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 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 4.11 (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) 2.6.10
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)
copy_file_range(2) 4.5
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
getunwind(2) 2.4.8 ia64; deprecated
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) 2.6.10
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
membarrier(2) 3.17
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
pkey_alloc(2) 4.8
pkey_free(2) 4.8
pkey_mprotect(2) 4.8
poll(2) 2.0.36; 2.2
ppc_rtas(2) 2.6.2 PowerPC only
ppc_swapcontext(2) 2.6.3 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
preadv2(2) 4.6
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
pwritev2(2) 4.6
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) 2.6.10
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
statx(2) 4.11
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) 4.3
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.
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 an 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.
intro(2), syscall(2), unimplemented(2), errno(3), libc(7), vdso(7)
This page is part of release 4.12 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
https://www.kernel.org/doc/man-pages/.
Linux 2017-05-03 SYSCALLS(2)
Pages that refer to this page: intro(2), syscall(2), unimplemented(2), stapprobes(3stap), libc(7), man-pages(7), vdso(7)