C Conflicting Data Types in int function between char[] and int

Question

As I am going through a sample code in the book (program that reads a set of text lines and prints the longest):

#include <stdio.h>
#define MAXLINE 1000

int getline(char line[], int maxline);
void copy(char to[], char from[]);

main()
{
        int len;
        int max;
        char line[MAXLINE];
        char longest[MAXLINE];

        max = 0;
        while ((len = getline(line, MAXLINE)) > 0)
                if (len > max){
                        max = len;
                        copy(longest, line);
                }
        if (max > 0)
                printf("%s", longest);
        return 0;
}

int getline(char s[], int lim)
{
        int c, i;

        for (i=0; i<lim-1 && (c=getchar()) !=EOF && c != '\n'; ++i)
                s[i] = c;
        if (c == '\n') {
                s[i] = c;
                ++i;
        }
        s[i] = '\0';
        return i;
}

void copy(char to[], char from[])
{
        int i;

        i = 0;
        while ((to[i] = from[i]) != '\0')
                ++i;
}

gcc spits out

longestline.c:4:5: error: conflicting types for ‘getline’
    4 | int getline(char line[], int maxline);
      |     ^~~~~~~
In file included from longestline.c:1:
/usr/include/stdio.h:616:18: note: previous declaration of ‘getline’ was here
  616 | extern __ssize_t getline (char **__restrict __lineptr,
      |                  ^~~~~~~
longestline.c:7:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
    7 | main()
      | ^~~~
longestline.c:25:5: error: conflicting types for ‘getline’
   25 | int getline(char s[], int lim)
      |     ^~~~~~~
In file included from longestline.c:1:
/usr/include/stdio.h:616:18: note: previous declaration of ‘getline’ was here
  616 | extern __ssize_t getline (char **__restrict __lineptr,
      |                  ^~~~~~~

that is the exact sample from the book, so what can be done here? I think this might be due to this book being quite outdated, what is the solution?

Answer 1

The problem is that there is a POSIX function called getline . Your compiler is defaulting to POSIX compatibility instead of ISO C compatibility. To work around it you can either:

• Call your function something else, or
• Invoke the compiler in ISO C mode.

For gcc use a switch such as -std=c11 ; or it is also possible to control dialect on a per-file basis with feature test macros .

Answer 2

It turns out getline is defined in stdio.h, simply renamed the function:

int gettline(char line[], int maxline);


int getline(char s[], int lim)

.

#include <stdio.h>
#define MAXLINE 1000

int gettline(char line[], int maxline);
void copy(char to[], char from[]);

main()
{
        int len;
        int max;
        char line[MAXLINE];
        char longest[MAXLINE];

        max = 0;
        while ((len = gettline(line, MAXLINE)) > 0)
                if (len > max){
                        max = len;
                        copy(longest, line);
                }
        if (max > 0)
                printf("%s", longest);
        return 0;
}

int gettline(char s[], int lim)
{
        int c, i;

        for (i=0; i<lim-1 && (c=getchar()) !=EOF && c != '\n'; ++i)
                s[i] = c;
        if (c == '\n') {
                s[i] = c;
                ++i;
        }
        s[i] = '\0';
        return i;
}

void copy(char to[], char from[])
{
        int i;

        i = 0;
        while ((to[i] = from[i]) != '\0')
                ++i;
}