strcat() and null chars question

**David Reghay** · 01-01-2013

Working through Teach Yourself C in my spare time and am now in chapter 5 which deals with arrays and strings. My question is fairly obvious but the author does not seem to address it in his treatment of strcat(). What I want to know is this: all strings contain a null character at the end, which strlen() does not return. When using strcat() to combine two strings, are both null characters preserved? Or is one of them lost, so that only one null character exists at the end of the new, larger string? Thanks in advance for your time and patience in answering this novice's question.

**laserlight** · 01-01-2013

Originally Posted by David Reghay

When using strcat() to combine two strings, are both null characters preserved? Or is one of them lost, so that only one null character exists at the end of the new, larger string?

One of them is "lost". After all, the second one wouldn't matter anyway since the string length, as you noted, is only until (and excluding) the first null character.

**fnprintf** · 01-02-2013

The man page for strcat(3) describes it well:

char *strcat(char *dest, const char *src);
...
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 man pages are great for reference. In Linux, just type "man <function>" in term, otherwise do an online search.

If you want to see how the strcat() function is coded, you can always download the source code and check it out. For glibc, it's located in "string/strcat.c".

**Malcolm McLean** · 01-02-2013

Originally Posted by fnprintf

The man page for strcat(3) describes it well:

The man pages are great for reference. In Linux, just type "man <function>" in term, otherwise do an online search.

If you want to see how the strcat() function is coded, you can always download the source code and check it out. For glibc, it's located in "string/strcat.c".

It's probably a better exercise to write it yourself. Call it mystrcat(), and make sure it always does exactly the same a strcat, even when passed an empty string or the null pointer. (You can't reliably replicate behaviour when passed a wild pointer or an unterminated string as that's undefined).

**grumpy** · 01-02-2013

Originally Posted by Malcolm McLean

It's probably a better exercise to write it yourself. Call it mystrcat(), and make sure it always does exactly the same a strcat, even when passed an empty string or the null pointer.

strcat() has undefined behaviour if passed a null pointer. So, if the only observable difference between mystrcat() and strcat() is in what happens when they are passed a null pointer, they are equivalent - at least, according to the standard.

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