how to use strxfrm in C language?

Question

I ask a question to know the usage of "strxfrm" in C.

I know the function is to transform a string according to current locale configuration.

but I don't know what "transform" is, and how this function transforms.

For example, I tried a code like below in macOS:

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

int main(int argc, char * argv[])
{
    char str1[512] = { 0x68, 0x6c, 0x61, 0x76, 0x61, 0x00 }; //"hlava";
    char str2[512] = { 0xc4, 0x8d, 0xc3, 0xad, 0xc5, 0xa1, 0x6e, 0xc3, 0xad, 0x6b, 0x00 }; //"číšník";
    char xfm1[512] = { '\0', };
    char xfm2[512] = { '\0', };
    char * result = NULL;
    size_t lxfm1 = 0;
    size_t lxfm2 = 0;

    result = setlocale(LC_ALL, "en_US.UTF-8");
    lxfm1 = strxfrm(xfm1, str1, sizeof xfm1);
    lxfm2 = strxfrm(xfm2, str2, sizeof xfm2);
    printf("<en-US>\n");
    printf("setlocale = \"%s\"\n", (result == NULL) ? "NULL" : result);
    printf("str1: \"%s\" --> \"%s\"\n", str1, xfm1);
    printf("str2: \"%s\" --> \"%s\"\n", str2, xfm2);
    printf("strcmp(str1, str2) = %d\n", strcmp(str1, str2));
    printf("strcmp(xfm1, xfm2) = %d\n", strcmp(xfm1, xfm2));
    printf("strcoll(xfm1, xfm2) = %d\n", strcoll(str1, str2));
    printf("returns of strxfrm: %zu / %zu\n", lxfm1, lxfm2);

    result = setlocale(LC_ALL, "cs_CZ.UTF-8");
    lxfm1 = strxfrm(xfm1, str1, sizeof xfm1);
    lxfm2 = strxfrm(xfm2, str2, sizeof xfm2);
    printf("<cs-CZ>\n");
    printf("setlocale = \"%s\"\n", result);
    printf("str1: \"%s\" --> \"%s\"\n", str1, xfm1);
    printf("str2: \"%s\" --> \"%s\"\n", str2, xfm2);
    printf("strcmp(str1, str2) = %d\n", strcmp(str1, str2));
    printf("strcmp(xfm1, xfm2) = %d\n", strcmp(xfm1, xfm2));
    printf("strcoll(xfm1, xfm2) = %d\n", strcoll(str1, str2));
    printf("returns of strxfrm: %zu / %zu\n", lxfm1, lxfm2);

    return 0;
}

I expected that the result of "strcmp(xfm1, xfm2)" would be positive integer because the character 'č' precedes 'h' in czech language.

However, the result is...

<en-US>
setlocale = "en_US.UTF-8"
str1: "hlava" --> "001Z001^001S001h001S0000001Z001^001S001h001S"
str2: "číšník" --> "0042003_0042001`003_001]0000008?003_009S001`003_001]"
strcmp(str1, str2) = -92
strcmp(xfm1, xfm2) = -3
strcoll(xfm1, xfm2) = -152
returns of strxfrm: 44 / 52
<cs-CZ>
setlocale = "cs_CZ.UTF-8"
str1: "hlava" --> "001Z001^001S001h001S0000001Z001^001S001h001S"
str2: "číšník" --> "0042003_0042001`003_001]0000008?003_009S001`003_001]"
strcmp(str1, str2) = -92
strcmp(xfm1, xfm2) = -3
strcoll(xfm1, xfm2) = -152
returns of strxfrm: 44 / 52

Am I misunderstanding about this function 'strxfrm'? Actually, I don't know the meaning of 'transform' clearly even now.

please let me know the right usage and purpose of the function.

Answer 1

Your usage of strxfrm is correct. The problem lies in the Mac OS X (and FreeBSD) locales implementation. It simply doesn't work properly with UTF-8. It's apparently a long standing bug/defect/inconsistency/quirk/whatever in the version of libc these operating systems use.