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While I was doing some reading on system calls, I did a search for syscalls.h to find the header file in LXR. The search results puzzled me. There is a dozen of syscalls.h files coming from directories under arch/_arch_name_/include/asm. These are ok, they are architecture specific definitions or something else needed. The question is why do we have two different syscalls.h headers under both include/linux and include/asm-generic?

Also, I want to find out that what include/linux headers are for and what include/asm-generic headers are for. How do they differentiate between each other? What is the logic behind having two separate header folders? How do they relate to each other?

Thanks

Pavan Manjunath
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dirtybit
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    Did you look inside each these files? – Basile Starynkevitch Mar 21 '12 at 06:18
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    In /include/linux/syscalls.h, there are several system call declarations. In /include/asm-generic/syscalls.h, all system calls are declared with accompanying macro definitions. But this applies to only syscalls.h, however, i'd like to understand big picture. – dirtybit Mar 21 '12 at 06:46

2 Answers2

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I've asked this question on Kernel Newbies ML. I got the following answer which makes things clear:

Cihangir Akturk wrote:

AFAIK, headers found in /include/asm-generic directory is for architechture independent (probably shared across architectures) code. Most likely you do not need to include these headers directly, instead we include these headers indirectly via architecthure dependent headers.

OTOH, /include/linux directory is for generic header files to define interfaces between components of the kernel. In most situations, you will find all the kernel functionalities you need in these headers.

regards, chngr.

Jacob Groundwater
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dirtybit
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The headers in directories arch/_arch_name_/include/asm are specific architecture dependent, such as for x86, ARM architectures. So the logic inside is platform-specific and highly depends on the underlying hardware. You will only include what your platform architecture is.

Headers in include/linux are common and platform independent. They're common logics and will be shared cross architectures.

Wayne
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  • The other question is what is the reason of having include/asm-generic in addition to include/linux headers? What is the relation between them? – dirtybit Mar 22 '12 at 07:33
  • My opinion is `include/asm-generic` provides a common and uniform interface to access hardware-related data structure. `include/asm-generic` allows developers control the underlying hardware much easier and no need to dig out the hardware spec detail. For example, `include/asm-generic/rtc.h` provide a uniform interface to retrieve rtc information, but its implementation includes architecture-dependent code, such as `CMOS_WRITE` and `CMOS_READ`. – Wayne Mar 22 '12 at 13:40