CODEWITHSUNDEEP
ccompilationuse-case
Peter Badida
Peter Badida

Reputation: 12189

What remains in C if I exclude libraries and compiler extensions?

Imagine a situation where you can't or don't want to use any of the libraries provided by the compiler as "standard", nor any external library. You can't use even the compiler extensions (such as gcc extensions).

What is the remaining part you get if you strip C language of all the things a lot of people use as a matter of course?

In such a way, probably a list of every callable function supported by any big C compiler (not only ANSI C) out-of-box would be satisfying as as answer as it'd at least approximately show the use-case of the language.

First I thought about sizeof() and printf() (those were already clarified in the comments - operator + stdio), so... what remains? In-line assembly seem like an extension too, so that pretty much strips even the option to use assembly with C if I'm right.

Probably in the matter of code it'd be easier to understand. Imagine a code compiled with only e.g. gcc main.c (output flag permitted) that has no #include, nor extern.

int main() {
    // replace_me
    return 0;
}

What can I call to actually do something else than "boring" type math and casting from type to type?

Note that switch, goto, if, loops and other constructs that do nothing and only allow repeating a piece of code aren't the thing I'm looking for (if it isn't obvious).

(Hopefully the edit clarified wtf I'm actually asking, but Matteo's answer pretty much did it.)

Upvotes: 0

Views: 293

Answers (3)

Clifford
Clifford

Reputation: 93486

C has no "built-in" functions as such. A compiler implementation may include "intrinsic" functions that are implemented directly by the compiler without provision of an external library, although a prototype declaration is still required for intrinsics, so you would still normally include a header file for such declarations.

C is a systems-level language with a minimal run-time and start-up requirement. Because it can directly access memory and memory mapped I/O there is very little that it cannot do (and what it cannot do is what you use assembly, in-line assembly or intrinsics for). For example, much of the library code you are wondering what you can do without is written in C. When running in an OS environment however (using C as an application-level rather then system-level language), you cannot practically use C in that manner - the OS has control over such things as I/O and memory-management and in modern systems will normally prevent unmediated access to such resources. Of course that OS itself is likely to largely written in C (and/or C++).

In a standalone of bare-metal environment with no OS, C is often used very early in the bootstrap process initialising hardware and establishing an application execution environment. In fact on ARM Cortex-M processors it is possible to boot directly into C code from reset, since the hardware loads an initial stack-pointer and start address from the vector table on start-up; this being enough to run C code that does not rely on library or static data initialisation - such initialisation can however be written in C before calling main().

Note that sizeof is not a function, it is an operator.

Upvotes: 3

Malcolm McLean
Malcolm McLean

Reputation: 6404

I don't think you really understand the situation.

You don't need a header to call a function in C. You can call with unchecked parameters - a bad idea and an obsolete feature, but still supported. And if a compiler links a library by default instead of only when you explicitly tell it to, that's only a little switch within the compiler to "link libc". Notoriously Unix compilers need to be told to link the math library, it wasn't linked by default because some very early programs didn't use floating point.

To be fair, some standard library functions like memcpy tend to be special-cased these days as they lend themselves to inlining and optimisation.

The standard library is documented and is usually available, though in effect deprecated by Microsoft for security reasons. You can write pretty much any function quite easily with only stdlib functions, what you can't do is fancy IO.

Upvotes: 0

Matteo Italia
Matteo Italia

Reputation: 126837

If you remove all libraries essentially you have something similar to a freestanding implementation of C (which still has to provide some libraries - say, string.h, but that's nothing you couldn't easily implement yourself in portable C), and that's what normally you start with when programming microcontrollers and other computers that don't have a ready-made operating system - and what operating system writers in general use when they compile their operating systems.

There you typically have two ways of doing stuff besides "raw" computation:

  • assembly blocks (where you can do literally anything the underlying machine can do);
  • memory mapped IO (you set a volatile pointer to some hardware dependent location and read/write from it; that affects hardware stuff).

That's really all you need to build anything - and after all, it all boils down to that stuff anyway, the C library of a regular hosted implementation is normally written in C itself, with some assembly used either for speed or to communicate with the operating system1 (typically the syscalls are invoked through some kind of interrupt).

Again, it's nothing you couldn't implement yourself. But the point of having a standard library is both to avoid to continuously reinvent the wheel, and to have a set of portable functions that spare you to have to rewrite everything knowing the details of each target platform.

  1. And mainstream operating systems, in turn, are generally written in a mix or C and assembly as well.

Upvotes: 7

Related Questions