在Ubuntu中的unistd_64.h

Question

unistd_64 as my understanding (with lots of limited) contains the system call number. When I search the file from terminal, it shows more than one results under different directories as below:

1. /usr/include/x86_64-linux-gnu/asm/unistd_64.h
2. /usr/src/linux-headers-3.5.0-23/arch/sh/include/asm/unistd_64.h
3. /usr/src/linux-headers-3.5.0-23-generic/arch/x86/include/generated/ asm/.unistd_64.h.cmd
4. /usr/src/linux-headers-3.5.0-23-generic/arch/x86/include/generated/asm/unistd_64.h

I don't understand the difference between these files and the use of each file. And the file number 3 has .cmd, what does it mean?

Answer 1

If you are writing an ordinary C program that needs to know system call numbers, you should not use any of those headers. Instead, you should use <sys/syscall.h> . Your C program does not need to know the full pathname of this header; #include <sys/syscall.h> is all that is necessary. However, if you want to read it, it will be found somewhere in /usr/include , probably either /usr/include/sys/syscall.h or /usr/include/x86_64-linux-gnu/sys/syscall.h .

Now, I will explain the files you found:

• /usr/include/x86_64-linux-gnu/asm/unistd_64.h : This is a header file that may be used internally by sys/syscall.h . You can read it, but do not include it directly in your program. It probably defines a whole bunch of names that begin with __NR_ . Those names should never be used in an ordinary, "userspace" program: always use the names beginning with SYS_ instead.

• /usr/src/linux-headers-3.5.0-23/arch/sh/include/asm/unistd_64.h and /usr/src/linux-headers-3.5.0-23-generic/arch/x86/include/generated/asm/unistd_64.h : These are private kernel headers. They exist for the sake of people trying to build kernel modules that are developed separately from the kernel proper. It's possible that one of them is textually the same as /usr/include/x86_64-linux-gnu/asm/unistd_64 but that is not something you should rely on.

• /usr/src/linux-headers-3.5.0-23-generic/arch/x86/include/generated/ asm/.unistd_64.h.cmd : This is not a header file at all, it is used by the Linux kernel's build system.

Answer 2

The first file, which resides in /usr/include (the system include directory) is the one you would include.

The others reside in /usr/src , which is a source code directory that should not be referenced.