previous declaration of 'function' was here in C [duplicate]

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Learning C by K&R, error when trying to compile programs from book with arrays and function calls

While learning The C Programming Language by Brian W. Kernighan and Dennis M. Ritchie, I tried the example in section 1.9 Character Arrays. Here are the codes:

/* read a set of text lines and print the longest */

#include <stdio.h>
#define MAXLINE 1000 /* maximum input line length */

/* declare functions: getline() and copy() */
int getline(char line[], int maxline); 
void copy(char to[], char from[]);

/* getline: read a line into array "s", return length */ 
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';  /* the null character whose value is 0 */
    return i;
}

/* copy: copy 'from' into 'to'; assume to is big enough */ 
/* the return type of copy is "void" -- no value is returned */
void copy(char to[], char from[])
{
    int i;
    i = 0;
    while ((to[i] = from[i]) != '\0')  /* terminated with a \0 */
        ++i; 
}

/* print the longest input line */
int main()
{
    int len;  /* current line length */ 
    int max;  /* maximum length seen so far */
    char line[MAXLINE];  /* current input line */
    char longest[MAXLINE];  /* longest line saved here */

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

There are two main errors:

1. error: conflicting types for ‘getline’
2. error: previous declaration of ‘getline’ was here

The complete error list is here:

/Users/C/Codes/Ritchie/array_char.c:8: error: conflicting types for ‘getline’
/usr/include/stdio.h:449: error: previous declaration of ‘getline’ was here
/Users/C/Codes/Ritchie/array_char.c:13: error: conflicting types for ‘getline’
/usr/include/stdio.h:449: error: previous declaration of ‘getline’ was here
/Users/C/Codes/Ritchie/array_char.c: In function ‘getline’:
/Users//C/Codes/Ritchie/array_char.c:17: warning: comparison between pointer and integer
/Users/C/Codes/Ritchie/array_char.c:17: warning: comparison with string literal results in unspecified behavior

I am not sure what went wrong, since it is exactly the same code from the book. Maybe the declaration of functions at the beginning:

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

is problematic? Thank you!

Answer 1

http://www.kernel.org/doc/man-pages/online/pages/man3/getline.3.html

getline already exists in stdio.h. That is why you are getting the error. Change the function name to something else like getline_my.

Also, you are comparing a character with a string in line 16. It should be
if(c == '\n')

NOT

if(c == "\n")

Answer 2

The problem is that there is likely a definition of getline in stdio.h. On my version of linux there is a getline function supplied by the C library (part of the POSIX standard I think). You can't have two functions with the same name in C, and this is your problem. Try renaming your version of getline to my_getline (both where you declare/define it and where you use it).

Answer 3

/usr/include/stdio.h:449: error: previous declaration of ‘getline’ was here

It's exactly as it says: getline is declared in stdio.h (because the standard library provides a function with that name). You cannot provide your own function with that name, because when you call getline, the compiler wouldn't know which one to use.

Answer 4

From the date when that book was written up to nowadays the Standard C library modified a little, and they are not consistent any more the old and the new one.

You must remove the declarations, and leave the declarations from the current stdio.h.