string/FreeBSD/strcat.3

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  32 .\"     @(#)strcat.3    8.1 (Berkeley) 6/4/93
  33 .\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.17 2009/12/01 07:28:56 brueffer Exp $
  34 .\"
  35 .Dd December 1, 2009
  36 .Dt STRCAT 3
  37 .Os
  38 .Sh NAME
  39 .Nm strcat ,
  40 .Nm strncat
  41 .Nd concatenate strings
  42 .Sh LIBRARY
  43 .Lb libc
  44 .Sh SYNOPSIS
  45 .In string.h
  46 .Ft char *
  47 .Fo strcat
  48 .Fa "char *restrict s1"
  49 .Fa "const char *restrict s2"
  50 .Fc
  51 .Ft char *
  52 .Fo strncat
  53 .Fa "char *restrict s1"
  54 .Fa "const char *restrict s2"
  55 .Fa "size_t n"
  56 .Fc
  57 .Sh DESCRIPTION
  58 The
  59 .Fn strcat
  60 and
  61 .Fn strncat
  62 functions
  63 append a copy of the null-terminated string
  64 .Fa s2
  65 to the end of the null-terminated string
  66 .Fa s1 ,
  67 then add a terminating
  68 .Ql \e0 .
  69 The string
  70 .Fa s1
  71 must have sufficient space to hold the result.
  72 .Pp
  73 The
  74 .Fn strncat
  75 function
  76 appends not more than
  77 .Fa n
  78 characters from
  79 .Fa s2 ,
  80 and then adds a terminating
  81 .Ql \e0 .
  82 .Pp
  83 The source and destination strings should not overlap, as the
  84 behavior is undefined.
  85 .Sh RETURN VALUES
  86 The
  87 .Fn strcat
  88 and
  89 .Fn strncat
  90 functions
  91 return the pointer
  92 .Fa s1 .
  93 .Sh SECURITY CONSIDERATIONS
  94 The
  95 .Fn strcat
  96 function is easily misused in a manner
  97 which enables malicious users to arbitrarily change
  98 a running program's functionality through a buffer overflow attack.
  99 (See
 100 the FSA.)
 101 .Pp
 102 Avoid using
 103 .Fn strcat .
 104 Instead, use
 105 .Fn strncat
 106 or
 107 .Fn strlcat
 108 and ensure that no more characters are copied to the destination buffer
 109 than it can hold.
 110 .Pp
 111 Note that
 112 .Fn strncat
 113 can also be problematic.
 114 It may be a security concern for a string to be truncated at all.
 115 Since the truncated string will not be as long as the original,
 116 it may refer to a completely different resource
 117 and usage of the truncated resource
 118 could result in very incorrect behavior.
 119 Example:
 120 .Bd -literal
 121 void
 122 foo(const char *arbitrary_string)
 123 {
 124         char onstack[8] = "";
 125
 126 #if defined(BAD)
 127         /*
 128          * This first strcat is bad behavior.  Do not use strcat!
 129          */
 130         (void)strcat(onstack, arbitrary_string);        /* BAD! */
 131 #elif defined(BETTER)
 132         /*
 133          * The following two lines demonstrate better use of
 134          * strncat().
 135          */
 136         (void)strncat(onstack, arbitrary_string,
 137             sizeof(onstack) - strlen(onstack) - 1);
 138 #elif defined(BEST)
 139         /*
 140          * These lines are even more robust due to testing for
 141          * truncation.
 142          */
 143         if (strlen(arbitrary_string) + 1 >
 144             sizeof(onstack) - strlen(onstack))
 145                 err(1, "onstack would be truncated");
 146         (void)strncat(onstack, arbitrary_string,
 147             sizeof(onstack) - strlen(onstack) - 1);
 148 #endif
 149 }
 150 .Ed
 151 .Sh SEE ALSO
 152 .Xr bcopy 3 ,
 153 .Xr memccpy 3 ,
 154 .Xr memcpy 3 ,
 155 .Xr memmove 3 ,
 156 .Xr strcpy 3 ,
 157 .Xr strlcat 3 ,
 158 .Xr strlcpy 3 ,
 159 .Xr wcscat 3
 160 .Sh STANDARDS
 161 The
 162 .Fn strcat
 163 and
 164 .Fn strncat
 165 functions
 166 conform to
 167 .St -isoC .