Why do C programmers use typedefs to rename basic types?

Question

Renaming types without changing their exposed semantics/characteristics doesn't make much sense. In your example

typedef unsigned char uch;
typedef unsigned long ulg;

belong to that category. I don't see the point, aside from making a shorter name.

But these ones

typedef uch UBYTE;
typedef unsigned int u32;
typedef signed short s16;

are a completely different story. For example, s16 stands for "signed 16 bit type". This type is not necessarily signed short. Which specific type will hide behind s16 is platform-dependent. Programmers introduce this extra level of naming indirection to simplify the support for multiple platforms. If on some other platform signed 16 bit type happens to be signed int, the programmer will only have to change one typedef definition. UBYTE apparently stands for an unsigned machine byte type, which is not necessarily unsigned char.

It's worth noting that the C99 specification already provides a standard nomenclature for integral types of specific width, like int16_t, uint32_t and so on. It probably makes more sense to stick with this standard naming convention on platforms that don't support C99.

This allows for portability. For example you need an unsigned 32-bit integer type. Which standard type is that? You don't know - it's implementation defined. That's why you typedef a separate type to be 32-bit unsigned integer and use the new type in your code. When you need to compile on another C implementation you just change the typedefs.

Sometimes it is used to reduce an unwieldy thing like volatile unsigned long to something a little more compact such as vuint32_t.

Other times it is to help with portability since types like int are not always the same on each platform. By using a typedef you can set the storage class you are interested in to the platform's closest match without changing all the source code.

There are many reasons to it. What I think is:

1. Typename becomes shorter and thus code also smaller and more readable.
2. Aliasing effect for longer structure names.
3. Convention used in particular team/companies/style.
4. Porting - Have same name across all OS and machine. Its native data-structure might be slightly different.

Following is a quote from The C Programming Language (K&R)

Besides purely aesthetic issues, there are two main reasons for using typedefs.

First- to parameterize a program

The first is to parameterize a program against portability problems. If typedefs are used for data types that may be machine-dependent, only the typedefs need change when the program is moved.

One common situation is to use typedef names for various integer quantities, then make an appropriate set of choices of short, int, and long for each host machine. Types like size_t and ptrdiff_t from the standard library are examples.

The italicized portions tells us that programmers typedef basic type for portability. If I want to make sure my program works on different platforms, using different compiler, I will try to ensure that its portability in every possible way and typedef is one of them.

When I started programming using Turbo C compiler on Windows platform, it gave us the size of int 2. When I moved to Linux platform and GCC complier, the size I get is 4. If I had developed a program using Turbo C which relied on the assertion that sizeof( int ) is always two, it would have not ported properly to my new platform.

Hope it helps.

Following quote from K&R is not related to your query but I have posted it too for the sake of completion.

Second- to provide better documentation

The second purpose of typedefs is to provide better documentation for a program - a type called Treeptr may be easier to understand than one declared only as a pointer to a complicated structure.