strncat(3) [freebsd man page]

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

NAME

     strcat, strncat -- concatenate strings

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <string.h>

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

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

DESCRIPTION

     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 terminating ''.  The string s must have sufficient space to hold the result.

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

RETURN VALUES

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

SEE ALSO

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

STANDARDS

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

SECURITY CONSIDERATIONS

     The strcat() function is easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality
     through a buffer overflow attack.

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

     Note that strncat() can also be problematic.  It may be a security concern 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 different resource and usage of the truncated resource could result in very
     incorrect behavior.  Example:

     void
     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);
     #endif
     }

BSD
								 December 1, 2009							       BSD

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

NAME

     strcat, strncat -- concatenate strings

LIBRARY

     Standard C Library (libc, -lc)

SYNOPSIS

     #include <string.h>

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

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

DESCRIPTION

     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 terminating ''.  The string s must have sufficient space to hold the result.

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

RETURN VALUES

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

SEE ALSO

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

STANDARDS

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

SECURITY CONSIDERATIONS

     The strcat() function is easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality
     through a buffer overflow attack.

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

     Note that strncat() can also be problematic.  It may be a security concern 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 different resource and usage of the truncated resource could result in very
     incorrect behavior.  Example:

     void
     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);
     #endif
     }

BSD
								 December 1, 2009							       BSD