# intro

## NAME



           intro - introduction to system calls
    


## DESCRIPTION



           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 call;
    
           •  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 section.
    
           For a list of the Linux system calls, see syscalls(2).
    


## RETURN VALUE



           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).
    


## STANDARDS



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


## NOTES



       **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!
    


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