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SYSCALLS(2)               Linux Programmer's Manual              SYSCALLS(2)

NAME         top

       syscalls - Linux system calls

SYNOPSIS         top

       Linux system calls.

DESCRIPTION         top

       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 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.

       *  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 3.15 (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)                  2.6.11
       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)
       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)
       creat(2)                    1.0
       create_module(2)                          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
       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           1.0
       ftruncate(2)                1.0
       ftruncate64(2)              2.4
       futex(2)                    2.6
       futimesat(2)                2.6.16
       get_kernel_syms(2)                        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
       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
       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_load(2)               2.6.13
       keyctl(2)                   2.6.11
       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
       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
       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 called
                                                 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
       poll(2)                     2.0.36; 2.2
       ppc_rtas(2)                               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
       prlimit(2)                  2.6.36
       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
       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)              2.6.11
       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
       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
       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)                                  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
       stime(2)                    1.0
       subpage_prot(2)             2.6.25        PowerPC if
                                                 CONFIG_PPC_64K_PAGES
       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        Architecture-specific
                                                 variant of
                                                 sync_file_range(2)
       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
       utime(2)                    1.0
       utimensat(2)                2.6.22
       utimes(2)                   2.2
       utrap_install(2)            2.2           Sparc
       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
       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         top

       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 superseded by newer ones, and this has been treated somewhat
       unsystematically.  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, 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 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 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         top

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

COLOPHON         top

       This page is part of release 3.72 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-07-08                      SYSCALLS(2)