为什么这两个程序在 VC++2008 中给出不同的输出？

Question

为什么这两个程序在 VC++2008 中给出不同的输出？

毕竟，比较的是相同的字符串。

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 保证结果将包括：

零值表示两个字符串相等。
大于零的值表示第一个不匹配的字符在 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，并且可以是任何值。唯一重要的部分是两者都返回 0、小于零或大于零。在这方面，他们给出了相同的答案。

Answer 2

来自 strncmp：

返回一个整数值，指示字符串之间的关系：
零值表示两个字符串中比较的字符全部相等。
大于零的值表示第一个不匹配的字符在 str1 中的值大于在 str2 中的值；小于零的值表示相反。

显然，对于不相等的字符，strcmp 始终返回 1 或 -1，而 strncmp 返回不相等字符之间的差。由于该行为未定义，因此不是问题。