string/FreeBSD/strlcpy.3

   1 .\" $OpenBSD: strlcpy.3,v 1.5 1999/06/06 15:17:32 aaron Exp $
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   3 .\" Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
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  28 .\" $FreeBSD: src/lib/libc/string/strlcpy.3,v 1.12 2002/12/18 12:45:11 ru 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 *dst" "const char *src" "size_t size"
  43 .Ft size_t
  44 .Fn strlcat "char *dst" "const char *src" "size_t size"
  45 .Sh DESCRIPTION
  46 The
  47 .Fn strlcpy
  48 and
  49 .Fn strlcat
  50 functions copy and concatenate strings respectively.  They are designed
  51 to be safer, more consistent, and less error prone replacements for
  52 .Xr strncpy 3
  53 and
  54 .Xr strncat 3 .
  55 Unlike those functions,
  56 .Fn strlcpy
  57 and
  58 .Fn strlcat
  59 take the full size of the buffer (not just the length) and guarantee to
  60 NUL-terminate the result (as long as
  61 .Fa size
  62 is larger than 0 or, in the case of
  63 .Fn strlcat ,
  64 as long as there is at least one byte free in
  65 .Fa dst ) .
  66 Note that you should include a byte for the NUL in
  67 .Fa size .
  68 Also note that
  69 .Fn strlcpy
  70 and
  71 .Fn strlcat
  72 only operate on true
  73 .Dq C
  74 strings.
  75 This means that for
  76 .Fn strlcpy
  77 .Fa src
  78 must be NUL-terminated and for
  79 .Fn strlcat
  80 both
  81 .Fa src
  82 and
  83 .Fa dst
  84 must be NUL-terminated.
  85 .Pp
  86 The
  87 .Fn strlcpy
  88 function copies up to
  89 .Fa size
  90 - 1 characters from the NUL-terminated string
  91 .Fa src
  92 to
  93 .Fa dst ,
  94 NUL-terminating the result.
  95 .Pp
  96 The
  97 .Fn strlcat
  98 function appends the NUL-terminated string
  99 .Fa src
 100 to the end of
 101 .Fa dst .
 102 It will append at most
 103 .Fa size
 104 - strlen(dst) - 1 bytes, NUL-terminating the result.
 105 .Sh RETURN VALUES
 106 The
 107 .Fn strlcpy
 108 and
 109 .Fn strlcat
 110 functions return the total length of the string they tried to
 111 create.  For
 112 .Fn strlcpy
 113 that means the length of
 114 .Fa src .
 115 For
 116 .Fn strlcat
 117 that means the initial length of
 118 .Fa dst
 119 plus
 120 the length of
 121 .Fa src .
 122 While this may seem somewhat confusing it was done to make
 123 truncation detection simple.
 124 .Pp
 125 Note however, that if
 126 .Fn strlcat
 127 traverses
 128 .Fa size
 129 characters without finding a NUL, the length of the string is considered
 130 to be
 131 .Fa size
 132 and the destination string will not be NUL-terminated (since there was
 133 no space for the NUL).
 134 This keeps
 135 .Fn strlcat
 136 from running off the end of a string.
 137 In practice this should not happen (as it means that either
 138 .Fa size
 139 is incorrect or that
 140 .Fa dst
 141 is not a proper
 142 .Dq C
 143 string).
 144 The check exists to prevent potential security problems in incorrect code.
 145 .Sh EXAMPLES
 146 The following code fragment illustrates the simple case:
 147 .Bd -literal -offset indent
 148 char *s, *p, buf[BUFSIZ];
 149
 150 \&...
 151
 152 (void)strlcpy(buf, s, sizeof(buf));
 153 (void)strlcat(buf, p, sizeof(buf));
 154 .Ed
 155 .Pp
 156 To detect truncation, perhaps while building a pathname, something
 157 like the following might be used:
 158 .Bd -literal -offset indent
 159 char *dir, *file, pname[MAXPATHLEN];
 160
 161 \&...
 162
 163 if (strlcpy(pname, dir, sizeof(pname)) >= sizeof(pname))
 164         goto toolong;
 165 if (strlcat(pname, file, sizeof(pname)) >= sizeof(pname))
 166         goto toolong;
 167 .Ed
 168 .Pp
 169 Since we know how many characters we copied the first time, we can
 170 speed things up a bit by using a copy instead of an append:
 171 .Bd -literal -offset indent
 172 char *dir, *file, pname[MAXPATHLEN];
 173 size_t n;
 174
 175 \&...
 176
 177 n = strlcpy(pname, dir, sizeof(pname));
 178 if (n >= sizeof(pname))
 179         goto toolong;
 180 if (strlcpy(pname + n, file, sizeof(pname) - n) >= sizeof(pname) - n)
 181         goto toolong;
 182 .Ed
 183 .Pp
 184 However, one may question the validity of such optimizations, as they
 185 defeat the whole purpose of
 186 .Fn strlcpy
 187 and
 188 .Fn strlcat .
 189 As a matter of fact, the first version of this manual page got it wrong.
 190 .Sh SEE ALSO
 191 .Xr snprintf 3 ,
 192 .Xr strncat 3 ,
 193 .Xr strncpy 3
 194 .Sh HISTORY
 195 The
 196 .Fn strlcpy
 197 and
 198 .Fn strlcat
 199 functions first appeared in
 200 .Ox 2.4 ,
 201 and made their appearance in
 202 .Fx 3.3 .