strcat Vs strncat - When should which function be used?

If you are absolutely sure about source buffer's size and that the source buffer contains a NULL-character terminating the string, then you can safely use strcat when the destination buffer is large enough.

I still recommend using strncat and give it the size of the destination buffer - length of the destination string - 1

Note: I edited this since comments noted that my previous answer was horribly wrong.

Concatenate two strings into a single string.

Prototypes

#include <string.h>

char * strcat(char *restrict s1, const char *restrict s2);

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

DESCRIPTION

The strcat() and strncat() functions append a copy of the null-terminated string s2 to the end of the null-terminated string s1, then add a terminating \0'. The string s1 must have sufficient space to hold the result.

The strncat() function appends not more than n characters from s2, and then adds a terminating \0'.

The source and destination strings should not overlap, as the behavior is undefined.

RETURN VALUES

 The `strcat()` and `strncat()` functions return the pointer s1.

SECURITY CONSIDERATIONS

The strcat() function is easily misused in a manner which enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack.

Avoid using strcat(). Instead, use strncat() or strlcat() and ensure that no more characters are copied to the destination buffer than it can hold.

Note that strncat() can also be problematic. It may be a security concern for a string to be truncated at all. Since the truncated string will not be as long as the original, it may refer to a completely different resource and usage of the truncated resource could result in very incorrect behavior. Example:

void
 foo(const char *arbitrary_string)
 {
         char onstack[8] = "";

 #if defined(BAD)
         /*
          * This first strcat is bad behavior.  Do not use strcat!
          */
         (void)strcat(onstack, arbitrary_string);        /* BAD! */
 #elif defined(BETTER)
         /*
          * The following two lines demonstrate better use of
          * strncat().
          */
         (void)strncat(onstack, arbitrary_string,
             sizeof(onstack) - strlen(onstack) - 1);
 #elif defined(BEST)
         /*
          * These lines are even more robust due to testing for
          * truncation.
          */
         if (strlen(arbitrary_string) + 1 >
             sizeof(onstack) - strlen(onstack))
                 err(1, "onstack would be truncated");
         (void)strncat(onstack, arbitrary_string,
             sizeof(onstack) - strlen(onstack) - 1);
 #endif
 }

Example

char dest[20] = "Hello";
char *src = ", World!";
char numbers[] = "12345678";

printf("dest before strcat: \"%s\"\n", dest); // "Hello"

strcat(dest, src);
printf("dest after strcat:  \"%s\"\n", dest); // "Hello, World!"

strncat(dest, numbers, 3); // strcat first 3 chars of numbers
printf("dest after strncat: \"%s\"\n", dest); // "Hello, World!123"

They don't do the same thing so they can't be substituted for one another. Both have different data models.

• A string for strcat is a null terminated string for which you (as the programmer) guarantee that it has enough space.
• A string for strncat is a sequence of char that is either terminated at the length you are indicating or by a null termination if it is supposed to be shorter than that length.

So the use of these functions just depends on the assumptions that you may (or want to) do about your data.