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NAME
syscalls - Linux system calls
SYNOPSIS
Linux system calls.
DESCRIPTION
The system call is the fundamental interface between an application and the Linux kernel.
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()
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.
System call list
Below is a list of the Linux system calls. 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, and the 4.x kernel series, which followed on from
kernel 3.19.
* 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 4.4 (or in a few cases only on
older kernels) is as follows:
System call Kernel Notes
────────────────────────────────────────────────────────────────────
_llseek(2) 1.2
_newselect(2) 2.0
_sysctl(2) 2.0
accept(2) 2.0 See notes on socketcall(2)
accept4(2) 2.6.28
access(2) 1.0
acct(2) 1.0
add_key(2) 2.6.11
adjtimex(2) 1.0
alarm(2) 1.0
alloc_hugepages(2) 2.5.36 Removed in 2.5.44
bdflush(2) 1.2 Deprecated (does nothing)
since 2.6
bind(2) 2.0 See notes on socketcall(2)
bpf(2) 3.18
brk(2) 1.0
cacheflush(2) 1.2 Not on x86
capget(2) 2.2
capset(2) 2.2
chdir(2) 1.0
chmod(2) 1.0
chown(2) 2.2 See chown(2) for
version details
chown32(2) 2.4
chroot(2) 1.0
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) 1.0
close(2) 1.0
connect(2) 2.0 See notes on socketcall(2)
creat(2) 1.0
create_module(2) 1.0 Removed in 2.6
delete_module(2) 1.0
dup(2) 1.0
dup2(2) 1.0
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) 1.0
execveat(2) 3.19
exit(2) 1.0
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) 1.0
fchmod(2) 1.0
fchmodat(2) 2.6.16
fchown(2) 1.0
fchown32(2) 2.4
fchownat(2) 2.6.16
fcntl(2) 1.0
fcntl64(2) 2.4
fdatasync(2) 2.0
fgetxattr(2) 2.6; 2.4.18
finit_module(2) 3.8
flistxattr(2) 2.6; 2.4.18
flock(2) 2.0
fork(2) 1.0
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) 1.0
fstat64(2) 2.4
fstatat64(2) 2.6.16
fstatfs(2) 1.0
fstatfs64(2) 2.6
fsync(2) 1.0
ftruncate(2) 1.0
ftruncate64(2) 2.4
futex(2) 2.6
futimesat(2) 2.6.16
get_kernel_syms(2) 1.0 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) 1.0
getegid32(2) 2.4
geteuid(2) 1.0
geteuid32(2) 2.4
getgid(2) 1.0
getgid32(2) 2.4
getgroups(2) 1.0
getgroups32(2) 2.4
getitimer(2) 1.0
getpeername(2) 2.0 See notes on socketcall(2)
getpagesize(2) 2.0 Not on x86
getpgid(2) 1.0
getpgrp(2) 1.0
getpid(2) 1.0
getppid(2) 1.0
getpriority(2) 1.0
getrandom(2) 3.17
getresgid(2) 2.2
getresgid32(2) 2.4
getresuid(2) 2.2
getresuid32(2) 2.4
getrlimit(2) 1.0
getrusage(2) 1.0
getsid(2) 2.0
getsockname(2) 2.0 See notes on socketcall(2)
getsockopt(2) 2.0 See notes on socketcall(2)
gettid(2) 2.4.11
gettimeofday(2) 1.0
getuid(2) 1.0
getuid32(2) 2.4
getxattr(2) 2.6; 2.4.18
init_module(2) 1.0
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) 1.0
ioperm(2) 1.0
iopl(2) 1.0
ioprio_get(2) 2.6.13
ioprio_set(2) 2.6.13
ipc(2) 1.0
kcmp(2) 3.5
kern_features(2) 3.7 Sparc64
kexec_file_load(2) 3.17
kexec_load(2) 2.6.13
keyctl(2) 2.6.11
kill(2) 1.0
lchown(2) 1.0 See chown(2) for
version details
lchown32(2) 2.4
lgetxattr(2) 2.6; 2.4.18
link(2) 1.0
linkat(2) 2.6.16
listen(2) 2.0 See notes on socketcall(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) 1.0
lsetxattr(2) 2.6; 2.4.18
lstat(2) 1.0
lstat64(2) 2.4
madvise(2) 2.4
mbind(2) 2.6.6
memfd_create(2) 3.17
migrate_pages(2) 2.6.16
mincore(2) 2.4
mkdir(2) 1.0
mkdirat(2) 2.6.16
mknod(2) 1.0
mknodat(2) 2.6.16
mlock(2) 2.0
mlock2(2) 4.4
mlockall(2) 2.0
mmap(2) 1.0
mmap2(2) 2.4
modify_ldt(2) 1.0
mount(2) 1.0
move_pages(2) 2.6.18
mprotect(2) 1.0
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) 2.6.6
mremap(2) 2.0
msgctl(2) 2.0 See notes on ipc(2)
msgget(2) 2.0 See notes on ipc(2)
msgrcv(2) 2.0 See notes on ipc(2)
msgsnd(2) 2.0 See notes on ipc(2)
msync(2) 2.0
munlock(2) 2.0
munlockall(2) 2.0
munmap(2) 1.0
name_to_handle_at(2) 2.6.39
nanosleep(2) 2.0
nfsservctl(2) 2.2 Removed in 3.1
nice(2) 1.0
oldfstat(2) 1.0
oldlstat(2) 1.0
oldolduname(2) 1.0
oldstat(2) 1.0
olduname(2) 1.0
open(2) 1.0
open_by_handle_at(2) 2.6.39
openat(2) 2.6.16
pause(2) 1.0
pciconfig_iobase(2) 2.2.15; 2.4 Not on x86
pciconfig_read(2) 2.0.26; 2.2 Not on x86
pciconfig_write(2) 2.0.26; 2.2 Not on x86
perf_event_open(2) 2.6.31 Was perf_counter_open() in
2.6.31; renamed in 2.6.32
personality(2) 1.2
perfctr(2) 2.2 Sparc; removed in 2.6.34
perfmonctl(2) 2.4 ia64
pipe(2) 1.0
pipe2(2) 2.6.27
pivot_root(2) 2.4
poll(2) 2.0.36; 2.2
ppc_rtas(2) 2.6.2 PowerPC only
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
prlimit64(2) 2.6.36
process_vm_readv(2) 3.2
process_vm_writev(2) 3.2
pselect6(2) 2.6.16
ptrace(2) 1.0
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) 1.0
read(2) 1.0
readahead(2) 2.4.13
readdir(2) 1.0
readlink(2) 1.0
readlinkat(2) 2.6.16
readv(2) 2.0
reboot(2) 1.0
recv(2) 2.0 See notes on socketcall(2)
recvfrom(2) 2.0 See notes on socketcall(2)
recvmsg(2) 2.0 See notes on socketcall(2)
recvmmsg(2) 2.6.33
remap_file_pages(2) 2.6 Deprecated since 3.16
removexattr(2) 2.6; 2.4.18
rename(2) 1.0
renameat(2) 2.6.16
renameat2(2) 3.15
request_key(2) 2.6.11
restart_syscall(2) 2.6
rmdir(2) 1.0
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
s390_runtime_instr(2) 3.7 s390 only
s390_pci_mmio_read(2) 3.19 s390 only
s390_pci_mmio_write(2) 3.19 s390 only
sched_get_priority_max(2) 2.0
sched_get_priority_min(2) 2.0
sched_getaffinity(2) 2.6
sched_getattr(2) 3.14
sched_getparam(2) 2.0
sched_getscheduler(2) 2.0
sched_rr_get_interval(2) 2.0
sched_setaffinity(2) 2.6
sched_setattr(2) 3.14
sched_setparam(2) 2.0
sched_setscheduler(2) 2.0
sched_yield(2) 2.0
seccomp(2) 3.17
select(2) 1.0
semctl(2) 2.0 See notes on ipc(2)
semget(2) 2.0 See notes on ipc(2)
semop(2) 2.0 See notes on ipc(2)
semtimedop(2) 2.6; 2.4.22
send(2) 2.0 See notes on socketcall(2)
sendfile(2) 2.2
sendfile64(2) 2.6; 2.4.19
sendmmsg(2) 3.0
sendmsg(2) 2.0 See notes on socketcall(2)
sendto(2) 2.0 See notes on socketcall(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
setdomainname(2) 1.0
setfsgid(2) 1.2
setfsgid32(2) 2.4
setfsuid(2) 1.2
setfsuid32(2) 2.4
setgid(2) 1.0
setgid32(2) 2.4
setgroups(2) 1.0
setgroups32(2) 2.4
sethostname(2) 1.0
setitimer(2) 1.0
setns(2) 3.0
setpgid(2) 1.0
setpriority(2) 1.0
setregid(2) 1.0
setregid32(2) 2.4
setresgid(2) 2.2
setresgid32(2) 2.4
setresuid(2) 2.2
setresuid32(2) 2.4
setreuid(2) 1.0
setreuid32(2) 2.4
setrlimit(2) 1.0
setsid(2) 1.0
setsockopt(2) 2.0 See notes on socketcall(2)
settimeofday(2) 1.0
setuid(2) 1.0
setuid32(2) 2.4
setup(2) 1.0 Removed in 2.2
setxattr(2) 2.6; 2.4.18
sgetmask(2) 1.0
shmat(2) 2.0 See notes on ipc(2)
shmctl(2) 2.0 See notes on ipc(2)
shmdt(2) 2.0 See notes on ipc(2)
shmget(2) 2.0 See notes on ipc(2)
shutdown(2) 2.0 See notes on socketcall(2)
sigaction(2) 1.0
sigaltstack(2) 2.2
signal(2) 1.0
signalfd(2) 2.6.22
signalfd4(2) 2.6.27
sigpending(2) 1.0
sigprocmask(2) 1.0
sigreturn(2) 1.0
sigsuspend(2) 1.0
socket(2) 2.0 See notes on socketcall(2)
socketcall(2) 1.0
socketpair(2) 2.0 See notes on socketcall(2)
splice(2) 2.6.17
spu_create(2) 2.6.16 PowerPC only
spu_run(2) 2.6.16 PowerPC only
ssetmask(2) 1.0
stat(2) 1.0
stat64(2) 2.4
statfs(2) 1.0
statfs64(2) 2.6
stime(2) 1.0
subpage_prot(2) 2.6.25 PowerPC only
swapoff(2) 1.0
swapon(2) 1.0
symlink(2) 1.0
symlinkat(2) 2.6.16
sync(2) 1.0
sync_file_range(2) 2.6.17
sync_file_range2(2) 2.6.22
syncfs(2) 2.6.39
sysfs(2) 1.2
sysinfo(2) 1.0
syslog(2) 1.0
tee(2) 2.6.17
tgkill(2) 2.6
time(2) 1.0
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) 1.0
tkill(2) 2.6; 2.4.22
truncate(2) 1.0
truncate64(2) 2.4
ugetrlimit(2) 2.4
umask(2) 1.0
umount(2) 1.0
umount2(2) 2.2
uname(2) 1.0
unlink(2) 1.0
unlinkat(2) 2.6.16
unshare(2) 2.6.16
uselib(2) 1.0
ustat(2) 1.0
userfaultfd(2) 4.2
utime(2) 1.0
utimensat(2) 2.6.22
utimes(2) 2.2
utrap_install(2) 2.2 Sparc only
vfork(2) 2.2
vhangup(2) 1.0
vm86old(2) 1.0 Was "vm86"; renamed in
2.0.28/2.2
vm86(2) 2.0.28; 2.2
vmsplice(2) 2.6.17
wait4(2) 1.0
waitid(2) 2.6.10
waitpid(2) 1.0
write(2) 1.0
writev(2) 2.0
On many platforms, including x86-32, 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.
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.
NOTES
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, certain architectures (notably,
longstanding 32-bit architectures such as i386) now have 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),
getdents64(2), stat64(2), statfs64(2), truncate64(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/GIDs (e.g., alpha,
ia64, s390x, x86-64), there is just a single version of the UID/GID and file access
system calls. On platforms (typically, 32-bit platforms) 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 in 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.
SEE ALSO
intro(2), syscall(2), unimplemented(2), errno(3), libc(7), vdso(7)
COLOPHON
This page is part of release 4.04 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this page, can be
found at http://www.kernel.org/doc/man-pages/.