string_copy
end = string_copy(outfile, file, sizeof(outfile));
string_copy(end, GZ_SUFFIX, sizeof(outfile) - (z_size_t)(end - outfile));
string_copy(buf, file, sizeof(buf));
string_copy(buf + len, GZ_SUFFIX, sizeof(buf) - len);
string_copy(outmode, "wb6 ", sizeof(outmode));