Question

Why do these two programs give different outputs in VC++2008?

After all, the same strings are compared.

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"));
}

Output

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));
}

Output

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

Answer 1

Both strcmp and strncmp provide the guarantee that the result will include:

A zero value indicates that both strings are equal.
A value greater than zero indicates that the first character that does not match has a greater value in str1 than in str2; And a value less than zero indicates the opposite.

The actual number returned (1/-1 or 12/-13) is implementation specific, and can be any value. The only portion that matters is that both return 0, less than zero, or greater than zero. In that respect, they provide the same answer.