.\" Copyright (c) 1990, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" This code is derived from software contributed to Berkeley by .\" Chris Torek and the American National Standards Committee X3, .\" on Information Processing Systems. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" from: @(#)strcpy.3 8.1 (Berkeley) 6/4/93 .\" from: NetBSD: strcpy.3,v 1.23 2015/04/01 20:18:17 riastradh Exp .\" $NetBSD: strncpy.3,v 1.16.4.2 2024/09/20 11:22:29 martin Exp $ .\" .Dd August 11, 2023 .Dt STRNCPY 3 .Os .Sh NAME .Nm stpncpy , .Nm strncpy .Nd copy fixed-width buffers with NUL padding .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In string.h .Ft char * .Fn stpncpy "char * restrict dst" "const char * restrict src" "size_t len" .Ft char * .Fn strncpy "char * restrict dst" "const char * restrict src" "size_t len" .Sh DESCRIPTION The .Fn stpncpy and .Fn strncpy functions fill a .Fa len Ns -byte buffer at .Fa dst by copying up to .Fa len .No non- Ns Tn NUL bytes from .Fa src followed by enough .Tn NUL bytes \(em possibly zero of them \(em to pad the remainder. .Pp The buffers .Fa src and .Fa dst may not overlap. .Pp The buffer .Fa src is not required to hold a .Tn NUL Ns -terminated string on input; it is only required .Em either to have at least .Fa len bytes allocated and initialized, .Em or to have a .Tn NUL byte if it is shorter than .Fa len bytes. .Pp The buffer .Fa dst is not guaranteed to hold a .Tn NUL Ns -terminated string on output; .Fn stpncpy and .Fn strncpy write exactly .Fa len bytes to it, which includes nonempty .Tn NUL padding only if a .Tn NUL byte appears in the first .Fa len bytes at .Fa src . .Sh RETURN VALUES The .Fn strncpy function returns .Fa dst . .Pp The .Fn stpncpy function returns a pointer to the byte after the last .No non- Ns Tn NUL byte of .Fa dst . This does not necessarily point to a .Tn NUL byte; .Fn stpncpy may return .Li \*(Am Ns Fa dst Ns Li "[" Fa len Ns Li "]" Ns , if all .Fa len bytes starting at .Fa src are .No non- Tn NUL . .Sh EXAMPLES The following logic fills a fixed-width field in a record that might appear on disk with the content of a caller-provided string .Dv str , padded to the end of the field with .Tn NUL bytes: .Bd -literal -offset indent struct record { uint16_t id; char name[6]; uint8_t tag; ... }; struct record *rec = ...; strncpy(rec->name, str, sizeof(rec->name)); .Ed .Pp The following values for .Dv str result in the following corresponding contents of .Dv rec Ns Li "->name" : .Bl -column -offset indent ".Li \*qabcdefghi\e0\*q" ".Li \*qabc\e0\e0\e0\*q" .It Dv str Ta Dv rec Ns Li "->name" .It Li \*qabc\e0\*q Ta Li \*qabc\e0\e0\e0\*q .It Li \*qabc\e0\e0\e0\*q Ta Li \*qabc\e0\e0\e0\*q .It Li \*qabcde\e0\*q Ta Li \*qabcde\e0\*q .It Li \*qabcdef\e0\*q Ta Li \*qabcdef\*q .It Li \*qabcdef\*q Ta Li \*qabcdef\*q .It Li \*qabcdefghi\e0\*q Ta Li \*qabcdef\*q .It Li \*qabcdefghi\*q Ta Li \*qabcdef\*q .El .Pp Note that when .Dv str has at least six .No non- Ns Tn NUL bytes, .Dv rec Ns Li "->name" is .Em not .Tn NUL Ns -terminated \(em it is only .Em padded with (possibly zero) .Tn NUL bytes to fill the fixed-width buffer. When .Dv str has .Em more than six .No non- Ns Tn NUL bytes, the additional ones are truncated. If .Dv str has space for .Em fewer than six bytes, and the last one is not .Tn NUL , using .Fn strncpy is undefined. .Pp Because .Fn strncpy does .Em not guarantee to .Tn NUL Ns -terminate the result, if .Tn NUL Ns -termination is required it must be done explicitly: .Bd -literal -offset indent char buf[1024]; strncpy(buf, input, sizeof(buf) - 1); buf[sizeof(buf) - 1] = '\e0'; .Ed .Pp If .Va input is guaranteed to be .Tn NUL Ns -terminated , and if .Va buf need only be .Tn NUL Ns -terminated , not fully initialized with .Tn NUL padding, this could be achieved using .Xr strlcpy 3 as follows: .Bd -literal -offset indent strlcpy(buf, input, sizeof(buf)); .Ed .Pp It is not enough for .Va input to have .Li sizeof(buf) bytes allocated; it MUST be .Tn NUL Ns -terminated for .Xr strlcpy 3 to be used. .Pp Note that because .Xr strlcpy 3 is not defined in any standards, it should only be used when portability is not a concern. .Pp .Sy WARNING : Because .Xr strlcpy 3 does not fully initialize .Fa dst , but does read all the way to a .Tn NUL terminator in .Fa src even past .Fa len bytes, .Xr strlcpy 3 is .Em not a safe .Tn NUL Ns -terminating replacement for .Fn strncpy . Naively replacing .Fn strncpy by .Xr strlcpy 3 can lead to crashes, undefined behaviour, and disclosure of secrets from uninitialized memory. .Sh SEE ALSO .Xr bcopy 3 , .Xr memccpy 3 , .Xr memcpy 3 , .Xr memmove 3 , .Xr strcpy 3 , .Xr strlcpy 3 , .Xr wcscpy 3 .Sh STANDARDS The .Fn strncpy function conforms to .St -isoC-99 . .Pp The .Fn stpncpy function conforms to .St -p1003.1-2008 . .Sh HISTORY The .Fn stpncpy function first appeared in .Nx 6.0 . .Sh SECURITY CONSIDERATIONS The .Fn stpncpy and .Fn strncpy functions are not guaranteed to .Tn NUL Ns -terminate the result.