Après m'être battu un moment avec iconv, voici un petit exemple de conversion ISO-8859-1 vers UTF-8 :

#include ‹iconv.h›

int translate_text(char** text, char free_text)
{
    char* tmp_in, *tmp_out, *original_out;
    size_t mult_size, size_in, size_out;

    if (!text || !*text) return 1;

    converter = iconv_open ("UTF-8", "ISO-8859-1");

    if (converter <= 0)
        return 1;

    size_in = strlen(*text);
    mult_size = size_out = size_in*4; // Because UTF8 is bigger thant ISO-XXX
    tmp_in = *text;
    original_out = tmp_out = malloc(size_out);

    iconv(converter, &tmp_in, &size_in, &tmp_out, &size_out); // All variables are modified by iconv

    original_out[mult_size-size_out] = 0;

    if (free_text) free(*text);

    *text = original_out;

    iconv_close(converter);

    return 0;
}