INTRO(2) Linux Programmer's Manual INTRO(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 library func-
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).
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 val-
ues from a successful call. The details are described in the individ-
ual 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 speci-
fied 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 fur-
ther 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).
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
_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),
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