{"id":4699,"date":"2021-04-17T00:01:57","date_gmt":"2021-04-17T07:01:57","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4699"},"modified":"2021-04-09T14:54:23","modified_gmt":"2021-04-09T21:54:23","slug":"zero-and-null-and-pointers-and-stuff","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4699","title":{"rendered":"Zero and NULL and Pointers and Stuff"},"content":{"rendered":"

The defined constant NULL<\/code> represents a null or empty pointer. I’ve written before<\/a> that the constant isn’t defined as zero, though such an assumption could lead you into trouble.
\n
\nKeep in mind that NULL<\/code> is a defined constant. It’s declared in the stdio.h<\/code> header file or, more likely, its definition is made in another header file that’s referenced from stdio.h<\/code>. Either way, the problem is that various C compilers can define NULL<\/code> differently.<\/p>\n

Before diving into the details, be aware that NULL<\/code> is a pointer constant. It’s not the same as integer value zero (though it might be defined that way). NULL<\/code> is used where a pointer, a memory address, is to be evaluated or set as an argument.<\/p>\n

Also, NULL<\/code> isn’t the same thing as the null character, \\0<\/code>. This character is ASCII code zero, but it’s not a pointer. It’s a character value. (In some of my earlier books, I referred to \\0<\/code> as the “NULL character,” which is incorrect.)<\/p>\n

Finally, NULL<\/code> isn’t memory location (address) zero. It may be the value zero as it’s defined, but it’s not a memory location. Instead, think of it as an “empty” location and not zero; NULL<\/code> indicates the absence of a memory location, which is important when comparing pointers: NULL<\/code> evaluates as FALSE.<\/p>\n

The point [sic] of this post is that despite all this NULL<\/code> nonsense, it’s unwise to assume NULL<\/code>‘s definition. From an email I recently received:<\/p>\n

The problem with NULL is that it’s either defined as an integer zero constant or as an integer zero constant casted to a void pointer…<\/p><\/blockquote>\n

The assumption that NULL<\/code> is always a pointer causes problems with some functions. The takeaway here is that it’s always a good idea to typecast NULL<\/code> when you can.<\/p>\n

For example, if a function requires a NULL<\/code> pointer as an argument, specify it like this:<\/p>\n

function(data, (char *)NULL);<\/code><\/p>\n

For fictitious function()<\/em> above, the second argument is a char<\/em> pointer. Yet, many compilers can swallow NULL<\/code> by itself as the argument, so this statement might be okay:<\/p>\n

function(data, NULL);<\/code><\/p>\n

But to be safe consider typecasting NULL<\/code> to the proper pointer data type, as in:<\/p>\n

(char *)NULL<\/code>.<\/p>\n

When the data type is unknown, use the void<\/em> cast:<\/p>\n

(void *)NULL<\/code><\/p>\n

You may never experience an issue with NULL<\/code> on your compiler. It may be liberal enough that it understands the difference between a pointer and the integer constant zero. Still, safe coding practice is always good. Since learning about this difference, I’ve double-checked my code to ensure that NULL<\/code>s are typecast wherever possible.<\/p>\n","protected":false},"excerpt":{"rendered":"

The defined constant NULL<\/code> isn’t zero, but is it always a pointer? Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-4699","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4699","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=4699"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4699\/revisions"}],"predecessor-version":[{"id":4718,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4699\/revisions\/4718"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4699"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4699"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4699"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}