##### Synopsis

    #include <string.h>
    size_t strspn(const char *s1, const char *s2);

##### Description

strspn computes the length of the maximum initial segment of the string pointed to by s1 which consists entirely of characters from the string pointed to by s2.

strspn returns the length of the segment.

##### Portability

strspn conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).