为什么这两个程序在VC ++ 2008中提供不同的输出？

Question

为什么这两个程序在VC ++ 2008中提供不同的输出？

毕竟，比较相同的字符串。

strcmp__usage.c

#include <stdio.h>
#include <string.h>

main() 
{
char targetString[] = "klmnop";

printf ("Compare = %d\n", strcmp(targetString, "abcdef"));
printf ("Compare = %d\n", strcmp(targetString, "abcdefgh"));
printf ("Compare = %d\n", strcmp(targetString, "jlmnop"));
printf ("Compare = %d\n", strcmp(targetString, "klmnop"));
printf ("Compare = %d\n", strcmp(targetString, "klmnoq"));
printf ("Compare = %d\n", strcmp(targetString, "uvwxyz"));
printf ("Compare = %d\n", strcmp(targetString, "xyz"));
}

输出

Compare = 1
Compare = 1
Compare = 1
Compare = 0
Compare = -1
Compare = -1
Compare = -1

strncmp_usage.c

#include <stdio.h>
#include <string.h>

main() 
{   
    char targetString[] = "klmnopqrstuvwxyz";   
    int n = 6;

    printf ("Compare = %d\n", strncmp(targetString, "abcdef", n));
    printf ("Compare = %d\n", strncmp(targetString, "abcdefgh", n));
    printf ("Compare = %d\n", strncmp(targetString, "jlmnop", n));
    printf ("Compare = %d\n", strncmp(targetString, "klmnop", n));
    printf ("Compare = %d\n", strncmp(targetString, "klmnoq", n));
    printf ("Compare = %d\n", strncmp(targetString, "uvwxyz", n));
    printf ("Compare = %d\n", strncmp(targetString, "xyz", n));
}

输出

Compare = 10
Compare = 10
Compare = 1
Compare = 0
Compare = -1
Compare = -10
Compare = -13

Answer 1

strcmp和strncmp都可以保证结果包括：

  

零值表示两个字符串相等。   大于零的值表示不匹配的第一个字符在str1中的值大于在str2中的值;小于零的值表示相反。

返回的实际数字（1 / -1或12 / -13）是特定于实现的，可以是任何值。唯一重要的部分是返回0，小于零或大于零。在这方面，他们提供了相同的答案。

Answer 2

来自strncmp：

返回一个表示字符串之间关系的整数值： 零值表示两个字符串中比较的字符都相等。 大于零的值表示不匹配的第一个字符在str1中的值大于在str2中的值;小于零的值表示相反。

显然strcmp总是为非等号字符返回1或-1，而strncmp返回不等字符之间的差异。由于该行为未定义，因此不存在问题。