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'\" t
.\" Copyright, the authors of the Linux man-pages project
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.TH strstr 3 2025-05-17 "Linux man-pages (unreleased)"
.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]\[rs]0\[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)
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