I am trying to reimplement the strcasecmp function in C and I noticed what appears to be an inconsistency in the comparison process.
From man strcmp
The strcmp() function compares the two strings s1 and s2. The locale is not taken into account (for a locale-aware comparison, see strcoll(3)). It returns an integer less than, equal to, or greater than zero if s1 is found, respectively, to be less than, to match, or be greater than s2.
From man strcasecmp
The strcasecmp() function performs a byte-by-byte comparison of the strings s1 and s2, ignoring the case of the characters. It returns an integer less than, equal to, or greater than zero if s1 is found, respectively, to be less than, to match, or be greater than s2.
int strcmp(const char *s1, const char *s2);
int strcasecmp(const char *s1, const char *s2);
Given, this information, I don't understand the result of the following code:
#include <stdio.h>
#include <string.h>
int main()
{
// ASCII values
// 'A' = 65
// '_' = 95
// 'a' = 97
printf("%i\n", strcmp("A", "_"));
printf("%i\n", strcmp("a", "_"));
printf("%i\n", strcasecmp("A", "_"));
printf("%i\n", strcasecmp("a", "_"));
return 0;
}
Ouput:
-1 # "A" is less than "_"
1 # "a" is more than "_"
2 # "A" is more than "_" with strcasecmp ???
2 # "a" is more than "_" with strcasecmp
It appears that, if the current character in s1 is a letter, it is always converted to lowercase, regardless of whether the current character in s2 is a letter or not.
Can someone explain this behaviour? Shouldn't the first and third lines be identical?
Thank you in advance!
PS:
I am using gcc 9.2.0 on Manjaro.
Also, when I compile with the -fno-builtin flag I get instead:
-30
2
2
2
I guess it's because the program does not use gcc's optimised functions, but the question remains.
