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