NULL v. zero [LWN.net]

Back in June, this page looked at the sparse utility, which is being used to search out various kinds of errors in the kernel code base. Recently, large numbers of patches have gone in to address one particular sparse complaint: using an integer 0 to represent a null pointer value. These patches (example) have struck some developers as useless code churn, leading to complaints like:

If you want people to conform people to a certain CodingStyle please document officially in the kernel, sparse isn't distributed with the kernel and the sparse police is silently changing the kernel all over the place with sometimes questionable benefit. Only the __user warnings had really found the bugs, but the rest I've seen changes perfectly legal code.

Linus responds that programmers who interchange NULL and zero are confused about the types they are using and are putting that confusion into the kernel. In his desire to enable the compiler (and other compile-time checkers) to find errors, he wants to separate the integer and pointer types as completely as possible. NULL is a pointer, while 0 can never be.

In other words:

	char * p = 0;	/* IS WRONG! DAMMIT! */
	int i = NULL;	/* THIS IS WRONG TOO! */

and anybody who writes code like the above either needs to get out of the kernel, or needs to get transported to the 21st century.

One might conclude from this statement that Linus is pretty well convinced that the current course of action is correct. He also states that, without exception, changing zero to NULL has resulted in better, more readable code. So use of NULL seems to have become part of the official kernel coding style, even if the CodingStyle document is still silent on the matter.

Index entries for this article
Kernel	Coding style
Kernel	NULL and zero

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