[SOLVED] Difference between Library API and System Call API
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Most of the time, code that invokes a system-call occurs within a library. This is done for two reasons: to conceal differences that ought to make no difference, and to provide "the rest of" the functionality needed to do this-or-that beyond the low level stuff provided directly by the operating system.
A true system-call always involves issuing some kind of (implementation specific) instruction that passes through a one-way gate into the privileged-mode world of the operating system. Macros and other features are provided for doing this, since the actual manner of invoking the gate is CPU-type and even CPU-model specific.
Last edited by sundialsvcs; 12-11-2012 at 08:38 AM.
Well, obviously, procedure names like fork() are historic names. Also consider that they are, most logically speaking, "subroutine calls."
Now, it so happens that the implementation of this function involves one-or-more system calls. But do you need to be concerned with "which ones," or even, from one implementation to another, "how many" or "what they do?" No. And that's why the library exists ... so that you can just call what you need to call, to do what you need to do, and "it just works."
System-calls are actually intended to be very low-level interfaces, to very specific functionality which your program cannot accomplish on its own. They're focused, "raw," "unfriendly," and maybe implementation-specific. They're part of the necessary voodoo that gets things done. The library functions that "wrap" them are the rest of the story ... and often, the part that actually does most of the work.
When you call 'fork' for example, it is actually a wrapper function in libc, that fills some registers and executes a special machine instruction (platform dependent, INT 80H on x86) to actually call the kernel.