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strstr

Section: C Library Functions (3)
Updated: 2026-02-08
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NAME

strstr, strcasestr - locate a substring  

LIBRARY

Standard C library (libc,~-lc)  

SYNOPSIS

#include <string.h>
char *strstr(const char *haystack, const char *needle);
#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <string.h>
char *strcasestr(const char *haystack, const char *needle);

 

DESCRIPTION

The strstr() function finds the first occurrence of the substring needle in the string haystack. The terminating null bytes ([aq][rs]0[aq]) are not compared. The strcasestr() function is like strstr(), but ignores the case of both arguments.  

RETURN VALUE

These functions return a pointer to the beginning of the located substring, or NULL if the substring is not found. If needle is the empty string, the return value is always haystack itself.  

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

Interface	Attribute	Value
strstr()	Thread safety	MT-Safe
strcasestr()	Thread safety	MT-Safe locale

 

STANDARDS

strstr()

C11, POSIX.1-2008.

strcasestr()

GNU.

 

HISTORY

strstr()

POSIX.1-2001, C89.

strcasestr()

GNU.

 

SEE ALSO

memchr(3), memmem(3), strcasecmp(3), strchr(3), string(3), strpbrk(3), strsep(3), strspn(3), strtok(3), wcsstr(3)

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Index

NAME

LIBRARY

SYNOPSIS

DESCRIPTION

RETURN VALUE

ATTRIBUTES

STANDARDS

HISTORY

SEE ALSO

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Time: 19:39:41 GMT, April 11, 2026