strcat, strncat - concatenate two strings

       For an explanation of the terms used in this section, see attributes(7).
       ┌─────────────────────┬───────────────┬─────────┐
       │ Interface           │ Attribute     │ Value   │
       ├─────────────────────┼───────────────┼─────────┤
       │ strcat(), strncat() │ Thread safety │ MT-Safe │
       └─────────────────────┴───────────────┴─────────┘

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GNU                                                2020-11-01                                          STRCAT(3)

       POSIX.1-2001, POSIX.1-2008, C89, C99, SVr4, 4.3BSD.

       The  strcat()  function  appends the src string to the dest string, overwriting 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; bufferoverrunsareafavoriteavenueforattackingsecureprograms.

       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 bytes.

       As with strcat(), the resulting string in dest is always null-terminated.

       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:

           char *
           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;
           }

       Because  strcat()  and  strncat()  must find the null byte that terminates the string dest using a search
       that starts at the beginning of the string, the execution time of these functions scales according to the
       length of the string dest.  This can be demonstrated by running the program below.  (If the  goal  is  to
       concatenate  many  strings  to  one target, then manually copying the bytes from each source string while
       maintaining a pointer to the end of the target string will provide better performance.)

   Programsource

       #include <stdint.h>
       #include <string.h>
       #include <time.h>
       #include <stdio.h>

       int
       main(int argc, char *argv[])
       {
       #define LIM 4000000
           char p[LIM + 1];    /* +1 for terminating null byte */
           time_t base;

           base = time(NULL);
           p[0] = '\0';

           for (int j = 0; j < LIM; j++) {
               if ((j % 10000) == 0)
                   printf("%d %jd\n", j, (intmax_t) (time(NULL) - base));
               strcat(p, "a");
           }
       }

       strcat, strncat - concatenate two strings

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

           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 terminating null byte to the result, unlesssize 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 create; if the return value is greater than or equal to size, data loss occurred.   If
       data  loss  matters,  the  caller  must  either check the arguments 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.

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

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

#include<string.h>char*strcat(char*dest,constchar*src);char*strncat(char*dest,constchar*src,size_tn);