Clang warns about this:

opensc-explorer.c:309:33: warning: format specifies type 'unsigned int *' but the argument has type 'u8 *' (aka 'unsigned char *') [-Wformat]
    if (sscanf(arg, "%02X%02X", &fid[0], &fid[1]) != 2) {
                     ~~~~       ^~~~~~~
                     %2s
opensc-explorer.c:309:42: warning: format specifies type 'unsigned int *' but the argument has type 'u8 *' (aka 'unsigned char *') [-Wformat]
    if (sscanf(arg, "%02X%02X", &fid[0], &fid[1]) != 2) {
                         ~~~~            ^~~~~~~
                         %2s

This writes useful values to fid[0] and fid[1] on little-endian architectures, and zero on big-endian architectures.