c - Can the input and output strings to sprintf() be the same?

Question

I have used this type of convention many times in my code in the past:

strcpy ( cTmpA, "hello" );
sprintf ( cTmpA, "%s world", cTmpA );

Recently I switched my legacy C compiler to Visual Studio 2005, and found I got a garbled string resulting from the above code. It then occurred to me that perhaps the behaviour of sprintf() is not rigidly defined where one of the inputs matches the output string.

Is the above code valid K&R C? If not, how do I find all the places in my code where this type of call to sprintf() has occurred?

Answer 1

While it is valid K&R C, you probably want to rather know if it's valid POSIX - see sprintf Specification. We read:

If copying takes place between objects that overlap as a result of a call to sprintf() or snprintf(), the results are undefined.