SYSCALLS(2) Linux Programmer's Manual SYSCALLS(2)
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 sys-
tem 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 wrap-
per 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 posi-
tive), 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 trun-
cate64(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 prob-
ably 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 initi-
ated 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 prob-
ably 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 initi-
ated 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 prob-
ably 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 initi-
ated 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 prob-
ably 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 initi-
ated 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 prob-
ably 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
s390_sthyi(2) 4.15 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 Sys-
tem V IPC calls are multiplexed through ipc(2).
Although slots are reserved for them in the system call table, the fol-
lowing 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 super-
seded by newer ones, and this has been treated somewhat unsystemati-
cally. 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 bina-
ries.) 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 cur-
rent. 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, replace-
ments were required for system calls that deal with file offsets and
sizes. Thus the following system calls were added: fcntl64(2), get-
dents64(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.
SEE ALSO
intro(2), syscall(2), unimplemented(2), errno(3), libc(7), vdso(7)
COLOPHON
This page is part of release 4.15 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/.
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