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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 .