NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.
In newer C++ (C++11 and higher), use nullptr (as pointed out in a comment, thanks).
Otherwise, add
#include <stddef.h>
to get the NULL definition.
NULL is not a built-in constant in the C or C++ languages. In fact, in C++ it's more or less obsolete, just use a plain literal 0 instead, the compiler will do the right thing depending on the context.
In newer C++ (C++11 and higher), use nullptr (as pointed out in a comment, thanks).
Otherwise, add
#include <stddef.h>
to get the NULL definition.
Do use NULL. It is just #defined as 0 anyway and it is very useful to semantically distinguish it from the integer 0.
There are problems with using 0 (and hence NULL). For example:
void f(int);
void f(void*);
f(0); // Ambiguous. Calls f(int).
The next version of C++ (C++0x) includes nullptr to fix this.
f(nullptr); // Calls f(void*).
'NULL' undeclared (first use in this function)
c - NUL undeclared- first use in this function - Stack Overflow
'NULL' undeclared...
error C2065: Undeclared Identifier
There's NULL and then there's NUL.
NULL is defined in stddef.h, is used very widely, and is a reference to a null pointer.
NUL is different - it is the first character in the standard ASCII character set, and more importantly, it is not a standard macro. You may have to define it yourself.
To define NUL, do:
#define NUL '\0'
No, that's not standard. Add this at the beginning of your code or just use 0:
#define NUL 0
I infered you're not looking for NULL since you're doing ch == NUL
In various texts it is quite frequent to refer to '\0' as NUL.
C 2011 Standard, online draft
6.3.2.3 Pointers
...
3 An integer constant expression with the value 0, or such an expression cast to typevoid *, is called a null pointer constant.66) If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
66) The macroNULLis defined in<stddef.h>(and other headers) as a null pointer constant; see 7.19.
The macro NULL is always defined as a zero-valued constant expression; it can be a naked 0, or 0 cast to void *, or some other integral expression that evaluates to 0. As far as your source code is concerned, NULL will always evaluate to 0.
Once the code has been translated, any occurrence of the null pointer constant (0, NULL, etc.) will be replaced with whatever the underlying architecture uses for a null pointer, which may or may not be 0-valued.
WhozCraig wrote these comments to a now-deleted answer, but it could be promoted to a full answer (and that's what I've done here). He notes:
Interesting note: AS/400 is a very unique platform where any non-valid pointer is considered equivalent to
NULL. The mechanics which they employ to do this are simply amazing. "Valid" in this sense is any 128-bit pointer (the platform uses a 128bit linear address space for everything) containing a "value" obtained by a known-trusted instruction set. Hard as it is to believe,int *p = (int *)1; if (p) { printf("foo"); }will not print "foo" on that platform. The value assigned to p is not source-trusted, and thus, considered "invalid" and thereby equivalent toNULL.It's frankly startling how it works. Each 16-byte paragraph in the mapped virtual address space of a process has a corresponding "bit" in a process-wide bitmap. All pointers must reside on one of these paragraph boundaries. If the bit is "lit", the corresponding pointer was stored from a trusted source, otherwise it is invalid and equivalent to NULL. Calls to malloc, pointer math, etc, are all scrutinized in determining whether that bit gets lit or not. And as you can imagine, putting pointers in structures brings a whole new world of hurt on the idea of structure packing.
This is marked community-wiki (it's not my answer — I shouldn't get the credit) but it can be deleted if WhozCraig writes his own answer.
What this shows is that there are real platforms with interesting pointer properties.
There have been platforms where #define NULL ((void *)0) is not the usual definition; on some platforms it can be just 0, on others, 0L or 0ULL or other appropriate values as long as the compiler understands it. C++ does not like ((void *)0) as a definition; systems where the headers interwork with C++ may well not use the void pointer version.
I learned C on a machine where the representation for the char * address for a given memory location was different from the int * address for the same memory location. This was in the days before void *, but it meant that you had to have malloc() properly declared (char *malloc(); — no prototypes either), and you had to explicitly cast the return value to the correct type or you got core dumps. Be grateful for the C standard (though the machine in question, an ICL Perq — badged hardware from Three Rivers — was largely superseded by the time the standard was defined).
I use the C/C++ extension for VSCode and I wanted to give a shot to c23, gcc does recognize nullptr but the VSCode IDE doesn't, and even tho my code works, I dont like the error message in the IDE. Any solutions??