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SYSCALLS(2) Linux Programmer's Manual SYSCALLS(2)
syscalls - Linux system calls
Linux system calls.
The system call is the fundamental interface between an application and the
Linux kernel.
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() which 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 truncate64(2),
and the glibc truncate() wrapper function checks which of those system
calls are provided by the kernel and determines which should be employed.
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 series.)
* 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 series.)
* 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 series.)
* 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 series.)
* 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 kernel
2.6.39.
* 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 3.1 (or in a few
cases only on older kernels) is as follows:
System call Kernel Notes
---------------------------------------------------------------
_llseek(2) 1.2
_newselect(2)
_sysctl(2)
accept(2)
accept4(2) 2.6.28
access(2)
acct(2)
add_key(2) 2.6.11
adjtimex(2)
alarm(2)
alloc_hugepages(2) 2.5.36 Removed in 2.5.44
bdflush(2) Deprecated (does
nothing) since 2.6
bind(2)
brk(2)
cacheflush(2) 1.2 Not on i386
capget(2) 2.2
capset(2) 2.2
chdir(2)
chmod(2)
chown(2)
chown32(2) 2.4
chroot(2)
clock_adjtime(2) 2.6.39
clock_getres(2) 2.6
clock_gettime(2) 2.6
clock_nanosleep(2) 2.6
clock_settime(2) 2.6
clone(2)
close(2)
connect(2)
creat(2)
create_module(2) Removed in 2.6
delete_module(2)
dup(2)
dup2(2)
dup3(2) 2.6.27
epoll_create(2) 2.6
epoll_create1(2) 2.6.27
epoll_ctl(2) 2.6
epoll_pwait(2) 2.6.19
epoll_wait(2) 2.6
eventfd(2) 2.6.22
eventfd2(2) 2.6.27
execve(2)
exit(2)
exit_group(2) 2.6
faccessat(2) 2.6.16
fadvise64(2) 2.6
fadvise64_64(2) 2.6
fallocate(2) 2.6.23
fanotify_init(2) 2.6.37
fanotify_mark(2) 2.6.37
fchdir(2)
fchmod(2)
fchmodat(2) 2.6.16
fchown(2)
fchown32(2) 2.4
fchownat(2) 2.6.16
fcntl(2)
fcntl64(2) 2.4
fdatasync(2)
fgetxattr(2) 2.6; 2.4.18
flistxattr(2) 2.6; 2.4.18
flock(2) 2.0
fork(2)
free_hugepages(2) 2.5.36 Removed in 2.5.44
fremovexattr(2) 2.6; 2.4.18
fsetxattr(2) 2.6; 2.4.18
fstat(2)
fstat64(2) 2.4
fstatat64(2) 2.6.16
fstatfs(2)
fstatfs64(2) 2.6
fsync(2)
ftruncate(2)
ftruncate64(2) 2.4
futex(2) 2.6
futimesat(2) 2.6.16
get_kernel_syms(2) Removed in 2.6
get_mempolicy(2) 2.6.6
get_robust_list(2) 2.6.17
get_thread_area(2) 2.6
getcpu(2) 2.6.19
getcwd(2) 2.2
getdents(2) 2.0
getdents64(2) 2.4
getegid(2)
getegid32(2) 2.4
geteuid(2)
geteuid32(2) 2.4
getgid(2)
getgid32(2) 2.4
getgroups(2)
getgroups32(2) 2.4
getitimer(2)
getpeername(2)
getpagesize(2) 2.0 Not on i386
getpgid(2)
getpgrp(2)
getpid(2)
getppid(2)
getpriority(2)
getresgid(2) 2.2
getresgid32(2) 2.4
getresuid(2) 2.2
getresuid32(2) 2.4
getrlimit(2)
getrusage(2)
getsid(2) 2.0
getsockname(2)
getsockopt(2)
gettid(2) 2.4.11
gettimeofday(2)
getuid(2)
getuid32(2) 2.4
getxattr(2) 2.6; 2.4.18
init_module(2)
inotify_add_watch(2) 2.6.13
inotify_init(2) 2.6.13
inotify_init1(2) 2.6.27
inotify_rm_watch(2) 2.6.13
io_cancel(2) 2.6
io_destroy(2) 2.6
io_getevents(2) 2.6
io_setup(2) 2.6
io_submit(2) 2.6
ioctl(2)
ioperm(2)
iopl(2)
ioprio_get(2) 2.6.13
ioprio_set(2) 2.6.13
ipc(2)
kexec_load(2) 2.6.13
keyctl(2) 2.6.11
kill(2)
lchown(2) 2.2
lchown32(2) 2.4
lgetxattr(2) 2.6; 2.4.18
link(2)
linkat(2) 2.6.16
listen(2)
listxattr(2) 2.6; 2.4.18
llistxattr(2) 2.6; 2.4.18
lookup_dcookie(2) 2.6
lremovexattr(2) 2.6; 2.4.18
lseek(2)
lsetxattr(2) 2.6; 2.4.18
lstat(2)
lstat64(2) 2.4
madvise(2) 2.4
madvise1(2) 2.4
mbind(2) 2.6.6
migrate_pages(2) 2.6.16
mincore(2) 2.4
mkdir(2)
mkdirat(2) 2.6.16
mknod(2)
mknodat(2) 2.6.16
mlock(2)
mlockall(2)
mmap(2)
mmap2(2) 2.4
modify_ldt(2)
mount(2)
move_pages(2) 2.6.18
mprotect(2)
mq_getsetattr(2) 2.6.6
mq_notify(2) 2.6.6
mq_open(2) 2.6.6
mq_timedreceive(2) 2.6.6
mq_timedsend(2) 2.6.6
mq_unlink(2)
mremap(2) 2.0
msgctl(2)
msgget(2)
msgrcv(2)
msgsnd(2)
msync(2) 2.0
munlock(2)
munlockall(2)
munmap(2)
name_to_handle_at(2) 2.6.39
nanosleep(2) 2.0
nfsservctl(2) 2.2 Removed in 3.1
nice(2)
oldfstat(2)
oldlstat(2)
oldolduname(2)
oldstat(2)
olduname(2)
open(2)
open_by_handle_at(2) 2.6.39
openat(2) 2.6.16
pause(2)
pciconfig_iobase(2) 2.2.15; 2.4 Not on i386
pciconfig_read(2) 2.0.26; 2.2 Not on i386
pciconfig_write(2) 2.0.26; 2.2 Not on i386
perf_event_open(2) 2.6.31 Was called
perf_counter_open()
in 2.6.31; renamed
in 2.6.32
personality(2) 1.2
pipe(2)
pipe2(2) 2.6.27
pivot_root(2) 2.4
poll(2) 2.2
ppoll(2) 2.6.16
prctl(2) 2.2
pread64(2) Added as "pread" in
2.2; renamed
"pread64" in 2.6
preadv(2) 2.6.30
prlimit(2) 2.6.36
process_vm_readv(2) 3.2
process_vm_writev(2) 3.2
pselect6(2) 2.6.16
ptrace(2)
pwrite64(2) Added as "pwrite" in
2.2; renamed
"pwrite64" in 2.6
pwritev(2) 2.6.30
query_module(2) 2.2 Removed in 2.6
quotactl(2)
read(2)
readahead(2) 2.4.13
readdir(2)
readlink(2)
readlinkat(2) 2.6.16
readv(2) 2.0
reboot(2)
recv(2)
recvfrom(2)
recvmsg(2)
recvmmsg(2) 2.6.33
remap_file_pages(2) 2.6
removexattr(2) 2.6; 2.4.18
rename(2)
renameat(2) 2.6.16
request_key(2) 2.6.11
restart_syscall(2) 2.6
rmdir(2)
rt_sigaction(2) 2.2
rt_sigpending(2) 2.2
rt_sigprocmask(2) 2.2
rt_sigqueueinfo(2) 2.2
rt_sigreturn(2) 2.2
rt_sigsuspend(2) 2.2
rt_sigtimedwait(2) 2.2
rt_tgsigqueueinfo(2) 2.6.31
sched_get_priority_max(2) 2.0
sched_get_priority_min(2) 2.0
sched_getaffinity(2) 2.6
sched_getparam(2) 2.0
sched_getscheduler(2) 2.0
sched_rr_get_interval(2) 2.0
sched_setaffinity(2) 2.6
sched_setparam(2) 2.0
sched_setscheduler(2) 2.0
sched_yield(2) 2.0
select(2)
semctl(2)
semget(2)
semop(2)
semtimedop(2) 2.6; 2.4.22
send(2)
sendfile(2) 2.2
sendfile64(2) 2.6; 2.4.19
sendmmsg(2) 3.0
sendmsg(2)
sendto(2)
set_mempolicy(2) 2.6.6
set_robust_list(2) 2.6.17
set_thread_area(2) 2.6
set_tid_address(2) 2.6
set_zone_reclaim(2) 2.6.13 Removed in 2.6.16
(was never available
to userspace)
setdomainname(2)
setfsgid(2) 1.2
setfsgid32(2) 2.4
setfsuid(2) 1.2
setfsuid32(2) 2.4
setgid(2)
setgid32(2) 2.4
setgroups(2)
setgroups32(2) 2.4
sethostname(2)
setitimer(2)
setns(2) 3.0
setpgid(2)
setpriority(2)
setregid(2)
setregid32(2) 2.4
setresgid(2) 2.2
setresgid32(2) 2.4
setresuid(2) 2.2
setresuid32(2) 2.4
setreuid(2)
setreuid32(2) 2.4
setrlimit(2)
setsid(2)
setsockopt(2)
settimeofday(2)
setuid(2)
setuid32(2) 2.4
setup(2) Removed in 2.2
setxattr(2) 2.6; 2.4.18
sgetmask(2)
shmat(2)
shmctl(2)
shmdt(2)
shmget(2)
shutdown(2)
sigaction(2)
sigaltstack(2) 2.2
signal(2)
signalfd(2) 2.6.22
signalfd4(2) 2.6.27
sigpending(2)
sigprocmask(2)
sigreturn(2)
sigsuspend(2)
socket(2)
socketcall(2)
socketpair(2)
splice(2) 2.6.17
spu_create(2) 2.6.16 PowerPC only
spu_run(2) 2.6.16 PowerPC only
ssetmask(2)
stat(2)
stat64(2) 2.4
statfs(2)
statfs64(2) 2.6
stime(2)
subpage_prot(2) 2.6.25 PowerPC if
CONFIG_PPC_64K_PAGES
swapoff(2)
swapon(2)
symlink(2)
symlinkat(2) 2.6.16
sync(2)
sync_file_range(2) 2.6.17
sync_file_range2(2) 2.6.22 Architecture-
specific variant of
sync_file_range(2)
syncfs(2) 2.6.39
sysfs(2) 1.2
sysinfo(2)
syslog(2)
tee(2) 2.6.17
tgkill(2) 2.6
time(2)
timer_create(2) 2.6
timer_delete(2) 2.6
timer_getoverrun(2) 2.6
timer_gettime(2) 2.6
timer_settime(2) 2.6
timerfd_create(2) 2.6.25
timerfd_gettime(2) 2.6.25
timerfd_settime(2) 2.6.25
times(2)
tkill(2) 2.6; 2.4.22
truncate(2)
truncate64(2) 2.4
ugetrlimit(2) 2.4
umask(2)
umount(2)
umount2(2) 2.2
uname(2)
unlink(2)
unlinkat(2) 2.6.16
unshare(2) 2.6.16
uselib(2)
ustat(2)
utime(2)
utimensat(2) 2.6.22
utimes(2) 2.2
vfork(2)
vhangup(2)
vm86old(2)
vmsplice(2) 2.6.17
wait4(2)
waitid(2) 2.6.10
waitpid(2)
write(2)
writev(2) 2.0
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:
afs_syscall(2), break(2), ftime(2), getpmsg(2), gtty(2), idle(2), lock(2),
madvise1(2), mpx(2), phys(2), prof(2), profil(2), putpmsg(2), security(2),
stty(2), tuxcall(2), ulimit(2), and vserver(2) (see also unimplemented(2)).
However, ftime(3), profil(3) and ulimit(3) exist as library routines. The
slot for phys(2) is in use since kernel 2.1.116 for umount(2); phys(2) will
never be implemented. The getpmsg(2) and putpmsg(2) calls are for kernels
patched to support STREAMS, and may never be in the standard kernel.
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 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 OS 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) and truncate64(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() and sys_vm86().
* 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 "rt_" prefix.
* 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.
syscall(2), unimplemented(2), libc(7)
This page is part of release 3.41 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can be
found at http://www.kernel.org/doc/man-pages/.
Linux 2012-03-23 SYSCALLS(2)
HTML rendering created 2012-05-11 by Michael Kerrisk,
author of
The Linux Programming Interface,
maintainer of the
Linux man-pages project