syscall(2) — Linux manual page


syscall(2)                 System Calls Manual                syscall(2)

NAME         top

       syscall - indirect system call

LIBRARY         top

       Standard C library (libc, -lc)

SYNOPSIS         top

       #include <sys/syscall.h>      /* Definition of SYS_* constants */
       #include <unistd.h>

       long syscall(long number, ...);

   Feature Test Macro Requirements for glibc (see

           Since glibc 2.19:
           Before glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION         top

       syscall() is a small library function that invokes the system
       call whose assembly language interface has the specified number
       with the specified arguments.  Employing syscall() is useful, for
       example, when invoking a system call that has no wrapper function
       in the C library.

       syscall() saves CPU registers before making the system call,
       restores the registers upon return from the system call, and
       stores any error returned by the system call in errno(3).

       Symbolic constants for system call numbers can be found in the
       header file <sys/syscall.h>.

RETURN VALUE         top

       The return value is defined by the system call being invoked.  In
       general, a 0 return value indicates success.  A -1 return value
       indicates an error, and an error number is stored in errno.

ERRORS         top

       ENOSYS The requested system call number is not implemented.

       Other errors are specific to the invoked system call.

NOTES         top

       syscall() first appeared in 4BSD.

   Architecture-specific requirements
       Each architecture ABI has its own requirements on how system call
       arguments are passed to the kernel.  For system calls that have a
       glibc wrapper (e.g., most system calls), glibc handles the
       details of copying arguments to the right registers in a manner
       suitable for the architecture.  However, when using syscall() to
       make a system call, the caller might need to handle architecture-
       dependent details; this requirement is most commonly encountered
       on certain 32-bit architectures.

       For example, on the ARM architecture Embedded ABI (EABI), a
       64-bit value (e.g., long long) must be aligned to an even
       register pair.  Thus, using syscall() instead of the wrapper
       provided by glibc, the readahead(2) system call would be invoked
       as follows on the ARM architecture with the EABI in little endian

           syscall(SYS_readahead, fd, 0,
                   (unsigned int) (offset & 0xFFFFFFFF),
                   (unsigned int) (offset >> 32),

       Since the offset argument is 64 bits, and the first argument (fd)
       is passed in r0, the caller must manually split and align the
       64-bit value so that it is passed in the r2/r3 register pair.
       That means inserting a dummy value into r1 (the second argument
       of 0).  Care also must be taken so that the split follows endian
       conventions (according to the C ABI for the platform).

       Similar issues can occur on MIPS with the O32 ABI, on PowerPC and
       parisc with the 32-bit ABI, and on Xtensa.

       Note that while the parisc C ABI also uses aligned register
       pairs, it uses a shim layer to hide the issue from user space.

       The affected system calls are fadvise64_64(2), ftruncate64(2),
       posix_fadvise(2), pread64(2), pwrite64(2), readahead(2),
       sync_file_range(2), and truncate64(2).

       This does not affect syscalls that manually split and assemble
       64-bit values such as _llseek(2), preadv(2), preadv2(2),
       pwritev(2), and pwritev2(2).  Welcome to the wonderful world of
       historical baggage.

   Architecture calling conventions
       Every architecture has its own way of invoking and passing
       arguments to the kernel.  The details for various architectures
       are listed in the two tables below.

       The first table lists the instruction used to transition to
       kernel mode (which might not be the fastest or best way to
       transition to the kernel, so you might have to refer to vdso(7)),
       the register used to indicate the system call number, the
       register(s) used to return the system call result, and the
       register used to signal an error.
       Arch/ABI    Instruction           System  Ret  Ret  Error    Notes
                                         call #  val  val2
       alpha       callsys               v0      v0   a4   a3       1, 6
       arc         trap0                 r8      r0   -    -
       arm/OABI    swi NR                -       r0   -    -        2
       arm/EABI    swi 0x0               r7      r0   r1   -
       arm64       svc #0                w8      x0   x1   -
       blackfin    excpt 0x0             P0      R0   -    -
       i386        int $0x80             eax     eax  edx  -
       ia64        break 0x100000        r15     r8   r9   r10      1, 6
       loongarch   syscall 0             a7      a0   -    -
       m68k        trap #0               d0      d0   -    -
       microblaze  brki r14,8            r12     r3   -    -
       mips        syscall               v0      v0   v1   a3       1, 6
       nios2       trap                  r2      r2   -    r7
       parisc      ble 0x100(%sr2, %r0)  r20     r28  -    -
       powerpc     sc                    r0      r3   -    r0       1
       powerpc64   sc                    r0      r3   -    cr0.SO   1
       riscv       ecall                 a7      a0   a1   -
       s390        svc 0                 r1      r2   r3   -        3
       s390x       svc 0                 r1      r2   r3   -        3
       superh      trapa #31             r3      r0   r1   -        4, 6
       sparc/32    t 0x10                g1      o0   o1   psr/csr  1, 6
       sparc/64    t 0x6d                g1      o0   o1   psr/csr  1, 6
       tile        swint1                R10     R00  -    R01      1
       x86-64      syscall               rax     rax  rdx  -        5
       x32         syscall               rax     rax  rdx  -        5
       xtensa      syscall               a2      a2   -    -


       •  On a few architectures, a register is used as a boolean (0
          indicating no error, and -1 indicating an error) to signal
          that the system call failed.  The actual error value is still
          contained in the return register.  On sparc, the carry bit
          (csr) in the processor status register (psr) is used instead
          of a full register.  On powerpc64, the summary overflow bit
          (SO) in field 0 of the condition register (cr0) is used.

       •  NR is the system call number.

       •  For s390 and s390x, NR (the system call number) may be passed
          directly with svc NR if it is less than 256.

       •  On SuperH additional trap numbers are supported for historic
          reasons, but trapa#31 is the recommended "unified" ABI.

       •  The x32 ABI shares syscall table with x86-64 ABI, but there
          are some nuances:

          •  In order to indicate that a system call is called under the
             x32 ABI, an additional bit, __X32_SYSCALL_BIT, is bitwise
             ORed with the system call number.  The ABI used by a
             process affects some process behaviors, including signal
             handling or system call restarting.

          •  Since x32 has different sizes for long and pointer types,
             layouts of some (but not all; struct timeval or struct
             rlimit are 64-bit, for example) structures are different.
             In order to handle this, additional system calls are added
             to the system call table, starting from number 512 (without
             the __X32_SYSCALL_BIT).  For example, __NR_readv is defined
             as 19 for the x86-64 ABI and as __X32_SYSCALL_BIT | 515 for
             the x32 ABI.  Most of these additional system calls are
             actually identical to the system calls used for providing
             i386 compat.  There are some notable exceptions, however,
             such as preadv2(2), which uses struct iovec entities with
             4-byte pointers and sizes ("compat_iovec" in kernel terms),
             but passes an 8-byte pos argument in a single register and
             not two, as is done in every other ABI.

       •  Some architectures (namely, Alpha, IA-64, MIPS, SuperH,
          sparc/32, and sparc/64) use an additional register ("Retval2"
          in the above table) to pass back a second return value from
          the pipe(2) system call; Alpha uses this technique in the
          architecture-specific getxpid(2), getxuid(2), and getxgid(2)
          system calls as well.  Other architectures do not use the
          second return value register in the system call interface,
          even if it is defined in the System V ABI.

       The second table shows the registers used to pass the system call
       Arch/ABI      arg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
       alpha         a0    a1    a2    a3    a4    a5    -
       arc           r0    r1    r2    r3    r4    r5    -
       arm/OABI      r0    r1    r2    r3    r4    r5    r6
       arm/EABI      r0    r1    r2    r3    r4    r5    r6
       arm64         x0    x1    x2    x3    x4    x5    -
       blackfin      R0    R1    R2    R3    R4    R5    -
       i386          ebx   ecx   edx   esi   edi   ebp   -
       ia64          out0  out1  out2  out3  out4  out5  -
       loongarch     a0    a1    a2    a3    a4    a5    a6
       m68k          d1    d2    d3    d4    d5    a0    -
       microblaze    r5    r6    r7    r8    r9    r10   -
       mips/o32      a0    a1    a2    a3    -     -     -     1
       mips/n32,64   a0    a1    a2    a3    a4    a5    -
       nios2         r4    r5    r6    r7    r8    r9    -
       parisc        r26   r25   r24   r23   r22   r21   -
       powerpc       r3    r4    r5    r6    r7    r8    r9
       powerpc64     r3    r4    r5    r6    r7    r8    -
       riscv         a0    a1    a2    a3    a4    a5    -
       s390          r2    r3    r4    r5    r6    r7    -
       s390x         r2    r3    r4    r5    r6    r7    -
       superh        r4    r5    r6    r7    r0    r1    r2
       sparc/32      o0    o1    o2    o3    o4    o5    -
       sparc/64      o0    o1    o2    o3    o4    o5    -
       tile          R00   R01   R02   R03   R04   R05   -
       x86-64        rdi   rsi   rdx   r10   r8    r9    -
       x32           rdi   rsi   rdx   r10   r8    r9    -
       xtensa        a6    a3    a4    a5    a8    a9    -


       •  The mips/o32 system call convention passes arguments 5 through
          8 on the user stack.

       Note that these tables don't cover the entire calling convention—
       some architectures may indiscriminately clobber other registers
       not listed here.

EXAMPLES         top

       #define _GNU_SOURCE
       #include <signal.h>
       #include <sys/syscall.h>
       #include <sys/types.h>
       #include <unistd.h>

           pid_t tid;

           tid = syscall(SYS_gettid);
           syscall(SYS_tgkill, getpid(), tid, SIGHUP);

SEE ALSO         top

       _syscall(2), intro(2), syscalls(2), errno(3), vdso(7)

COLOPHON         top

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       user-space interface documentation) project.  Information about
       the project can be found at 
       ⟨⟩.  If you have a bug report
       for this manual page, see
       This page was obtained from the tarball man-pages-6.9.1.tar.gz
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       ⟨⟩ on
       2024-06-26.  If you discover any rendering problems in this HTML
       version of the page, or you believe there is a better or more up-
       to-date source for the page, or you have corrections or
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       part of the original manual page), send a mail to

Linux man-pages 6.9.1          2024-05-02                     syscall(2)

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