'\" t
.\" Copyright 1993 David Metcalfe (david@prism.demon.co.uk)
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" References consulted:
.\"     Linux libc source code
.\"     Lewine's _POSIX Programmer's Guide_ (O'Reilly & Associates, 1991)
.\"     386BSD man pages
.\" Modified Sat Jul 24 17:56:43 1993 by Rik Faith (faith@cs.unc.edu)
.\" Added history, aeb, 980113.
.\" 2005-05-05 mtk: added strcasestr()
.\"
.TH strstr 3 2023-10-31 "Linux man-pages 6.7"
.SH NAME
strstr, strcasestr \- locate a substring
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <string.h>
.P
.BI "char *strstr(const char *" haystack ", const char *" needle );
.P
.BR "#define _GNU_SOURCE" "         /* See feature_test_macros(7) */"
.B #include <string.h>
.P
.BI "char *strcasestr(const char *" haystack ", const char *" needle );
.fi
.SH DESCRIPTION
The
.BR strstr ()
function finds the first occurrence of the substring
.I needle
in the string
.IR haystack .
The terminating null bytes (\[aq]\e0\[aq]) are not compared.
.P
The
.BR strcasestr ()
function is like
.BR strstr (),
but ignores the case of both arguments.
.SH RETURN VALUE
These functions return a pointer to the beginning of the
located substring, or NULL if the substring is not found.
.P
If
.I needle
is the empty string,
the return value is always
.I haystack
itself.
.SH ATTRIBUTES
For an explanation of the terms used in this section, see
.BR attributes (7).
.TS
allbox;
lbx lb lb
l l l.
Interface	Attribute	Value
T{
.na
.nh
.BR strstr ()
T}	Thread safety	MT-Safe
T{
.na
.nh
.BR strcasestr ()
T}	Thread safety	MT-Safe locale
.TE
.SH STANDARDS
.TP
.BR strstr ()
C11, POSIX.1-2008.
.TP
.BR strcasestr ()
GNU.
.SH HISTORY
.TP
.BR strstr ()
POSIX.1-2001, C89.
.TP
.BR strcasestr ()
GNU.
.SH SEE ALSO
.BR memchr (3),
.BR memmem (3),
.BR strcasecmp (3),
.BR strchr (3),
.BR string (3),
.BR strpbrk (3),
.BR strsep (3),
.BR strspn (3),
.BR strtok (3),
.BR wcsstr (3)