strncat man page on FreeBSD

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STRCAT(3)		 BSD Library Functions Manual		     STRCAT(3)

     strcat, strncat — concatenate strings

     Standard C Library (libc, -lc)

     #include <string.h>

     char *
     strcat(char * restrict s, const char * restrict append);

     char *
     strncat(char * restrict s, const char * restrict append, size_t count);

     The strcat() and strncat() functions append a copy of the null-terminated
     string append to the end of the null-terminated string s, then add a ter‐
     minating ‘\0’.  The string s must have sufficient space to hold the

     The strncat() function appends not more than count characters from
     append, and then adds a terminating ‘\0’.

     The strcat() and strncat() functions return the pointer s.

     The strcat() function is easily misused in a manner which enables mali‐
     cious users to arbitrarily change a running program's functionality
     through a buffer overflow attack.	(See the FSA.)

     Avoid using strcat().  Instead, use strncat() or strlcat() and ensure
     that no more characters are copied to the destination buffer than it can

     Note that strncat() can also be problematic.  It may be a security con‐
     cern for a string to be truncated at all.	Since the truncated string
     will not be as long as the original, it may refer to a completely differ‐
     ent resource and usage of the truncated resource could result in very
     incorrect behavior.  Example:

     foo(const char *arbitrary_string)
	     char onstack[8];

     #if defined(BAD)
	      * This first strcat is bad behavior.  Do not use strcat!
	     (void)strcat(onstack, arbitrary_string);	     /* BAD! */
     #elif defined(BETTER)
	      * The following two lines demonstrate better use of
	      * strncat().
	     (void)strncat(onstack, arbitrary_string,
		 sizeof(onstack) - strlen(onstack) - 1);
     #elif defined(BEST)
	      * These lines are even more robust due to testing for
	      * truncation.
	     if (strlen(arbitrary_string) + 1 >
		 sizeof(onstack) - strlen(onstack))
		     err(1, "onstack would be truncated");
	     (void)strncat(onstack, arbitrary_string,
		 sizeof(onstack) - strlen(onstack) - 1);

     bcopy(3), memccpy(3), memcpy(3), memmove(3), strcpy(3), strlcat(3),
     strlcpy(3), wcscat(3)

     The strcat() and strncat() functions conform to ISO/IEC 9899:1990
     (“ISO C90”).

BSD			       December 1, 2009				   BSD

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