strstr(3) Library Functions Manual strstr(3) strstr, strcasestr - C (libc, -lc) #include char *strstr(const char *haystack, const char *needle); #define _GNU_SOURCE /* . feature_test_macros(7) */ #include char *strcasestr(const char *haystack, const char *needle); The strstr() function finds the first occurrence of the substring needle in the string haystack. The terminating null bytes ('\0') are not compared. It is equivalent to memmem(haystack, strlen(haystack), needle, strlen(needle)) strcasestr() strstr(), . NULL, . If needle is the empty string, the return value is always haystack itself. attributes(7). +----------------------------+----------------------------------------------------------+--------------------------+ | | | | +----------------------------+----------------------------------------------------------+--------------------------+ |strstr() | | MT-Safe | +----------------------------+----------------------------------------------------------+--------------------------+ |strcasestr() | | MT-Safe locale | +----------------------------+----------------------------------------------------------+--------------------------+ strstr() C11, POSIX.1-2008. strcasestr() GNU. strstr() POSIX.1-2001, C89. strcasestr() GNU. memchr(3), memmem(3), strcasecmp(3), strchr(3), string(3), strpbrk(3), strsep(3), strspn(3), strtok(3), wcsstr(3) () Alexander Golubev , Azamat Hackimov , Hotellook, Nikita , Spiros Georgaras , Vladislav , Yuri Kozlov ; GNU (GNU General Public License - GPL, 3 ) , - . - , , () () () . Linux man-pages 6.18 25 2026 . strstr(3)