strspn / strcspn / strpbrk (3) - glibc

Split from the chapter glibc: strings, ctype and stdlib arithmetic into the per-function layout; see the method page and the glibc ledger for coverage status.

Declaration and gates

#include <string.h>

size_t strspn(const char *s, const char *accept);
size_t strcspn(const char *s, const char *reject);
char  *strpbrk(const char *s, const char *accept);

No feature-test gates; all three are ISO C (/usr/include/string.h lines 291-325, all marked __attribute_pure__).

Semantics

strspn returns the length in bytes of the initial segment of s made up entirely of bytes from accept; strcspn the length of the initial segment made up entirely of bytes not in reject (strspn(3)). strpbrk returns a pointer to the first byte in s that appears in accept, or NULL if none does (strpbrk(3)). The manual groups all three in the Search Functions node of string.texi.

Gotchas and portability

• strcspn(s, "") is the allocation-free strlen(s); strcspn(s, "\n") strips a trailing newline index for line buffers. The empty-set edge works because the terminator counts as a member of every reject set.
• strspn/strcspn return lengths, not pointers; a common bug is testing them for truthiness instead of comparing against strlen(s) to decide “string consists solely of …”.
• strpbrk is strchr generalized to a set; when the set has one member it degenerates to strchr, and strcspn(s, reject) equals strpbrk(s, reject) ? strpbrk(s, reject) - s : strlen(s).
• These functions scan accept/reject for every position of s, so cost is O(len(s) * len(set)); glibc’s ifunc-selected implementations mitigate but do not change the bound (nm shows i/ifunc symbols).
• All three are const-correct and do not modify their inputs; they are safe building blocks for lexers, but they cannot express “not in set A and in set B” without composition.
• musl 1.2.5 ships equivalent implementations (musl src/string/strspn.c, strcspn.c, strpbrk.c).

References

• glibc manual: string.texi, node Search Functions
• man3: strspn(3) (covers strcspn via .so alias strcspn.3), strpbrk(3)
• header: /usr/include/string.h
• symbols: nm -D --with-symbol-versions libc.so.6