strncat man page on Archlinux

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STRCAT(3)		   Linux Programmer's Manual		     STRCAT(3)

       strcat, strncat - concatenate two strings

       #include <string.h>

       char *strcat(char *dest, const char *src);

       char *strncat(char *dest, const char *src, size_t n);

       The  strcat() function appends the src string to the dest string, over‐
       writing the terminating null byte ('\0') at the end of dest,  and  then
       adds  a	terminating  null  byte.  The strings may not overlap, and the
       dest string must have enough space for the  result.   If	 dest  is  not
       large  enough, program behavior is unpredictable; buffer overruns are a
       favorite avenue for attacking secure programs.

       The strncat() function is similar, except that

       *  it will use at most n bytes from src; and

       *  src does not need to be null-terminated if it	 contains  n  or  more

       As  with	 strcat(),  the resulting string in dest is always null-termi‐

       If src contains n or more bytes, strncat() writes n+1 bytes to dest  (n
       from  src plus the terminating null byte).  Therefore, the size of dest
       must be at least strlen(dest)+n+1.

       A simple implementation of strncat() might be:

	   strncat(char *dest, const char *src, size_t n)
	       size_t dest_len = strlen(dest);
	       size_t i;

	       for (i = 0 ; i < n && src[i] != '\0' ; i++)
		   dest[dest_len + i] = src[i];
	       dest[dest_len + i] = '\0';

	       return dest;

       The strcat() and strncat() functions return a pointer to the  resulting
       string dest.

   Multithreading (see pthreads(7))
       The strcat() and strncat() functions are thread-safe.

       SVr4, 4.3BSD, C89, C99.

       Some  systems  (the  BSDs,  Solaris,  and others) provide the following

	   size_t strlcat(char *dest, const char *src, size_t size);

       This function appends the null-terminated  string  src  to  the	string
       dest,  copying  at  most	 size-strlen(dest)-1 from src, and adds a null
       terminator to the result, unless size is less than strlen(dest).	  This
       function	 fixes	the buffer overrun problem of strcat(), but the caller
       must still handle the possibility of data loss if size  is  too	small.
       The  function  returns the length of the string strlcat() tried to cre‐
       ate; if the return value is greater than or equal to  size,  data  loss
       occurred.  If data loss matters, the caller must either check the argu‐
       ments before the call, or test the function return value.  strlcat() is
       not present in glibc and is not standardized by POSIX, but is available
       on Linux via the libbsd library.

       bcopy(3),  memccpy(3),  memcpy(3),  strcpy(3),  string(3),  strncpy(3),
       wcscat(3), wcsncat(3)

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

GNU				  2014-01-20			     STRCAT(3)

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