Section: Linux Programmer's Manual (2)
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
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;
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
On error, most system calls return a negative error number
(i.e., the negated value of one of the constants described in
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
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
In such cases, the required macro is described in the man page.
For further information on feature test macros, see
Certain terms and abbreviations are used to indicate UNIX variants
and standards to which calls in this section conform.
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
Historically, this was also possible using one of the _syscall macros
Authors and copyright conditions
Look at the header of the manual page source for the author(s) and copyright
Note that these can be different from page to page!
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