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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 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.
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 2.6.33 (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) 2.6.13
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
perf_event_open(2) 2.6.31 Was called perf_counter_open()
in 2.6.31; renamed in 2.6.32
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
prlimit(2) 2.6.36
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)
recvmmsg(2) 2.6.33
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
rt_tgsigqueueinfo(2) 2.6.31
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
subpage_prot(2) 2.6.25 PowerPC if CONFIG_PPC_64K_PAGES
swapoff(2)
swapon(2)
symlink(2)
symlinkat(2) 2.6.16
sync(2)
sync_file_range(2) 2.6.17
sync_file_range2(2) 2.6.22 Architecture-specific variant
of sync_file_range(2)
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) 2.2
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).
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.
syscall(2), unimplemented(2), libc(7)
This page is part of release 3.32 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/.
Linux 2010-11-06 SYSCALLS(2)
HTML rendering created 2010-12-03 by Michael Kerrisk, author of The Linux Programming Interface