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 chdir() which invokes the
underlying "chdir" 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 on architectures without a separate error
register/flag, as noted in syscall(2); 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; and the 5.x
kernel series, which followed on from kernel 4.20.
* 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 5.11 (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 Removed in 5.5
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
arc_gettls(2) 3.9 ARC only
arc_settls(2) 3.9 ARC only
arc_usr_cmpxchg(2) 4.9 ARC only
arch_prctl(2) 2.6 x86_64, x86 since 4.12
atomic_barrier(2) 2.6.34 m68k only
atomic_cmpxchg_32(2) 2.6.34 m68k only
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
breakpoint(2) 2.2 ARM OABI only, defined with
__ARM_NR prefix
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
clone2(2) 2.4 IA-64 only
clone(2) 1.0
clone3(2) 5.3
close(2) 1.0
close_range(2) 5.9
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_pwait2(2) 5.11
epoll_wait(2) 2.6
eventfd(2) 2.6.22
eventfd2(2) 2.6.27
execv(2) 2.0 SPARC/SPARC64 only, for
compatibility with SunOS
execve(2) 1.0
execveat(2) 3.19
exit(2) 1.0
exit_group(2) 2.6
faccessat(2) 2.6.16
faccessat2(2) 5.8
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
fsconfig(2) 5.2
fsetxattr(2) 2.6; 2.4.18
fsmount(2) 5.2
fsopen(2) 5.2
fspick(2) 5.2
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
get_tls(2) 4.15 ARM OABI only, has __ARM_NR
prefix
getcpu(2) 2.6.19
getcwd(2) 2.2
getdents(2) 2.0
getdents64(2) 2.4
getdomainname(2) 2.2 SPARC, SPARC64; available
as osf_getdomainname(2) on
Alpha since Linux 2.0
getdtablesize(2) 2.0 SPARC (removed in 2.6.26),
available on Alpha as
osf_getdtablesize(2)
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
gethostname(2) 2.0 Alpha, was available on
SPARC up to Linux 2.6.26
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 IA-64 only; deprecated
getxattr(2) 2.6; 2.4.18
getxgid(2) 2.0 Alpha only; see NOTES
getxpid(2) 2.0 Alpha only; see NOTES
getxuid(2) 2.0 Alpha only; see NOTES
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_pgetevents(2) 4.18
io_setup(2) 2.6
io_submit(2) 2.6
io_uring_enter(2) 5.1
io_uring_register(2) 5.1
io_uring_setup(2) 5.1
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 only
kexec_file_load(2) 3.17
kexec_load(2) 2.6.13
keyctl(2) 2.6.10
kill(2) 1.0
landlock_add_rule(2) 5.13
landlock_create_ruleset(2) 5.13
landlock_restrict_self(2) 5.13
landlock_add_rule(2) 5.13
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
memory_ordering(2) 2.2 SPARC64 only
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_mount(2) 5.2
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
newfstatat(2) 2.6.16 See stat(2)
nfsservctl(2) 2.2 Removed in 3.1
nice(2) 1.0
old_adjtimex(2) 2.0 Alpha only; see NOTES
old_getrlimit(2) 2.4 Old variant of getrlimit(2)
that used a different value
for RLIM_INFINITY
oldfstat(2) 1.0
oldlstat(2) 1.0
oldolduname(2) 1.0
oldstat(2) 1.0
oldumount(2) 2.4.116 Name of the old umount(2)
syscall on Alpha
olduname(2) 1.0
open(2) 1.0
open_by_handle_at(2) 2.6.39
open_tree(2) 5.2
openat(2) 2.6.16
openat2(2) 5.6
or1k_atomic(2) 3.1 OpenRISC 1000 only
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 only; removed in
2.6.34
perfmonctl(2) 2.4 IA-64 only; removed in 5.10
pidfd_getfd(2) 5.6
pidfd_send_signal(2) 5.1
pidfd_open(2) 5.3
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
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_madvise(2) 5.10
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
quotactl_fd(2) 5.14
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
riscv_flush_icache(2) 4.15 RISC-V only
rmdir(2) 1.0
rseq(2) 4.18
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
rtas(2) 2.6.2 PowerPC/PowerPC64 only
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
s390_sthyi(2) 4.15 s390 only
s390_guarded_storage(2) 4.12 s390 only
sched_get_affinity(2) 2.6 Name of
sched_getaffinity(2) on
SPARC and SPARC64
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_set_affinity(2) 2.6 Name of
sched_setaffinity(2) on
SPARC and SPARC64
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
set_tls(2) 2.6.11 ARM OABI/EABI only
(constant has __ARM_NR
prefix)
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
sethae(2) 2.0 Alpha only; see NOTES
sethostname(2) 1.0
setitimer(2) 1.0
setns(2) 3.0
setpgid(2) 1.0
setpgrp(2) 2.0 Alternative name for
setpgid(2) on Alpha
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)
spill(2) 2.6.13 Xtensa only
splice(2) 2.6.17
spu_create(2) 2.6.16 PowerPC/PowerPC64 only
spu_run(2) 2.6.16 PowerPC/PowerPC64 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/PowerPC64 only
swapcontext(2) 2.6.3 PowerPC/PowerPC64 only
switch_endian(2) 4.1 PowerPC64 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
sys_debug_setcontext(2) 2.6.11 PowerPC only
syscall(2) 1.0 Still available on ARM OABI
and MIPS O32 ABI
sysfs(2) 1.2
sysinfo(2) 1.0
syslog(2) 1.0
sysmips(2) 2.6.0 MIPS only
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
usr26(2) 2.4.8.1 ARM OABI only
usr32(2) 2.4.8.1 ARM OABI only
utime(2) 1.0
utimensat(2) 2.6.22
utimes(2) 2.2
utrap_install(2) 2.2 SPARC64 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
xtensa(2) 2.6.13 Xtensa only
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.
System calls on removed ports
Some system calls only ever existed on Linux architectures that
have since been removed from the kernel:
AVR32 (port removed in Linux 4.12)
* pread(2)
* pwrite(2)
Blackfin (port removed in Linux 4.17)
* bfin_spinlock(2) (added in Linux 2.6.22)
* dma_memcpy(2) (added in Linux 2.6.22)
* pread(2) (added in Linux 2.6.22)
* pwrite(2) (added in Linux 2.6.22)
* sram_alloc(2) (added in Linux 2.6.22)
* sram_free(2) (added in Linux 2.6.22)
Metag (port removed in Linux 4.17)
* metag_get_tls(2) (add in Linux 3.9)
* metag_set_fpu_flags(2) (add in Linux 3.9)
* metag_set_tls(2) (add in Linux 3.9)
* metag_setglobalbit(2) (add in Linux 3.9)
Tile (port removed in Linux 4.17)
* cmpxchg_badaddr(2) (added in Linux 2.6.36)
