1.\" Copyright (c) 2005 Pascal Gloor <pascal.gloor@spale.com> 2.\" 3.\" Redistribution and use in source and binary forms, with or without 4.\" modification, are permitted provided that the following conditions 5.\" are met: 6.\" 1. Redistributions of source code must retain the above copyright 7.\" notice, this list of conditions and the following disclaimer. 8.\" 2. Redistributions in binary form must reproduce the above copyright 9.\" notice, this list of conditions and the following disclaimer in the 10.\" documentation and/or other materials provided with the distribution. 11.\" 3. The name of the author may not be used to endorse or promote 12.\" products derived from this software without specific prior written 13.\" permission. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND 16.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 17.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 18.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 19.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 20.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 21.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 22.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 23.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 24.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 25.\" SUCH DAMAGE. 26.\" 27.Dd June 21, 2026 28.Dt MEMMEM 3 29.Os 30.Sh NAME 31.Nm memmem 32.Nd "locate a byte subsequence in a byte sequence" 33.Sh LIBRARY 34.Lb libc 35.Sh SYNOPSIS 36.In string.h 37.Ft "QVoid *" 38.Fo memmem 39.Fa "QVoid *big" "size_t big_len" 40.Fa "const void *little" "size_t little_len" 41.Fc 42.Sh DESCRIPTION 43The 44.Fn memmem 45function 46locates the first occurrence of the byte sequence 47.Fa little 48in the byte sequence 49.Fa big . 50.Sh RETURN VALUES 51If 52.Fa little_len 53is zero 54.Fa big 55is returned (that is, an empty little is deemed to match at the beginning of 56big); 57if 58.Fa little 59occurs nowhere in 60.Fa big , 61.Dv NULL 62is returned; 63otherwise a pointer to the first character of the first occurrence of 64.Fa little 65is returned. 66.Sh SEE ALSO 67.Xr memchr 3 , 68.Xr strchr 3 , 69.Xr strstr 3 70.Sh STANDARDS 71.Fn memmem 72conforms to 73.St -p1003.1-2024 . 74Like the qualifier-preserving functions in 75.St -isoC-2023 , 76.Fx implements 77.Fn memmem 78as qualifier-preserving as well. 79.Sh HISTORY 80The 81.Fn memmem 82function first appeared in 83.Fx 6.0 . 84It was replaced with an optimized O(n) implementation from the musl libc 85project in 86.Fx 12.0 . 87.An Pascal Gloor Aq Mt pascal.gloor@spale.com 88provided this man page along with the previous implementation. 89.Sh BUGS 90This function was broken in Linux libc up to and including version 5.0.9 91and in GNU libc prior to version 2.1. 92Prior to 93.Fx 11.0 94.Nm 95returned 96.Dv NULL 97when 98.Fa little_len 99equals 0. 100