冲突类型

Question

I'm working through K&R C and GCC continues to give me this error for example 1.9:

arrays.c:4:5: error: conflicting types for ‘getline’
/usr/include/stdio.h:675:20: note: previous declaration of ‘getline’ was here
arrays.c:27:5: error: conflicting types for ‘getline’
/usr/include/stdio.h:675:20: note: previous declaration of ‘getline’ was here
make: *** [arrays] Error 1

My code is:

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

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

/* print 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;
}

/* getline: read a line into 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';
    return i;
}

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

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

I realize that the error suggests some discrepancy between the 'getline' function prototype and 'getline' function definition. I've copied and pasted the same code from another person's question from here to check if I had made a typo. It returned the same error message. I don't know if I'm getting this error because K&R's code has become outdated or if it's something to do with how GCC compiles the code. Please help me find what I am doing wrong.

Answer 1

Rename your getline function to something else. You're having a naming error with the getline function as defined in stdio.h . Note that the error says: " /usr/include/stdio.h :675:20: note: previous declaration of 'getline' was here"

getline is defined in stdio.h with the following signature:

ssize_t getline(char **lineptr, size_t *n, FILE *stream);

Because C has no namespacing, the declarations in stdio.h are copied verbatim during compilation, thus causing a type mismatch.

I'm not sure of the history, but most likely getline was not part of the standard library at the time K&R was written (in fact, as pointed out by @NigelHarper, it still isn't part of the C standard; it's part of POSIX).