syscalls man page

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 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.
  • 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 4.19 (or in a few cases only on older kernels) is as follows:

System callKernelNotes
_llseek(2)1.2
_newselect(2)2.0
_sysctl(2)2.0
accept(2)2.0See 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)2.5.36Removed in 2.5.44
arc_gettls(2)3.9ARC only
arc_settls(2)3.9ARC only
arc_usr_cmpxchg(2)4.9ARC only
arch_prctl(2)2.6x86_64, x86 since 4.12
atomic_barrier(2)2.6.34m68k only
atomic_cmpxchg_32(2)2.6.34m68k only
bdflush(2)1.2Deprecated (does nothing)
since 2.6
bfin_spinlock(2)2.6.22Blackfin only (port removed
in Linux 4.17)
bind(2)2.0See notes on socketcall(2)
bpf(2)3.18
brk(2)1.0
breakpoint(2)2.2ARM OABI only, defined with
__ARM_NR prefix
cacheflush(2)1.2Not on x86
capget(2)2.2
capset(2)2.2
chdir(2)1.0
chmod(2)1.0
chown(2)2.2See 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)2.4IA-64 only
clone(2)1.0
close(2)1.0
cmpxchg_badaddr(2)2.6.36Tile only (port removed
in Linux 4.17)
connect(2)2.0See notes on socketcall(2)
copy_file_range(2)4.5
creat(2)1.0
create_module(2)1.0Removed in 2.6
delete_module(2)1.0
dma_memcpy(2)2.6.22Blackfin only (port removed
in Linux 4.17)
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
execv(2)2.0SPARC/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
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)2.5.36Removed 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)1.0Removed 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)4.15ARM 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)2.2SPARC, SPARC64; available
as osf_getdomainname(2)
on Alpha since Linux 2.0
getdtablesize(2)2.0SPARC (removed in 2.6.26),
available since Linux 2.0 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)2.0Alpha, was available on
SPARC up to Linux 2.6.26
getitimer(2)1.0
getpeername(2)2.0See notes on socketcall(2)
getpagesize(2)2.0Not 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)2.0See notes on socketcall(2)
getsockopt(2)2.0See notes on socketcall(2)
gettid(2)2.4.11
gettimeofday(2)1.0
getuid(2)1.0
getuid32(2)2.4
getunwind(2)2.4.8IA-64 only; deprecated
getxattr(2)2.6; 2.4.18
getxgid(2)2.0Alpha only; see Notes
getxpid(2)2.0Alpha only; see Notes
getxuid(2)2.0Alpha 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
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)3.7SPARC64 only
kexec_file_load(2)3.17
kexec_load(2)2.6.13
keyctl(2)2.6.10
kill(2)1.0
lchown(2)1.0See 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)2.0See 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)2.2SPARC64 only
metag_get_tls(2)3.9Metag only (port removed
in Linux 4.17)
metag_set_fpu_flags(2)3.9Metag only (port removed
in Linux 4.17)
metag_set_tls(2)3.9Metag only (port removed
in Linux 4.17)
metag_setglobalbit(2)3.9Metag only (port removed
in Linux 4.17)
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)2.0See notes on ipc(2)
msgget(2)2.0See notes on ipc(2)
msgrcv(2)2.0See notes on ipc(2)
msgsnd(2)2.0See 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)2.6.16See stat(2)
nfsservctl(2)2.2Removed in 3.1
nice(2)1.0
old_adjtimex(2)2.0Alpha only; see Notes
old_getrlimit(2)2.4Old 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)2.4.116Name of the old umount(2)
syscall on Alpha
olduname(2)1.0
open(2)1.0
open_by_handle_at(2)2.6.39
openat(2)2.6.16
or1k_atomic(2)3.1OpenRISC 1000 only
pause(2)1.0
pciconfig_iobase(2)2.2.15; 2.4Not on x86
pciconfig_read(2)2.0.26; 2.2Not on x86
pciconfig_write(2)2.0.26; 2.2Not on x86
perf_event_open(2)2.6.31Was perf_counter_open() in
2.6.31; renamed in 2.6.32
personality(2)1.2
perfctr(2)2.2SPARC only; removed in 2.6.34
perfmonctl(2)2.4IA-64 only
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
pread(2)Used for pread64(2) on AVR32
(port removed in Linux 4.12)
and Blackfin (port removed
in Linux 4.17)
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
pwrite(2)Used for pwrite64(2) on AVR32 (port removed in Linux 4.12) and Blackfin (port removed in Linux 4.17)
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)2.2Removed 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)2.0See notes on socketcall(2)
recvfrom(2)2.0See notes on socketcall(2)
recvmsg(2)2.0See notes on socketcall(2)
recvmmsg(2)2.6.33
remap_file_pages(2)2.6Deprecated 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)4.15RISC-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)2.6.2PowerPC/PowerPC64 only
s390_runtime_instr(2)3.7s390 only
s390_pci_mmio_read(2)3.19s390 only
s390_pci_mmio_write(2)3.19s390 only
s390_sthyi(2)4.15s390 only
s390_guarded_storage(2)4.12s390 only
sched_get_affinity(2)2.6Name 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)2.6Name 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)2.0See notes on ipc(2)
semget(2)2.0See notes on ipc(2)
semop(2)2.0See notes on ipc(2)
semtimedop(2)2.6; 2.4.22
send(2)2.0See notes on socketcall(2)
sendfile(2)2.2
sendfile64(2)2.6; 2.4.19
sendmmsg(2)3.0
sendmsg(2)2.0See notes on socketcall(2)
sendto(2)2.0See 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)2.6.11ARM 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)2.0Alpha only; see Notes
sethostname(2)1.0
setitimer(2)1.0
setns(2)3.0
setpgid(2)1.0
setpgrp(2)2.0Alternative 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)2.0See notes on socketcall(2)
settimeofday(2)1.0
setuid(2)1.0
setuid32(2)2.4
setup(2)1.0Removed in 2.2
setxattr(2)2.6; 2.4.18
sgetmask(2)1.0
shmat(2)2.0See notes on ipc(2)
shmctl(2)2.0See notes on ipc(2)
shmdt(2)2.0See notes on ipc(2)
shmget(2)2.0See notes on ipc(2)
shutdown(2)2.0See 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)2.0See notes on socketcall(2)
socketcall(2)1.0
socketpair(2)2.0See notes on socketcall(2)
spill(2)2.6.13Xtensa only
splice(2)2.6.17
spu_create(2)2.6.16PowerPC/PowerPC64 only
spu_run(2)2.6.16PowerPC/PowerPC64 only
sram_alloc(2)2.6.22Blackfin (port removed
in Linux 4.17)
sram_free(2)2.6.22Blackfin (port removed
in Linux 4.17)
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)2.6.25PowerPC/PowerPC64 only
swapcontext(2)2.6.3PowerPC/PowerPC64 only
switch_endian(2)4.1PowerPC64 only
swapcontext(2)2.6.3PowerPC 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)2.6.11PowerPC only
syscall(2)1.0Still available on ARM OABI
and MIPS O32 ABI
sysfs(2)1.2
sysinfo(2)1.0
syslog(2)1.0
sysmips(2)2.6.0MIPS 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)2.4.8.1ARM OABI only
usr32(2)2.4.8.1ARM OABI only
utime(2)1.0
utimensat(2)2.6.22
utimes(2)2.2
utrap_install(2)2.2SPARC64 only
vfork(2)2.2
vhangup(2)1.0
vm86old(2)1.0Was "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)2.6.13Xtensa 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.

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(). 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 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:

Architecture-specific details: Alpha

  • getxgid(2) returns a pair of GID and effective GID via registers r0 and r20; it is provided instead of getgid(2) and getegid(2).
  • getxpid(2) returns a pair of PID and parent PID via registers r0 and r20; it is provided instead of getpid(2) and getppid(2).
  • old_adjtimex(2) is a variant of adjtimex(2) that uses struct timeval32, for compatibility with OSF/1.
  • getxuid(2) returns a pair of GID and effective GID via registers r0 and r20; it is provided instead of getuid(2) and geteuid(2).
  • sethae(2) is used for configuring the Host Address Extension register on low-cost Alphas in order to access address space beyond first 27 bits.

See Also

intro(2), syscall(2), unimplemented(2), errno(3), libc(7), vdso(7)

Colophon

This page is part of release 5.02 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/.

Referenced By

intro(2), libc(7), man-pages(7), stapprobes.3stap(3), syscall(2), unimplemented(2), vdso(7).

2019-03-06 Linux Programmer's Manual