() is c++ not C. There is no standard C getline() function.

   ---

3. 01-26-2006

   
   

   #3

   

   
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   getline() is also a GNU provided extension available under #include <stdio.h>. It will not appear if you use the -ansi flag, because the -ansi flag excludes all GNU extensions. It’s not part of standard C, as Ancient Dragon said.

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4. 01-26-2006

   
   

   #4

   

   
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   replace getline() with fgets() and your program will (maybe) work.

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5. 01-27-2006

   
   

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   What’s happening is that you have tried to use a function called getline() that hasn’t been declared anywhere (eg in a header file) and the compiler takes your attempt to call it as an implicit declaration.

   As other have sort of said, your real problem is probably that you’re calling a function that doesn’t exist and you need to work out what function you really intend to call. If the function is one you actually know you’re trying to call, find the corresponding header file and #include it.

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6. 01-28-2006

   
   

   #6

   

   
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   Code:

   $ grep -n getline /usr/include/*.h

   

   If you’re trying to use the C++ getline, then you need <iosteam> etc.

   dwk

   Seek and ye shall find. quaere et invenies.

   «Simplicity does not precede complexity, but follows it.» — Alan Perlis
   «Testing can only prove the presence of bugs, not their absence.» — Edsger Dijkstra
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7. 08-07-2006

   
   

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   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h. As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension. In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).
   Hope this clarifies the issue.
   — LT

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8. 08-07-2006

   
   

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   Originally Posted by Lord Thunder

   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   Depends on your perspective. I consider your post to be incorrect and misleading, as you make some statements early on which are compiler/library specific but you state them as generalities. Yes you correct them later, sort of, but the information you give is not generally true.

   Originally Posted by Lord Thunder

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h.

   No. The functions that are declared in <stdio.h> are specified in the ISO C standard. They are not required to be defined in stdio.h.

   Reliance on stdio.h declaring any function that is not specified in the C standard is invalid, unless you are prepared to constrain yourself to only one compiler and it’s library (and possibly to a particular version of that compiler and library).

   Originally Posted by Lord Thunder

   As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension.

   Exactly my point. There are a few commonly used compilers that are produced by vendors not associated with GNU. Most of those compilers do not support GNU extensions and are not bunded with glibc.

   Originally Posted by Lord Thunder

   In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).

   Sort of. glibc, in practical terms, often relies on using a correct version of gcc (the GNU compiler collection). Several versions of glibc only work with particular versions of gcc. This is the reason that the gnu compiler and glibc are often bundled together, particularly with recent versions. Both the gnu compiler and library are supported on more systems than linux; cygwin is just one of several windows ports.

   ---

9. 08-07-2006

   
   

   #9

   

   
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   Originally Posted by Lord Thunder

   Sorry if I am a topic digging here, but I came across this thread while searching for the answer for exactly this problem and I noticed noone here gave the right answer.

   The answer turned out pretty simple:
   getline() IS a valid C function which is defined in stdio.h. As some people rightfully suggested it is not a function that is defined in the ANSI standard, but it is a GNU extension. In order to correctly include these extensions you have to add a define:

   Code:

   #define _GNU_SOURCE
   #include <stdio.h>
   
   /* functions like getline() and getdelim() should be defined now */

   Ofcourse this only works if you are actually using glibc (e.g. on Linux or cygwin).
   Hope this clarifies the issue.
   — LT

   Why is it exactly that you are dredging up a 7 month old thread? Was the urge so strong to blabber that you could not resist? Give me a break.

   Forum Guidelines. Read before posting:

   5. Don’t bump threads. (Bumping: posting messages on threads to move them up the list or to post on a thread that has been inactive for two weeks or longer).

   Any questions?

   ---

TL;DR: POSIX says getline() should be on my system, but when I use -std=c89 on GNU/Linux I get «implicit declaration» and when I do the same on OpenBSD 6.5, it works with without any error or warning. Why?

Here is a tiny program using getline() and ssize_t:

#include <stdio.h>

int main() {
        char *s = NULL;
        size_t len = 0;
        ssize_t n;
        n = getline(&s, &len, stdin);
        printf("bytes read: %dn", (int) n);
        printf("%s", s);
        return 0;
}

(sure, it is flawed, but that is not the point)

Now, if I compile it on my OpenBSD 6.5 with both gcc (4.2.1 20070719) and clang (7.0.1) with various standards, it always works.

$ clang -Wall -Wextra -pedantic -std=c89 -o gl gl.c
$ clang -Wall -Wextra -pedantic -std=c99 -o gl gl.c
$ gcc -Wall -Wextra -pedantic -std=c89 -o gl gl.c
$ gcc -Wall -Wextra -pedantic -std=c99 -o gl gl.c

(no errors or warnings)

Now, if I try the same on GNU/Linux (Archlinux).

With GCC 8.3.0, I get this (both c89 and c99):

$ gcc -Wall -Wextra -pedantic -std=c89 -o gl gl.c
gl.c: In function ‘main’:
gl.c:6:9: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
         ssize_t n;
         ^~~~~~~
         size_t
gl.c:7:13: warning: implicit declaration of function ‘getline’ [-Wimplicit-function-declaration]
         n = getline(&s, &len, stdin);

and on clang (8.0.0) I get this

$ clang -Wall -Wextra -pedantic -std=c99 -o gl gl.c
gl.c:6:9: error: unknown type name 'ssize_t'; did you mean 'size_t'?
        ssize_t n;
        ^~~~~~~
        size_t
/usr/lib/clang/8.0.0/include/stddef.h:62:23: note: 'size_t' declared here
typedef __SIZE_TYPE__ size_t;
                      ^
gl.c:7:13: warning: implicit declaration of function 'getline' is invalid in C99
      [-Wimplicit-function-declaration]
        n = getline(&s, &len, stdin);
            ^
1 warning and 1 error generated.
$ gcc -Wall -Wextra -pedantic -std=c89 -o gl gl.c
gl.c: In function ‘main’:
gl.c:6:9: error: unknown type name ‘ssize_t’; did you mean ‘size_t’?
         ssize_t n;
         ^~~~~~~
         size_t
gl.c:7:13: warning: implicit declaration of function ‘getline’ [-Wimplicit-function-declaration]
         n = getline(&s, &len, stdin);
             ^~~~~~~

If I use -std=gnu89, both clang and GCC produce no errors or warnings.

Now, the Linux man pages (5.01, http://man7.org/linux/man-pages/index.html) say about getline()

$ man getline | grep -A5 ^CONFORMING
CONFORMING TO
       Both getline() and getdelim() were  originally  GNU  extensions.   They
       were standardized in POSIX.1-2008.

EXAMPLE
       #define _GNU_SOURCE
$

The OpenBSD 6.5 manpage says

$ man getline | grep -A1 ^STANDARDS
STANDARDS
       The getdelim() and getline() functions conform to IEEE Std 1003.1-2008 (“POSIX.1”). 

And when it comes to ssize_t, the POSIX.1 [0] also say the sys/types.h shall define it. If I explicitly include the header, it does indeed resolve the ssize_t problem.

$ ed gl.c
228
1a
#include <sys/types.h>
.
wq
251
$ gcc -Wall -Wextra -pedantic -std=c89 -o gl gl.c
gl.c: In function ‘main’:
gl.c:8:13: warning: implicit declaration of function ‘getline’ [-Wimplicit-function-declaration]
         n = getline(&s, &len, stdin);
             ^~~~~~~
$

When POSIX says there has to be a function (getline()) and my system is (or partially is) compliant to POSIX, I would expect the function to be available, regardless of what C standard I use. But that seems to be different on GNU/Linux and OpenBSD. So which system does do it right? Should the range of symbols available be restricted to the C standard used or it should be enriched with what POSIX says should be available?

Also, why is the error saying the declaration is implicit? I included the stdio.h, just as the (Linux) man page says.

Thank you for all the answers!

[0] IEEE Std 1003.1-2008 (“POSIX.1”): http://pubs.opengroup.org/onlinepubs/9699919799/

make[4]: Entering directory `/boot/home/src/vlc/src' 
 CC config/libvlccore_la-dirs_xdg.lo | config/dirs_xdg.c: In function 'config_GetTypeDir': config/dirs_xdg.c:141: |
error: implicit declaration of function 'getline'

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

int main(void)
{
    char *line = NULL;
    size_t linesize = 0;
    ssize_t linelen;

    while ((linelen = getline(&line, &linesize, stdin)) !=-1)
    fwrite(line, 1, linelen, stdout);

    free(line);

    if (ferror(stdin))
    {
       perror("Standard input");
       return 1;
     }
      return 0;
}

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