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

NAME
       strcat, strncat - concatenate two strings

SYNOPSIS
       #include <string.h>

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

DESCRIPTION
       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  unpre‐
       dictable; 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 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;
           }

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

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┬───────────────┬─────────┐
       │Interface                                                                                                                             │ Attribute     │ Value   │
       ├──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┼───────────────┼─────────┤
       │strcat(), strncat()                                                                                                                   │ Thread safety │ MT-Safe │
       └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┴───────────────┴─────────┘

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

NOTES
       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,  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 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.

EXAMPLES
       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 concate‐
       nate 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.)

   Program source

       #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");
           }
       }

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

GNU                                                                            2021-03-22                                                                      STRCAT(3)