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intro() - Unix, Linux System Call
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
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
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 non-zero
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.
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 Terms
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!
This page is part of release 3.00 of the Linux
A description of the project,
and information about reporting bugs,
can be found at