intro(2) System Calls Manual intro(2)
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 li-
brary wrapper functions which perform the steps required (e.g., trapping
to kernel mode) in order to invoke the system call. Thus, making a sys-
tem 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 gen-
erally 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 li-
brary wrapper hides this detail from the caller: when a system call re-
turns 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 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 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 further in-
formation 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 copy-
right conditions. Note that these can be different from page to page!
SEE ALSO
_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), symlink(7), system_data_types(7),
sysvipc(7), time(7)
Linux man-pages 6.9.1 2024-05-02 intro(2)
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