string/FreeBSD/strlcpy.3

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   3 .\" Copyright (c) 1998, 2000 Todd C. Miller <Todd.Miller@courtesan.com>
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  28 .\" $FreeBSD: src/lib/libc/string/strlcpy.3,v 1.16 2009/04/07 13:42:53 trasz Exp $
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  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
  46 The
  47 .Fn strlcpy
  48 and
  49 .Fn strlcat
  50 functions copy and concatenate strings respectively.
  51 They are designed
  52 to be safer, more consistent, and less error prone replacements for
  53 .Xr strncpy 3
  54 and
  55 .Xr strncat 3 .
  56 Unlike those functions,
  57 .Fn strlcpy
  58 and
  59 .Fn strlcat
  60 take the full size of the buffer (not just the length) and guarantee to
  61 NUL-terminate the result (as long as
  62 .Fa size
  63 is larger than 0 or, in the case of
  64 .Fn strlcat ,
  65 as long as there is at least one byte free in
  66 .Fa dst ) .
  67 Note that a byte for the NUL should be included in
  68 .Fa size .
  69 Also note that
  70 .Fn strlcpy
  71 and
  72 .Fn strlcat
  73 only operate on true
  74 .Dq C
  75 strings.
  76 This means that for
  77 .Fn strlcpy
  78 .Fa src
  79 must be NUL-terminated and for
  80 .Fn strlcat
  81 both
  82 .Fa src
  83 and
  84 .Fa dst
  85 must be NUL-terminated.
  86 .Pp
  87 The
  88 .Fn strlcpy
  89 function copies up to
  90 .Fa size
  91 - 1 characters from the NUL-terminated string
  92 .Fa src
  93 to
  94 .Fa dst ,
  95 NUL-terminating the result.
  96 .Pp
  97 The
  98 .Fn strlcat
  99 function appends the NUL-terminated string
 100 .Fa src
 101 to the end of
 102 .Fa dst .
 103 It will append at most
 104 .Fa size
 105 - strlen(dst) - 1 bytes, NUL-terminating the result.
 106 .Sh RETURN VALUES
 107 The
 108 .Fn strlcpy
 109 and
 110 .Fn strlcat
 111 functions return the total length of the string they tried to
 112 create.
 113 For
 114 .Fn strlcpy
 115 that means the length of
 116 .Fa src .
 117 For
 118 .Fn strlcat
 119 that means the initial length of
 120 .Fa dst
 121 plus
 122 the length of
 123 .Fa src .
 124 While this may seem somewhat confusing, it was done to make
 125 truncation detection simple.
 126 .Pp
 127 Note however, that if
 128 .Fn strlcat
 129 traverses
 130 .Fa size
 131 characters without finding a NUL, the length of the string is considered
 132 to be
 133 .Fa size
 134 and the destination string will not be NUL-terminated (since there was
 135 no space for the NUL).
 136 This keeps
 137 .Fn strlcat
 138 from running off the end of a string.
 139 In practice this should not happen (as it means that either
 140 .Fa size
 141 is incorrect or that
 142 .Fa dst
 143 is not a proper
 144 .Dq C
 145 string).
 146 The check exists to prevent potential security problems in incorrect code.
 147 .Sh EXAMPLES
 148 The following code fragment illustrates the simple case:
 149 .Bd -literal -offset indent
 150 char *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
 158 To detect truncation, perhaps while building a pathname, something
 159 like the following might be used:
 160 .Bd -literal -offset indent
 161 char *dir, *file, pname[MAXPATHLEN];
 162
 163 \&...
 164
 165 if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
 166         goto toolong;
 167 if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
 168         goto toolong;
 169 .Ed
 170 .Pp
 171 Since it is known how many characters were copied the first time, things
 172 can be sped up a bit by using a copy instead of an append
 173 .Bd -literal -offset indent
 174 char *dir, *file, pname[MAXPATHLEN];
 175 size_t n;
 176
 177 \&...
 178
 179 n = strlcpy(pname, dir, sizeof(pname));
 180 if (n >= sizeof(pname))
 181         goto toolong;
 182 if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
 183         goto toolong;
 184 .Ed
 185 .Pp
 186 However, one may question the validity of such optimizations, as they
 187 defeat the whole purpose of
 188 .Fn strlcpy
 189 and
 190 .Fn strlcat .
 191 As 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
 198 The
 199 .Fn strlcpy
 200 and
 201 .Fn strlcat
 202 functions first appeared in
 203 .Ox 2.4 ,
 204 and made their appearance in
 205 .Fx 3.3 .