STRNLEN(3) Linux Programmer's Manual STRNLEN(3)
strnlen - determine the length of a fixed-size string
size_t strnlen(const char *s, size_t maxlen);
The strnlen function returns the number of characters in the string
pointed to by s, not including the terminating '\0' character, but at
most maxlen. In doing this, strnlen looks only at the first maxlen
characters at s and never beyond s+maxlen.
The strnlen function returns strlen(s), if that is less than maxlen, or
maxlen if there is no '\0' character among the first maxlen characters
pointed to by s.
This function is a GNU extension.
GNU 1999-07-25 STRNLEN(3)