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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 s1, const char *restrict s2);

     char *
     strncat(char *restrict s1, const char *restrict s2, size_t n);

     The strcat() and strncat() functions append a copy of the null-terminated
     string s2 to the end of the null-terminated string s1, then add a termi-nating terminating
     nating `\0'.  The string s1 must have sufficient space to hold the

     The strncat() function appends not more than n characters from s2, and
     then adds a terminating `\0'.

     The strcat() and strncat() functions return the pointer s1.

     The strcat() function is easily misused in a manner which enables mali-cious malicious
     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 concern
     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 different
     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),

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

BSD                              June 4, 1993                              BSD