Why is C platform-dependent? [duplicate]

Answer 1

Doesn't each architecture essentially go through the most basic commands in the same way?

For the most part. But those basic commands are represented or implemented differently. The C compiler is responsible for making sure that the correct representations and implementations are used.

Answer 2

Each try to abstract some features, like every language do, needs to be tuned to the concrete architecture, or create a virtual machine (for each architecture) that can run it.

Answer 3

1. Different CPUs have different instruction architectures (e.g., x86 vs ARM).
   • Early Macs used the Motorola 68k architecture; later ones used PowerPC; and still later ones used x86. During each of these transitions, developers had to ship their executables as fat binaries, which would contain object code for both architectures.
2. Current x86 CPUs have 32-bit and 64-bit modes.
   • This is why you have 32-bit and 64-bit versions of Windows, Ubuntu, etc.
3. Different operating systems provide different system calls, libraries, etc.
   • Different OS versions can provide different system calls, libraries, etc. also (although OS vendors do aim to be backward compatible as much as possible).
4. Even on the same operating system, the calling convention is not guaranteed to be the same between different compilers.
   • Even on the same OS, different executable file formats may be in use. For example, on many Unix systems, a.out used to be the format used, but most eventually switched to ELF. During the transition period, libraries had to be provided in both formats.

Answer 4

But every machine has its own instruction set

Answer 5

Yes, but all these loads and jumps etc. look different on different platforms. Each CPU family has its own command set.

On a higher level, if we have the same hardware but different OSes (such as Linux/Windows), we have different libraries, different executable file formats and so on.

All these are things which must be obeyed by the compiler and the linker.