string/FreeBSD/strcat.3

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  36 .\"     @(#)strcat.3    8.1 (Berkeley) 6/4/93
  37 .\" $FreeBSD: src/lib/libc/string/strcat.3,v 1.13 2002/09/06 11:24:06 tjr Exp $
  38 .\"
  39 .Dd June 4, 1993
  40 .Dt STRCAT 3
  41 .Os
  42 .Sh NAME
  43 .Nm strcat
  44 .Nd concatenate strings
  45 .Sh LIBRARY
  46 .Lb libc
  47 .Sh SYNOPSIS
  48 .In string.h
  49 .Ft char *
  50 .Fn strcat "char * restrict s" "const char * restrict append"
  51 .Ft char *
  52 .Fn strncat "char * restrict s" "const char * restrict append" "size_t count"
  53 .Sh DESCRIPTION
  54 The
  55 .Fn strcat
  56 and
  57 .Fn strncat
  58 functions
  59 append a copy of the null-terminated string
  60 .Fa append
  61 to the end of the null-terminated string
  62 .Fa s ,
  63 then add a terminating
  64 .Ql \e0 .
  65 The string
  66 .Fa s
  67 must have sufficient space to hold the result.
  68 .Pp
  69 The
  70 .Fn strncat
  71 function
  72 appends not more than
  73 .Fa count
  74 characters from
  75 .Fa append ,
  76 and then adds a terminating
  77 .Ql \e0 .
  78 .Sh RETURN VALUES
  79 The
  80 .Fn strcat
  81 and
  82 .Fn strncat
  83 functions
  84 return the pointer
  85 .Fa s .
  86 .Sh SECURITY CONSIDERATIONS
  87 The
  88 .Fn strcat
  89 function is easily misused in a manner
  90 which enables malicious users to arbitrarily change
  91 a running program's functionality through a buffer overflow attack.
  92 (See
  93 the FSA.)
  94 .Pp
  95 Avoid using
  96 .Fn strcat .
  97 Instead, use
  98 .Fn strncat
  99 or
 100 .Fn strlcat
 101 and ensure that no more characters are copied to the destination buffer
 102 than it can hold.
 103 .Pp
 104 Note that
 105 .Fn strncat
 106 can also be problematic.
 107 It may be a security concern for a string to be truncated at all.
 108 Since the truncated string will not be as long as the original,
 109 it may refer to a completely different resource
 110 and usage of the truncated resource
 111 could result in very incorrect behavior.
 112 Example:
 113 .Bd -literal
 114 void
 115 foo(const char *arbitrary_string)
 116 {
 117         char onstack[8];
 118
 119 #if defined(BAD)
 120         /*
 121          * This first strcat is bad behavior.  Do not use strcat!
 122          */
 123         (void)strcat(onstack, arbitrary_string);        /* BAD! */
 124 #elif defined(BETTER)
 125         /*
 126          * The following two lines demonstrate better use of
 127          * strncat().
 128          */
 129         (void)strncat(onstack, arbitrary_string,
 130             sizeof(onstack) - strlen(onstack) - 1);
 131 #elif defined(BEST)
 132         /*
 133          * These lines are even more robust due to testing for
 134          * truncation.
 135          */
 136         if (strlen(arbitrary_string) + 1 >
 137             sizeof(onstack) - strlen(onstack))
 138                 err(1, "onstack would be truncated");
 139         (void)strncat(onstack, arbitrary_string,
 140             sizeof(onstack) - strlen(onstack) - 1);
 141 #endif
 142 }
 143 .Ed
 144 .Sh SEE ALSO
 145 .Xr bcopy 3 ,
 146 .Xr memccpy 3 ,
 147 .Xr memcpy 3 ,
 148 .Xr memmove 3 ,
 149 .Xr strcpy 3 ,
 150 .Xr strlcat 3 ,
 151 .Xr strlcpy 3
 152 .Rs
 153 .%T "The FreeBSD Security Architecture"
 154 .Re
 155 (See
 156 .Pa "/usr/share/doc/{to be decided}" . )
 157 .Sh STANDARDS
 158 The
 159 .Fn strcat
 160 and
 161 .Fn strncat
 162 functions
 163 conform to
 164 .St -isoC .