iconv library improperly converts UTF-8 to KOI8-R

Question

I am trying to use the GNU iconv library to convert a UTF-8 encoded string to KOI8-R. My minimal example is

#include <iconv.h>
#include <stdio.h>
#include <stdlib.h>

int main() {
    /* The letter П in UTF-8. */
    char* buffer = "\xd0\x9f";
    size_t len = 2;
    /* Note: since KOI8-R is an 8-bit encoding, the buffer should only need a length of 1, but
     * iconv returns -1 if the buffer is any smaller than 4 bytes,
     */
    size_t len_in_koi = 4;
    char* buffer_in_koi = malloc(len_in_koi+1);
    /* A throwaway copy to give to iconv. */
    char* buffer_in_koi_copy = buffer_in_koi;
    iconv_t cd = iconv_open("UTF-8", "KOI8-R");
    if (cd == (iconv_t) -1) {
        fputs("Error while initializing iconv_t handle.\n", stderr);
        return 2;
    }
    if (iconv(cd, &buffer, &len, &buffer_in_koi_copy, &len_in_koi) != (size_t) -1) {
        /* Expecting f0 but get d0. */
        printf("Conversion successful! The byte is %x.\n", (unsigned char)(*buffer_in_koi));
    } else {
        fputs("Error while converting buffer to KOI8-R.\n", stderr);
        return 3;
    }
    iconv_close(cd);
    free(buffer_in_koi);
    return 0;
}

which (besides not working when my KOI8-R buffer is smaller than four bytes, although it should only need a single byte) incorrectly prints d0 (the correct encoding of 'П' in KOI8-R is f0).

iconv gives me the correct answer from the command line (e.g., echo П | iconv -t KOI8-R | hexdump), so what am I doing wrong in using its C interface?

Answer 1

You mixed up the "to" and "from" character set arguments to iconv_open. It just so happens that the character in slot D0 in KOI8-R has D0 as the first byte of its UTF-8 encoding.