I have read several times before that the length of the data type in C depends on the architecture on the PC.

But recently I have read documentation of a compiler and this documentation specifies the data types that you can access and the length of this data type.

So my question is, the data type length depends on the compiler your using, the architecture of the computer or both?

CodePudding user response：

The final determination of the size of each type is up to the C implementation, subject to the constraints of the C standard. (The standard specifies things such as that char is one “byte,” and a byte is eight bits or more, and that certain types are the same size as each other.)

A C implementation typically consists of a compiler, header files, libraries, a linker, and a target system to run on, and possibly other components. The compiler has a large influence on the characteristics of types, especially the core types of C, but sometimes header files complete the definitions of types, so that one compiler may be used with different sets of header files that provide different types.

Most C implementations are influenced by the characteristics of the execution architecture they are designed for. An int will often be the same size as a general processor register, and a pointer (of any type) will often be the smallest size that is a power-of-two number of bytes and has enough bits to span the address space of the target architecture.

However, C implementations may be designed for other purposes. I have seen a C implementation that used a 32-bit pointer size for a 64-bit architecture, for the purpose of reducing memory usage in programs that had a great many pointers. A C implementation could also be designed to run older C code on a new processor, so the C implementation used the sizes of types that the older code was written for rather than sizes that would be more “natural” on the new processor.

CodePudding user response：

It can depend on either. Of course the executable produced by the compiler will need to run efficiently on the CPU architecture that it is compiled for, so if (for example) you are cross-compiling software to run it on a 16-bit microcontroller, than it is very likely that sizeof(int)==2 in the compiled code, even if you are using a modern 64-bit desktop PC to do the compilation.

Simultaneously to that, it's also up to the compiler writers (within limits) to decide what sizes various non-width-specific data types should have in their compiler. So for example, some compilers set sizeof(int)==4 while others set sizeof(int)==8, for reasons particular to their own history and user's needs.