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Re: C library and C implement

Eric Sosman
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On 1/1/2014 9:31 AM, gyl wrote:
> I am not cleared with the concept of "language library" and "language
> implement".
> Here are my questions:
> 1. what is a C library used for?

To accomplish things that can't be done in unaided C itself,
or things that would be tedious and expensive if you had to re-
implement them for every program that needed them. For an example
of the first, consider the exit() function: The incantations that
shut down programs in Windows, Unix, i5/OS, OpenVMS, ... are all
different, and can't be done given only `for' and `switch' and so
on. For an example of the latter, consider the atan2() function:
It is more convenient (and less costly, and less error-prone) to
use the pre-written and pre-tested atan2() function from the library
than to write your own arctangent every time you need one.

> 2. Is it true that a C implement contains a C library?

Every hosted C implementation provides the library functions
(and macros and so on) specified by the Standard. Many provide
additional libraries for things like graphics or networking or
other areas not addressed by the Standard.

A "freestanding" C implementation need not provide all of the
Standard library; indeed, it might provide none at all.

> 3. What is POSIX? is it a library standard?

There's lots of information at <>.

> 4. What is the difference between glibc and MSVC's c runtime library?

What is the difference between a Ford and a Jeep? Both are
automobiles, but built by different companies and with different
purposes and audiences in mind.

> 5. How can I do some real programming with a C library?

Write C, and you'll find yourself using the Standard library
as soon as you call printf() -- or exit(), or sqrt(), or malloc(),
or ...

Using other libraries -- whether provided by third parties or
composed of your own components -- depends a lot on the environment:
Different tool sets have different ways of saying "Put this into my
program, please." In most C environments, your source code will use
the #include directive to obtain the compile-time declarations and
stuff that the library components need, and at "link time" you will
indicate where the pre-compiled library can be found. Variations

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