Question

I am new to C and have hit a problem with an example form K&R C ( section 1.9) not working properly. Here is the code I have copied from the example, and gone over once looking for discrepancies:

#include <stdio.h>
#define MAXLINE 1000

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

// print longest input line

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

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

// getline: read a line into s, return length
int mygetline(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;
}

// copy: copy 'from' onto 'to'; assume to is big enough
void copy(char to[], char from[]) {
    int i;

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

When I compile I get the following:

cc -Wall -g test.c -o test
test.c:9:1: warning: return type defaults to ‘int’ [-Wreturn-type]
test.c: In function ‘copy’:
test.c:45:30: warning: comparison between pointer and integer [enabled by default]
test.c:45:30: warning: comparison with string literal results in unspecified behavior [-Waddress]

When I run the program, this happens:

j

on@jon-G31M-ES2L:~/c$ ./test
Hello, does this work?
Segmentation fault (core dumped)

I am using gcc as my compiler.

Answer 1

The problem lies in the below line:

while ((to[i] = from[i]) != "\0") 

It should be

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

Now, as to why the first one was wrong. “\0” is actually a string literal which is stored in some memory (let’s say the address is 0x7d5678 just for example), whereas the left side of != is an integer. The compiler automatically knows that such a comparison is wrong and can never pass, so it gives you the warning

warning: comparison between pointer and integer [enabled by default]

Also, when you have a string literal in your code, you have no control as to, at what address will it be stored, so the compiler again knows that any comparison with a string literal is wrong hence flags the warning

warning: comparison between pointer and integer [enabled by default]

Also, as an additional note, always remember that strings in C can be compared & manipulated only using the strcmp or strncmp group of string functions. You can never do an “=” comparison on a string.
Hope this helps in understanding.