syscalls - Linux system calls
Linux system calls.
The system call is the fundamental interface between an application and the
System calls and library wrapper functions¶
System calls are generally not invoked directly, but rather via wrapper
functions in glibc (or perhaps some other library). For details of direct
invocation of a system call, see intro(2)
. Often, but not always, the
name of the wrapper function is the same as the name of the system call that
it invokes. For example, glibc contains a function truncate
invokes the underlying "truncate" system call.
Often the glibc wrapper function is quite thin, doing little work other than
copying arguments to the right registers before invoking the system call, and
then setting errno
appropriately after the system call has returned.
(These are the same steps that are performed by syscall(2)
, which can
be used to invoke system calls for which no wrapper function is provided.)
Note: system calls indicate a failure by returning a negative error number to
the caller; when this happens, the wrapper function negates the returned error
number (to make it positive), copies it to errno
, and returns -1 to the
caller of the wrapper.
Sometimes, however, the wrapper function does some extra work before invoking
the system call. For example, nowadays there are (for reasons described below)
two related system calls, truncate(2)
, and the
() wrapper function checks which of those system calls
are provided by the kernel and determines which should be employed.
System call list¶
Below is a list of those system calls that are common to most platforms. In the
list, the Kernel
column indicates the kernel version for those system
calls that were new in Linux 2.2, or have appeared since that kernel version.
Note the following points:
- Where no kernel version is indicated, the system call
appeared in kernel 1.0 or earlier.
- Where a system call is marked "1.2" this means
the system call probably appeared in a 1.1.x kernel version, and first
appeared in a stable kernel with 1.2. (Development of the 1.2 kernel was
initiated from a branch of kernel 1.0.6 via the 1.1.x unstable kernel
- Where a system call is marked "2.0" this means
the system call probably appeared in a 1.3.x kernel version, and first
appeared in a stable kernel with 2.0. (Development of the 2.0 kernel was
initiated from a branch of kernel 1.2.x, somewhere around 1.2.10, via the
1.3.x unstable kernel series.)
- Where a system call is marked "2.2" this means
the system call probably appeared in a 2.1.x kernel version, and first
appeared in a stable kernel with 2.2.0. (Development of the 2.2 kernel was
initiated from a branch of kernel 2.0.21 via the 2.1.x unstable kernel
- Where a system call is marked "2.4" this means
the system call probably appeared in a 2.3.x kernel version, and first
appeared in a stable kernel with 2.4.0. (Development of the 2.4 kernel was
initiated from a branch of kernel 2.2.8 via the 2.3.x unstable kernel
- Where a system call is marked "2.6" this means
the system call probably appeared in a 2.5.x kernel version, and first
appeared in a stable kernel with 2.6.0. (Development of kernel 2.6 was
initiated from a branch of kernel 2.4.15 via the 2.5.x unstable kernel
- Starting with kernel 2.6.0, the development model changed,
and new system calls may appear in each 2.6.x release. In this case, the
exact version number where the system call appeared is shown. This
convention continues with the 3.x kernel series, which followed on from
- In some cases, a system call was added to a stable kernel
series after it branched from the previous stable kernel series, and then
backported into the earlier stable kernel series. For example some system
calls that appeared in 2.6.x were also backported into a 2.4.x release
after 2.4.15. When this is so, the version where the system call appeared
in both of the major kernel series is listed.
The list of system calls that are available as at kernel 3.5 (or in a few cases
only on older kernels) is as follows:
On many platforms, including i386, socket calls are all multiplexed (via glibc
wrapper functions) through socketcall(2)
and similarly System V IPC
calls are multiplexed through ipc(2)
Although slots are reserved for them in the system call table, the following
system calls are not implemented in the standard kernel:
(2), and vserver(2)
(see also unimplemented(2)
library routines. The slot for phys(2)
is in use since kernel 2.1.116
will never be implemented. The
calls are for kernels patched to
support STREAMS, and may never be in the standard kernel.
There was briefly set_zone_reclaim
(2), added in Linux 2.6.13, and removed
in 2.6.16; this system call was never available to user space.
Roughly speaking, the code belonging to the system call with number __NR_xxx
defined in /usr/include/asm/unistd.h
can be found in the Linux kernel
source in the routine sys_xxx
(). (The dispatch table for i386 can be
found in /usr/src/linux/arch/i386/kernel/entry.S
.) There are many
exceptions, however, mostly because older system calls were superseded by
newer ones, and this has been treated somewhat unsystematically. On platforms
with proprietary operating-system emulation, such as parisc, sparc, sparc64
and alpha, there are many additional system calls; mips64 also contains a full
set of 32-bit system calls.
Over time, changes to the interfaces of some system calls have been necessary.
One reason for such changes was the need to increase the size of structures or
scalar values passed to the system call. Because of these changes, there are
now various groups of related system calls (e.g., truncate(2)
) which perform similar tasks, but which vary in details
such as the size of their arguments. (As noted earlier, applications are
generally unaware of this: the glibc wrapper functions do some work to ensure
that the right system call is invoked, and that ABI compatibility is preserved
for old binaries.) Examples of systems calls that exist in multiple versions
are the following:
- By now there are three different versions of
stat(2): sys_stat() (slot __NR_oldstat),
sys_newstat() (slot __NR_stat), and sys_stat64()
(slot __NR_stat64), with the last being the most current. A similar
story applies for lstat(2) and fstat(2).
- Similarly, the defines __NR_oldolduname,
__NR_olduname, and __NR_uname refer to the routines
sys_olduname(), sys_uname() and sys_newuname().
- In Linux 2.0, a new version of vm86(2) appeared,
with the old and the new kernel routines being named sys_vm86old()
- In Linux 2.4, a new version of getrlimit(2)
appeared, with the old and the new kernel routines being named
sys_old_getrlimit() (slot __NR_getrlimit) and
sys_getrlimit() (slot __NR_ugetrlimit).
- Linux 2.4 increased the size of user and group IDs from 16
to 32 bits. To support this change, a range of system calls were added
(e.g., chown32(2), getuid32(2), getgroups32(2),
setresuid32(2)), superseding earlier calls of the same name without
the "32" suffix.
- Linux 2.4 added support for applications on 32-bit
architectures to access large files (i.e., files for which the sizes and
file offsets can't be represented in 32 bits.) To support this change,
replacements were required for system calls that deal with file offsets
and sizes. Thus the following system calls were added: fcntl64(2),
ftruncate64(2), getdents64(2), stat64(2),
statfs64(2), and their analogs that work with file descriptors or
symbolic links. These system calls supersede the older system calls which,
except in the case of the "stat" calls, have the same name
without the "64" suffix.
On newer platforms that only have 64-bit file access and 32-bit uids (e.g.,
alpha, ia64, s390x) there are no *64 or *32 calls. Where the *64 and *32
calls exist, the other versions are obsolete.
- The rt_sig* calls were added in kernel 2.2 to
support the addition of real-time signals (see signal(7)). These
system calls supersede the older system calls of the same name without the
- The select(2) and mmap(2) system calls use
five or more arguments, which caused problems the way argument passing on
the i386 used to be set up. Thus, while other architectures have
sys_select() and sys_mmap() corresponding to
__NR_select and __NR_mmap, on i386 one finds
old_select() and old_mmap() (routines that use a pointer to
a argument block) instead. These days passing five arguments is not a
problem any more, and there is a __NR__newselect that corresponds
directly to sys_select() and similarly __NR_mmap2.
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