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STRSPN(3) Library Functions Manual STRSPN(3)

strspn
span a string

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

The strspn() function spans the initial part of the NUL-terminated string s as long as the characters from s occur in string charset.

The strspn() function returns the number of characters spanned.

The following call to strspn() will return 3, since the first three characters of string s are part of string charset:
char *s = "foobar"; 
char *charset = "of"; 
size_t span; 
 
span = strspn(s, charset);

memchr(3), strchr(3), strcspn(3), strpbrk(3), strrchr(3), strsep(3), strstr(3), strtok(3), wcsspn(3)

The strspn() function conforms to ANSI X3.159-1989 (“ANSI C89”).

The strspn() function first appeared in AT&T System III UNIX and was reimplemented for 4.3BSD-Tahoe.
June 5, 2013 OpenBSD-current