When compiling with -Wall -Wextra under Debian Buster, I get the following error message:

keys.c: In function ‘disable_old_keys’:
keys.c:26:12: warning: comparison of integer expressions of different signedness: ‘int’ and ‘long unsigned int’ [-Wsign-compare]
  if(result < sizeof(tmpfile)) {

snprintf returns a negative value on output error. In the current implementation, there is no error handling for negative return
values. Therefore in this context, it is acceptable to convert int to size_t.