What is the difference between the various unistd.h under /usr/include in Linux?

Under the /usr/include directory in Linux i entered the command: find -type f -name unistd.h which gave the following output:

./unistd.h ./linux/unistd.h ./asm-generic/unistd.h ./bits/unistd.h ./asm/unistd.h ./sys/unistd.h

my question is, what is the purpose of each unistd.h, since there is only one definiton of that file in th开发者_如何学编程e single unix specification ?

Thanks in advance.

linux/unistd.h actually points to asm/unistd.h, which in turn points to either asm/unistd_32.h or asm/unistd_64.h, which is where system call numbers are defined and presented to user space depending on the system's architecture. These come from the kernel.

bits/unistd.h is a collection of macros that augment unistd.h (mostly stuff to help prevent buffer overflows), which is conditionally included via:

/* Define some macros helping to catch buffer overflows.  */
#if __USE_FORTIFY_LEVEL > 0 && defined __extern_always_inline
# include <bits/unistd.h>
#endif

In essence, the only POSIX required header is in fact, just unistd.h, the rest are either extensions, or definitions from the kernel. So, just including unistd.h is all you have to worry about doing, everything you need will be pulled in depending on your architecture and whatever build options you've selected.

It's a common technique in C and C++ - you have a single file with the "standard" name in the "standard" place, in this case ./unistd.h, and then have that file include one or more implementation specific files, depending on preprocessor macros. If you look at almost any "standard" C or C++ header files, you will see it including other files not mentioned in any standard.

Basically think of /usr/include/unistd.h as a smart symbolic link. It will point to a correct implementation depending on what your operating conditions are.

That said, it makes difficult sometimes to figure out what that correct implementation is.