How do I "get" a null pointer in my programs?

Answer: According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal:

char *p = 0;
if(p != 0)

However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 "means" a null pointer. For instance, the Unix system call "execl" takes a variable-length, null-pointer-terminated list of character pointer arguments. To generate a null pointer in a function call context, an explicit cast is typically required:

execl("/bin/sh", "sh", "-c", "ls", (char *)0);

If the (char *) cast were omitted, the compiler would not know to pass a null pointer, and would pass an integer 0 instead. (Note that many Unix manuals get this example wrong.)

When function prototypes are in scope, argument passing becomes an "assignment context," and most casts may safely be omitted, since the prototype tells the compiler that a pointer is required, and of which type, enabling it to correctly cast unadorned 0's. Function prototypes cannot provide the types for variable arguments in variable-length argument lists, however, so explicit casts are still  required for those arguments. It is safest always to cast null pointer function arguments, to guard against varargs functions or those without prototypes, to allow interim use of non-ANSI compilers, and to demonstrate that you know what you are doing.

Summary:
Unadorned 0 okay: Explicit cast required:
initialization function call, no prototype in scope assignment variable argument in comparison arargs function call, prototype in scope, fixed argument

References: K&R I Sec. A7.7 p. 190, Sec. A7.14 p. 192; K&R II
Sec. A7.10 p. 207, Sec. A7.17 p. 209; H&S Sec. 4.6.3 p. 72; ANSI
Sec. 3.2.2.3 .

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