Question

我新到C, 遇到一个问题, 例如 K& R C (第1. 9节) 表格不正确。以下是我从示例中复制的代码, 有一次我翻过一次, 寻找差异 :

#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!= 
 ; ++i)
        s[i] = c;
    if (c ==  
 ) {
        s[i] = c;
        ++i;
    }
    s[i] =   ;
    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]) != "")
        ++i;
}

当我编集时,我得到以下资料:

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]

当我管理这个程序时,这种情况会发生:

j) j j

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

我用Gcc做我的编译器

Answer 1

问题在于以下几行:

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

应该是

while ((to[i] = from[i]) !=   )

至于为什么第一个错误。 “\\\\\\\\\ ” 实际上是一个字符串的字串, 存储在某种内存中( 请举个例子) 。而! = 的左侧是一个整数。编译器自动知道这样的比较是错误的, 并且永远无法通过, 所以它会警告您。

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

另外,当您在代码中有一个字符字符串字典时, 您无法控制它存储的地址, 因此编译器再次知道与字符字符串字典的任何比较是错误的, 因此您可以挂出警告。

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.

Answer 2

在复制函数中将 "\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\需要字符字符字符字符。"\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

Answer 3

您的 copy 函数有一个错误, \\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\\

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