strncat Function

Append a specified number of characters from the source string to the destination string.

Include

<string.h>

Prototype

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

Arguments

s1
destination string to copy to
s2
source string to copy from
n
maximum number of characters to append

Return Value

Returns a pointer to the destination string.

Remarks

This function appends up to n characters (a null character and characters that follow it are not appended) from the source string to the end of the destination string. If a null character is not encountered, then a terminating null character is appended to the result. If the strings overlap, the behavior is undefined.

Example

See the notes at the beginning of this chapter or section for information on using printf() or scanf() (and other functions reading and writing the stdin or stdout streams) in the example code.

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

int main(void)
{
  char buf1[50] = "We're here";
  char buf2[50] = "Where is the time?";
  char buf3[50] = "Why?";

  printf("buf1 : %s\n", buf1);
  printf("\t(%d characters)\n\n", strlen(buf1));
  printf("buf2 : %s\n", buf2);
  printf("\t(%d characters)\n\n", strlen(buf2));
  printf("buf3 : %s\n", buf3);
  printf("\t(%d characters)\n\n\n", strlen(buf3));

  strncat(buf1, buf2, 6);
  printf("buf1 after strncat of 6 characters "
         "of buf2: \n\t%s\n", buf1);
  printf("\t(%d characters)\n", strlen(buf1));
  printf("\n");

  strncat(buf1, buf2, 25);
  printf("buf1 after strncat of 25 characters "
         "of buf2: \n\t%s\n", buf1);
  printf("\t(%d characters)\n", strlen(buf1));
  printf("\n");

  strncat(buf1, buf3, 4);
  printf("buf1 after strncat of 4 characters "
         "of buf3: \n\t%s\n", buf1);
  printf("\t(%d characters)\n", strlen(buf1));
}

Example Output

buf1 : We’re here
        (10 characters)

buf2 : Where is the time?
        (18 characters)

buf3 : Why?
        (4 characters)

buf1 after strncat of 6 characters of buf2:
        We’re hereWhere
        (16 characters)

buf1 after strncat of 25 characters of buf2:
        We’re hereWhere Where is the time?
        (34 characters)

buf1 after strncat of 4 characters of buf3:
   We’re hereWhere Where is the time?Why?
        (38 characters)