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
- 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
values from a successful call. The details are described in the individual
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).
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).
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),
path_resolution(7), pipe(7), pty(7),
sem_overview(7), shm_overview(7), signal(7),
socket(7), standards(7), symlink(7), sysvipc(7),
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