Why are particular C data types declared in more than one standard header file?

Question

For example, C11 dictates that size_t should be declared in the following header files:

• stddef.h
• stdio.h
• stdlib.h
• string.h
• time.h
• uchar.h
• wchar.h

When reading C11, I found there are many other data types declared in more than one standard header files.

Questions

1. Let's say in the case of size_t. Why not just in stddef.h for simplicity?
2. Let's say a C compiler implements size_t in those header files. Are they guaranteed to have the same definition in those header files?

Answer 1

As an example of a function declared in stdio.h that requires size_t be predeclared, consider snprintf(). As it is, if you want to use it in your code, all you need to do is #include <stdio.h>. If size_t were declared only in stddef.h, you would have to

#include <stddef.h> #include <stdio.h> 

Not only that, but since stdio.h declares snprintf whether you use it or not, you would have to include both files every time you needed anything in stdio.h to avoid compiler errors; stdio.h would have an artificial dependency on stddef.h. That causes your source code to become longer and more brittle (notice that if you reverse the order of the two directives it would also break). Instead, we write header files so that they stand alone and do not depend on other headers, and this is what the C standardization committee decided on for the standard library.

Answer 2

Let's say in case of size_t. Why not just in stddef.h for simplicity?

The type is used in the declaration of functions in all those files. If it wasn't declared in <stdio.h> you would get a compilation error unless you first include <stddef.h>.

Let's say a C compiler implements size_t in those header files. Are they guaranteed to have the same definition in those header files?

Yes, they will have the same definition. Usually, the value is defined in a single place in a separate include file that is included by the others.

In some cases it may be possible to modify the definition with compiler options or defines, for example a compiler that allows 32/64 bit compilation may define size_t as a 32 or 64 bit unsigned entity depending on the target defined on the compiler command line.