#include <string.h> size_t strnlen(const char *s, size_t maxlen);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.10: _POSIX_C_SOURCE >= 200809L Before glibc 2.10: _GNU_SOURCE
The strnlen() function returns the number of bytes in the string pointed to by s, excluding the terminating null byte ('\0'), but at most maxlen. In doing this, strnlen() looks only at the first maxlen characters in the string pointed to by s and never beyond s[maxlen-1].
The strnlen() function returns strlen(s), if that is less than maxlen, or maxlen if there is no null terminating ('\0') among the first maxlen characters pointed to by s.
For an explanation of the terms used in this section, see attributes(7).
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signal-safety(7), strlen(3), wcsnlen(3).