# syscall

## NAME



           syscall - indirect system call
    


## LIBRARY



           Standard C library (_libc_ , _-lc_)
    


## 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_.
    


## ERRORS



           **ENOSYS** The requested system call number is not implemented.
    
           Other errors are specific to the invoked system call.
    


## 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
           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               eax     rax  rdx  -        5
           x32         syscall               eax     rax  rdx  -        5
           xtensa      syscall               a2      a2   -    -
    
           Notes:
    
           •  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
           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  -
           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    -
    
           Notes:
    
           •  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 <signal.h>
           #include <sys/syscall.h>
           #include <sys/types.h>
           #include <unistd.h>
    
           int
           main(void)
           {
               pid_t tid;
    
               tid = syscall(SYS_gettid);
               syscall(SYS_tgkill, getpid(), tid, SIGHUP);
           }
    


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