Re: Unnecessary hard drive spin-ups

["George N. White III" <gnwiii@xxxxxxxxx>]

On Thu, 13 Feb 2020 at 11:35, Andy Paterson via users <users@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

Except the system call version doesn't need you to include any header files!

Which is NOT clear from the man page!

The man page says "On Linux, the kernel provides a getcwd() system call, which the functions 

described in this page will use if possible." so it seems applications are meant to use the

header files and let the library do the system call "if possible".

 

On 13 Feb 2020, at 13:53, George N. White III <gnwiii@xxxxxxxxx> wrote:



On Thu, 13 Feb 2020 at 06:40, Andy Paterson via users <users@xxxxxxxxxxxxxxxxxxxxxxx> wrote:

[...] 

I am a little confused by getcwd(), on linux it is supposed to be a system call .. I expect to find a manual page for it in section 2 (man 2 getcwd)

instead it gives me the library function manual page from section 3 (man 3 getcwd)

$ man -k getcwd

getcwd (2)           - get current working directory

getcwd (3)           - get current working directory

getcwd (3p)          - get the pathname of the current working directory

$ gzip -dc /usr/share/man/man2/getcwd.2.gz

.so man3/getcwd.3

 

I would fully expect a libc version of getcwd to make many system calls to traverse the current path but here getcwd() is a system call with all work done internally in the kernel, so I wouldnt expect anything to probe the path elements.

When I knock up a simple program to do a getcwd() and strace it I find that getcwd (on FC31 anyway) IS a system call.

Why does "man getcwd" give me a page from section 3?

The man page explains the differences, so it makes sense that "man 2" and "man 3" give the same text:

 C library/kernel differences

       On Linux, the kernel provides a getcwd() system call, which  the  func‐

       tions  described  in  this  page will use if possible.  The system call

       takes the same arguments as the library function of the same name,  but

       is  limited  to  returning at most PATH_MAX bytes. 

The getcwd() system call saves overhead but should make the same storage accesses as the

library version.

--

George N. White III

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George N. White III

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