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 those system calls that are common to most
platforms. 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.
* 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 2.6.28 (or in
a few cases only on older kernels) is as follows:
System call Kernel Notes
-----------------------------------------------------------------------
_llseek(2) 1.2
_newselect(2)
_sysctl(2)
accept(2)
accept4(2) 2.6.28
access(2)
acct(2)
add_key(2) 2.6.11
adjtimex(2)
afs_syscall(2) Not implemented
alarm(2)
alloc_hugepages(2) 2.5.36 Removed in 2.5.44
bdflush(2)
bind(2)
break(2) Not implemented
brk(2)
cacheflush(2) 1.2 Not on i386
capget(2) 2.2
capset(2) 2.2
chdir(2)
chmod(2)
chown(2)
chown32(2) 2.4
chroot(2)
clock_getres(2) 2.6
clock_gettime(2) 2.6
clock_nanosleep(2) 2.6
clock_settime(2) 2.6
clone(2)
close(2)
connect(2)
creat(2)
create_module(2)
delete_module(2)
dup(2)
dup2(2)
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)
exit(2)
exit_group(2) 2.6
faccessat(2) 2.6.16
fadvise64(2) 2.6
fadvise64_64(2) 2.6
fallocate(2) 2.6.23
fchdir(2)
fchmod(2)
fchmodat(2) 2.6.16
fchown(2)
fchown32(2) 2.4
fchownat(2) 2.6.16
fcntl(2)
fcntl64(2) 2.4
fdatasync(2)
fgetxattr(2) 2.6; 2.4.18
flistxattr(2) 2.6; 2.4.18
flock(2) 2.0
fork(2)
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)
fstat64(2) 2.4
fstatat64(2) 2.6.16
fstatfs(2)
fstatfs64(2) 2.6
fsync(2)
ftime(2) Not implemented
ftruncate(2)
ftruncate64(2) 2.4
futex(2) 2.6
futimesat(2) 2.6.16
get_kernel_syms(2)
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)
getegid32(2) 2.4
geteuid(2)
geteuid32(2) 2.4
getgid(2)
getgid32(2) 2.4
getgroups(2)
getgroups32(2) 2.4
getitimer(2)
getpeername(2)
getpagesize(2) 2.0 Not on i386
getpgid(2)
getpgrp(2)
getpid(2)
getpmsg(2) Not implemented
getppid(2)
getpriority(2)
getresgid(2) 2.2
getresgid32(2) 2.4
getresuid(2) 2.2
getresuid32(2) 2.4
getrlimit(2)
getrusage(2)
getsid(2) 2.0
getsockname(2)
getsockopt(2)
gettid(2) 2.4.11
gettimeofday(2)
getuid(2)
getuid32(2) 2.4
getxattr(2) 2.6; 2.4.18
gtty(2) Not implemented
idle(2) Not implemented
init_module(2)
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)
ioperm(2)
iopl(2)
ioprio_get(2) 2.6.13
ioprio_set(2) 2.6.13
ipc(2)
kexec_load(2) 2.6.7
keyctl(2) 2.6.11
kill(2)
lchown(2) 2.2
lchown32(2) 2.4
lgetxattr(2) 2.6; 2.4.18
link(2)
linkat(2) 2.6.16
listen(2)
listxattr(2) 2.6; 2.4.18
llistxattr(2) 2.6; 2.4.18
lock(2) Not implemented
lookup_dcookie(2) 2.6
lremovexattr(2) 2.6; 2.4.18
lseek(2)
lsetxattr(2) 2.6; 2.4.18
lstat(2)
lstat64(2) 2.4
madvise(2) 2.4
madvise1(2) 2.4
mbind(2) 2.6.6
migrate_pages(2) 2.6.16
mincore(2) 2.4
mkdir(2)
mkdirat(2) 2.6.16
mknod(2)
mknodat(2) 2.6.16
mlock(2)
mlockall(2)
mmap(2)
mmap2(2) 2.4
modify_ldt(2)
mount(2)
move_pages(2) 2.6.18
mprotect(2)
mpx(2) Not implemented
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)
mremap(2) 2.0
msgctl(2)
msgget(2)
msgrcv(2)
msgsnd(2)
msync(2) 2.0
munlock(2)
munlockall(2)
munmap(2)
nanosleep(2) 2.0
nfsservctl(2) 2.2
nice(2)
oldfstat(2)
oldlstat(2)
oldolduname(2)
oldstat(2)
olduname(2)
open(2)
openat(2) 2.6.16
pause(2)
pciconfig_iobase(2) 2.2.15; 2.4 Not on i386
pciconfig_read(2) 2.0.26; 2.2 Not on i386
pciconfig_write(2) 2.0.26; 2.2 Not on i386
personality(2) 1.2
phys(2) Not implemented
pipe(2)
pipe2(2) 2.6.27
pivot_root(2) 2.4
poll(2) 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
prof(2) Not implemented
profil(2) Not implemented
pselect6(2) 2.6.16
ptrace(2)
putpmsg(2) Not implemented
pwrite64(2) Added as "pwrite" in 2.2;
renamed "pwrite64" in 2.6
pwritev(2) 2.6.30
query_module(2) 2.2
quotactl(2)
read(2)
readahead(2) 2.4.13
readdir(2)
readlink(2)
readlinkat(2) 2.6.16
readv(2) 2.0
reboot(2)
recv(2)
recvfrom(2)
recvmsg(2)
remap_file_pages(2) 2.6
removexattr(2) 2.6; 2.4.18
rename(2)
renameat(2) 2.6.16
request_key(2) 2.6.11
restart_syscall(2) 2.6
rmdir(2)
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
sched_get_priority_max(2) 2.0
sched_get_priority_min(2) 2.0
sched_getaffinity(2) 2.6
sched_getparam(2) 2.0
sched_getscheduler(2) 2.0
sched_rr_get_interval(2) 2.0
sched_setaffinity(2) 2.6
sched_setparam(2) 2.0
sched_setscheduler(2) 2.0
sched_yield(2) 2.0
security(2) Not implemented
select(2)
semctl(2)
semget(2)
semop(2)
semtimedop(2) 2.6; 2.4.22
send(2)
sendfile(2) 2.2
sendfile64(2) 2.6; 2.4.19
sendmsg(2)
sendto(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_zone_reclaim(2) 2.6.13 Removed in 2.6.16 (was never
available to userspace)
setdomainname(2)
setfsgid(2) 1.2
setfsgid32(2) 2.4
setfsuid(2) 1.2
setfsuid32(2) 2.4
setgid(2)
setgid32(2) 2.4
setgroups(2)
setgroups32(2) 2.4
sethostname(2)
setitimer(2)
setpgid(2)
setpriority(2)
setregid(2)
setregid32(2) 2.4
setresgid(2) 2.2
setresgid32(2) 2.4
setresuid(2) 2.2
setresuid32(2) 2.4
setreuid(2)
setreuid32(2) 2.4
setrlimit(2)
setsid(2)
setsockopt(2)
settimeofday(2)
setuid(2)
setuid32(2) 2.4
setup(2) Removed in 2.2
setxattr(2) 2.6; 2.4.18
sgetmask(2)
shmat(2)
shmctl(2)
shmdt(2)
shmget(2)
shutdown(2)
sigaction(2)
sigaltstack(2) 2.2
signal(2)
signalfd(2) 2.6.22
signalfd4(2) 2.6.27
sigpending(2)
sigprocmask(2)
sigreturn(2)
sigsuspend(2)
socket(2)
socketcall(2)
socketpair(2)
splice(2) 2.6.17
spu_create(2) 2.6.16 PowerPC only
spu_run(2) 2.6.16 PowerPC only
ssetmask(2)
stat(2)
stat64(2) 2.4
statfs(2)
statfs64(2) 2.6
stime(2)
stty(2) Not implemented
swapoff(2)
swapon(2)
symlink(2)
symlinkat(2) 2.6.16
sync(2)
sync_file_range(2) 2.6.17
sysfs(2) 1.2
sysinfo(2)
syslog(2)
tee(2) 2.6.17
tgkill(2) 2.6
time(2)
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)
tkill(2) 2.6; 2.4.22
truncate(2)
truncate64(2) 2.4
tuxcall(2) Not implemented
ugetrlimit(2) 2.4
ulimit(2) Not implemented
umask(2)
umount(2)
umount2(2) 2.2
uname(2)
unlink(2)
unlinkat(2) 2.6.16
unshare(2) 2.6.16
uselib(2)
ustat(2)
utime(2)
utimensat(2) 2.6.22
utimes(2) 2.6
vfork(2)
vhangup(2)
vm86old(2)
vmsplice(2) 2.6.17
vserver(2) Not implemented
wait4(2)
waitid(2) 2.6.10
waitpid(2)
write(2)
writev(2) 2.0
On many platforms, including i386, socket calls are all multiplexed
(via glibc wrapper functions) through socketcall(2) and similarly
System V IPC calls are multiplexed through ipc(2).
NOTES
Note the following points:
* 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.
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
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 OS 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, there are now 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), ftruncate64(2), getdents64(2), stat64(2), statfs64(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
(e.g., alpha, ia64, s390x) there are no *64 or *32 calls. 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 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
syscall(2), unimplemented(2), libc(7)
COLOPHON
This page is part of release 3.24 of the Linux man-pages project. A
description of the project, and information about reporting bugs, can
be found at http://www.kernel.org/doc/man-pages/.