In C programming language, string manipulation is not an easy task. Concatenating or adding strings together
can prove to be a challenge, too. Luckily, there are implemented library functions in C language that make
programmers life a bit easier.
In this article, we will review the strncat function. This function
is defined in the standard string.h library. Below you will find a
non-standard yet effective implementation of the strncat function,
too.
Return: Pointer to destination array (char *dest).
Parameter list: (char *dest) - destination array to which elements are
copied. (char *src) - source array from which elements are
copied. (size_t n) - number of elements to be copied.
Header file:<string.h>.
1. The strncat function in C
The problem that this function helps to solve is of string (or character array) concatenation. If you
have a character array that has more memory than it uses. And you would like to use this memory instead
and replace unused bytes with desired characters, then strncat
is a perfect candidate. To see this in a
more concrete example, consider this code:
Example 1: Usage of the strncat in C.
#include <stdio.h>
#include <string.h>
intmain()
{
char dest[24] = "Practice makes " ;
char dest2[24] = "Practice makes " ;
char *src = "perfect!" ;
strncat(dest, src, 5);
strncat(dest2, src, 8);
printf("When n is 5: %s\n" , dest);
printf("When n is 8: %s\n" , dest2);
return 0;
}
Possible output:
When n is 5: Practice makes perfe
When n is 8: Practice makes perfect!
From the example above, we can already make a couple of deductions about the behavior of this function.
(1) The strncat function modifies the destination array that we
pass as the first argument. (2) Number of elements that are copied from the source string depend on the
third argument. When we passed 5 as an argument (on line 8), we copied perfe. When we passed 8 as an argument (on line 9), we
copied perfect!.
How lovely this behavior may seem, there are a couple of drawbacks that are worthy of knowing before
you add the strncat function to your project. The drawbacks
manifest when (1) the destination and the source arrays overlap in memory, (2) when the destination
array does not have enough memory to contain characters from the source string.
2. How to implement the strncat function in
C
?
In order to implement the strncat function, we would need to
replicate the following behaviors:
Check that the source and destination arrays are not equal to 0.
Get length of the destination array.
Copy characters from the source to the destination array up until the
value n that is provided as the third parameter. Given that
source contains enough characters, otherwise, copy til the end of the source array.
Terminate with a terminating null character after copying from the
source array.
When n is 5: Practice makes perfe
When n is 8: Practice makes perfect!
Code snippet above demonstrates one of many ways how the strncat
function can be implemented and rewritten. With this implementation you can concatenate character
arrays, given, of course, that appropriate arguments are passed to this function.
