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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 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 argu‐
       ments 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 per‐
       formed by syscall(2), which can be used to invoke system calls for which no wrapper  func‐
       tion  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

       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

   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  ear‐

       *  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 ker‐
          nel 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 ker‐
          nel 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
       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
       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)                   2.6.11
       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
       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
       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
                                                 in 2.6.31; renamed in
       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
       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)                                  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
       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
       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
       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

       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 unimple‐
       mented(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.

       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/ker‐
       nel/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 plat‐
       forms 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  rou‐
          tines 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),  get‐
          groups32(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), 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" suf‐

          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 prob‐
          lems 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 sim‐
          ilarly __NR_mmap2.

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

       This  page  is  part of release 3.74 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                                       2014-10-02                                SYSCALLS(2)

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