Why don't you read the documentation (or source) from whomever
supplied you that (non-standard) function?
# 7.21.5.6 The strspn function
#
# Synopsis
#
# [#1]
#
# #include <string.h>
# size_t strspn(const char *s1, const char *s2);
#
# Description
#
# [#2] The strspn function 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.
#
# Returns
#
# [#3] The strspn function returns the length of the segment.
I don't see any description of what "spn" means in there.
dave