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       intro - introduction to system calls


       Section 2 of the manual describes the Linux system calls.  A system call is an entry point
       into the Linux kernel.  Usually, system calls are  not  invoked  directly:  instead,  most
       system  calls  have  corresponding  C  library  wrapper  functions which perform the steps
       required (e.g., trapping to kernel mode) in order to invoke the system call.  Thus, making
       a system call looks the same as invoking a normal library function.

       In many cases, the C library wrapper function does nothing more than:

       *  copying  arguments  and the unique system call number to the registers where the kernel
          expects them;

       *  trapping to kernel mode, at which point the kernel does the real  work  of  the  system

       *  setting  errno  if  the system call returns an error number when the kernel returns the
          CPU to user mode.

       However, in a few cases, a wrapper function may do rather more  than  this,  for  example,
       performing  some  preprocessing  of  the  arguments  before  trapping  to  kernel mode, or
       postprocessing of values returned by the system call.  Where this is the case, the  manual
       pages  in  Section 2 generally try to note the details of both the (usually GNU) C library
       API interface and the raw system call.  Most commonly, the main DESCRIPTION will focus  on
       the  C  library  interface,  and  differences for the system call are covered in the NOTES

       For a list of the Linux system calls, see syscalls(2).


       On error, most system calls return a negative error number (i.e., the negated value of one
       of the constants described in errno(3)).  The C library wrapper hides this detail from the
       caller: when a system call returns a negative value, the wrapper copies the absolute value
       into the errno variable, and returns -1 as the return value of the wrapper.

       The  value  returned  by  a successful system call depends on the call.  Many system calls
       return 0 on success, but some can return nonzero  values  from  a  successful  call.   The
       details are described in the individual manual pages.

       In  some  cases,  the  programmer  must define a feature test macro in order to obtain the
       declaration of a system call from the header file  specified  in  the  man  page  SYNOPSIS
       section.   (Where required, these feature test macros must be defined before including any
       header files.)  In such cases, the required macro is  described  in  the  man  page.   For
       further information on feature test macros, see feature_test_macros(7).


       Certain  terms and abbreviations are used to indicate UNIX variants and standards to which
       calls in this section conform.  See standards(7).


   Calling directly
       In most cases, it is unnecessary to invoke a system call directly,  but  there  are  times
       when  the  Standard C library does not implement a nice wrapper function for you.  In this
       case, the programmer must manually invoke the system call using syscall(2).  Historically,
       this was also possible using one of the _syscall macros described in _syscall(2).

   Authors and copyright conditions
       Look  at  the header of the manual page source for the author(s) and copyright conditions.
       Note that these can be different from page to page!


       _syscall(2), syscall(2), syscalls(2), errno(3), intro(3), capabilities(7), credentials(7),
       feature_test_macros(7), mq_overview(7), path_resolution(7), pipe(7), pty(7),
       sem_overview(7), shm_overview(7), signal(7), socket(7), standards(7), svipc(7),
       symlink(7), time(7)


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