1.\" $OpenBSD: strlcpy.3,v 1.19 2007/05/31 19:19:32 jmc Exp $ 2.\" 3.\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com> 4.\" 5.\" Permission to use, copy, modify, and distribute this software for any 6.\" purpose with or without fee is hereby granted, provided that the above 7.\" copyright notice and this permission notice appear in all copies. 8.\" 9.\" THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES 10.\" WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF 11.\" MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR 12.\" ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES 13.\" WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN 14.\" ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF 15.\" OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. 16.\" 17.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, 18.\" INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY 19.\" AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL 20.\" THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 21.\" EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, 22.\" PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; 23.\" OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, 24.\" WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR 25.\" OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF 26.\" ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. 27.\" 28.\" $FreeBSD$ 29.\" 30.Dd June 22, 1998 31.Dt STRLCPY 3 32.Os 33.Sh NAME 34.Nm strlcpy , 35.Nm strlcat 36.Nd size-bounded string copying and concatenation 37.Sh LIBRARY 38.Lb libc 39.Sh SYNOPSIS 40.In string.h 41.Ft size_t 42.Fn strlcpy "char * restrict dst" "const char * restrict src" "size_t size" 43.Ft size_t 44.Fn strlcat "char * restrict dst" "const char * restrict src" "size_t size" 45.Sh DESCRIPTION 46The 47.Fn strlcpy 48and 49.Fn strlcat 50functions copy and concatenate strings respectively. 51They are designed 52to be safer, more consistent, and less error prone replacements for 53.Xr strncpy 3 54and 55.Xr strncat 3 . 56Unlike those functions, 57.Fn strlcpy 58and 59.Fn strlcat 60take the full size of the buffer (not just the length) and guarantee to 61NUL-terminate the result (as long as 62.Fa size 63is larger than 0 or, in the case of 64.Fn strlcat , 65as long as there is at least one byte free in 66.Fa dst ) . 67Note that a byte for the NUL should be included in 68.Fa size . 69Also note that 70.Fn strlcpy 71and 72.Fn strlcat 73only operate on true 74.Dq C 75strings. 76This means that for 77.Fn strlcpy 78.Fa src 79must be NUL-terminated and for 80.Fn strlcat 81both 82.Fa src 83and 84.Fa dst 85must be NUL-terminated. 86.Pp 87The 88.Fn strlcpy 89function copies up to 90.Fa size 91- 1 characters from the NUL-terminated string 92.Fa src 93to 94.Fa dst , 95NUL-terminating the result. 96.Pp 97The 98.Fn strlcat 99function appends the NUL-terminated string 100.Fa src 101to the end of 102.Fa dst . 103It will append at most 104.Fa size 105- strlen(dst) - 1 bytes, NUL-terminating the result. 106.Sh RETURN VALUES 107The 108.Fn strlcpy 109and 110.Fn strlcat 111functions return the total length of the string they tried to 112create. 113For 114.Fn strlcpy 115that means the length of 116.Fa src . 117For 118.Fn strlcat 119that means the initial length of 120.Fa dst 121plus 122the length of 123.Fa src . 124While this may seem somewhat confusing, it was done to make 125truncation detection simple. 126.Pp 127Note however, that if 128.Fn strlcat 129traverses 130.Fa size 131characters without finding a NUL, the length of the string is considered 132to be 133.Fa size 134and the destination string will not be NUL-terminated (since there was 135no space for the NUL). 136This keeps 137.Fn strlcat 138from running off the end of a string. 139In practice this should not happen (as it means that either 140.Fa size 141is incorrect or that 142.Fa dst 143is not a proper 144.Dq C 145string). 146The check exists to prevent potential security problems in incorrect code. 147.Sh EXAMPLES 148The following code fragment illustrates the simple case: 149.Bd -literal -offset indent 150char *s, *p, buf[BUFSIZ]; 151 152\&... 153 154(void)strlcpy(buf, s, sizeof(buf)); 155(void)strlcat(buf, p, sizeof(buf)); 156.Ed 157.Pp 158To detect truncation, perhaps while building a pathname, something 159like the following might be used: 160.Bd -literal -offset indent 161char *dir, *file, pname[MAXPATHLEN]; 162 163\&... 164 165if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname)) 166 goto toolong; 167if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname)) 168 goto toolong; 169.Ed 170.Pp 171Since it is known how many characters were copied the first time, things 172can be sped up a bit by using a copy instead of an append 173.Bd -literal -offset indent 174char *dir, *file, pname[MAXPATHLEN]; 175size_t n; 176 177\&... 178 179n = strlcpy(pname, dir, sizeof(pname)); 180if (n >= sizeof(pname)) 181 goto toolong; 182if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n) 183 goto toolong; 184.Ed 185.Pp 186However, one may question the validity of such optimizations, as they 187defeat the whole purpose of 188.Fn strlcpy 189and 190.Fn strlcat . 191As a matter of fact, the first version of this manual page got it wrong. 192.Sh SEE ALSO 193.Xr snprintf 3 , 194.Xr strncat 3 , 195.Xr strncpy 3 , 196.Xr wcslcpy 3 197.Sh HISTORY 198The 199.Fn strlcpy 200and 201.Fn strlcat 202functions first appeared in 203.Ox 2.4 , 204and made their appearance in 205.Fx 3.3 . 206