strerror

strcspn

#include <string.h>size_t strcspn(const char *str1, const char *str2);

The strcspn( ) function returns the length of the initial substring of the string pointed to by str1 that is made up of only those characters not contained in the string pointed to by str2. Stated differently, strcspn( ) returns the index of the first character in the string pointed to by str1 that matches any of the characters in the string pointed to by str2.

Related functions are strrchr( ), strpbrk( ), strstr( ), and strtok( ).