syscall

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



NAME
       syscall - indirect system call

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

       long syscall(long number, ...);

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

       syscall():
           Since glibc 2.19:
               _DEFAULT_SOURCE
           Before glibc 2.19:
               _BSD_SOURCE || _SVID_SOURCE

DESCRIPTION
       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
       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.

NOTES
       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 mode:

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

       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
       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   -    -

       Notes:

       [1] 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.

       [2] NR is the system call number.

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

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

       [5] 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.

       [6] 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
       arguments.

       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  -
       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    -

       Notes:

       [1] 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
       #define _GNU_SOURCE
       #include <unistd.h>
       #include <sys/syscall.h>
       #include <sys/types.h>
       #include <signal.h>

       int
       main(int argc, char *argv[])
       {
           pid_t tid;

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

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

COLOPHON
       This page is part of release 5.13 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
       https://www.kernel.org/doc/man-pages/.



Linux                              2021-03-22                         SYSCALL(2)