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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 wrap-
per function is the same as the name of the system call that it invokes. For example, glibc contains a function trun-
cate() 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 2.6.33 (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) 2.6.7
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
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
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
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) 2.2
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).
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 unim-
plemented(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 ker-
nel.
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 emula-
tion, 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 sys-
tem calls that deal with file offsets and sizes. Thus the following system calls were added: fcntl64(2), ftrun-
cate64(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 cor-
responds directly to sys_select() and similarly __NR_mmap2.
SEE ALSO
syscall(2), unimplemented(2), libc(7)
COLOPHON
This page is part of release 3.25 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 2010-03-06 SYSCALLS(2)
