[thirdparty/man-pages.git] / man2 / syscall.2

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.\"     @(#)syscall.2   8.1 (Berkeley) 6/16/93
.\"
.\"
.\" 2002-03-20  Christoph Hellwig <hch@infradead.org>
.\"     - adopted for Linux
.\" 2015-01-17, Kees Cook <keescook@chromium.org>
.\"     Added mips and arm64.
.\"
.TH SYSCALL 2 2020-02-09 "Linux" "Linux Programmer's Manual"
.SH NAME
syscall \- indirect system call
.SH SYNOPSIS
.nf
.B #include <unistd.h>
.BR "#include <sys/syscall.h>   "  "/* For SYS_xxx definitions */"
.PP
.BI "long syscall(long " number ", ...);"
.fi
.PP
.in -4n
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.in
.BR syscall ():
.PD 0
.ad l
.RS 4
.TP 4
Since glibc 2.19:
_DEFAULT_SOURCE
.TP
Before glibc 2.19:
_BSD_SOURCE || _SVID_SOURCE
.RE
.ad
.PD
.SH DESCRIPTION
.BR syscall ()
is a small library function that invokes
the system call whose assembly language
interface has the specified
.I number
with the specified arguments.
Employing
.BR syscall ()
is useful, for example,
when invoking a system call that has no wrapper function in the C library.
.PP
.BR syscall ()
saves CPU registers before making the system call,
restores the registers upon return from the system call,
and stores any error code returned by the system call in
.BR errno (3)
if an error occurs.
.PP
Symbolic constants for system call numbers can be found in the header file
.IR <sys/syscall.h> .
.SH 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 code is stored in
.IR errno .
.SH NOTES
.BR syscall ()
first appeared in
4BSD.
.SS 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
.BR 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.
.PP
For example, on the ARM architecture Embedded ABI (EABI), a
64-bit value (e.g.,
.IR "long long" )
must be aligned to an even register pair.
Thus, using
.BR syscall ()
instead of the wrapper provided by glibc,
the
.BR readahead ()
system call would be invoked as follows on the ARM architecture with the EABI
in little endian mode:
.PP
.in +4n
.EX
syscall(SYS_readahead, fd, 0,
        (unsigned int) (offset & 0xFFFFFFFF),
        (unsigned int) (offset >> 32),
        count);
.EE
.in
.PP
Since the offset argument is 64 bits, and the first argument
.RI ( fd )
is passed in
.IR r0 ,
the caller must manually split and align the 64-bit value
so that it is passed in the
.IR r2 / r3
register pair.
That means inserting a dummy value into
.I 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).
.PP
Similar issues can occur on MIPS with the O32 ABI,
on PowerPC and parisc with the 32-bit ABI, and on Xtensa.
.\" Mike Frysinger: this issue ends up forcing MIPS
.\" O32 to take 7 arguments to syscall()
.PP
.\" See arch/parisc/kernel/sys_parisc.c.
Note that while the parisc C ABI also uses aligned register pairs,
it uses a shim layer to hide the issue from user space.
.PP
The affected system calls are
.BR fadvise64_64 (2),
.BR ftruncate64 (2),
.BR posix_fadvise (2),
.BR pread64 (2),
.BR pwrite64 (2),
.BR readahead (2),
.BR sync_file_range (2),
and
.BR truncate64 (2).
.PP
.\" You need to look up the syscalls directly in the kernel source to see if
.\" they should be in this list.  For example, look at fs/read_write.c and
.\" the function signatures that do:
.\" ..., unsigned long, pos_l, unsigned long, pos_h, ...
.\" If they use off_t, then they most likely do not belong in this list.
This does not affect syscalls that manually split and assemble 64-bit values
such as
.BR _llseek (2),
.BR preadv (2),
.BR preadv2 (2),
.BR pwritev (2),
and
.BR pwritev2 (2).
Welcome to the wonderful world of historical baggage.
.SS 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.
.PP
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
.BR 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.
.if t \{\
.ft CW
\}
.TS
l2      l2      l2      l2      l1      l2      l.
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  trap #0x17      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      -       -
.TE
.PP
Notes:
.IP [1] 4
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
.RI ( csr )
in the processor status register
.RI ( psr )
is used instead of a full register.
On powerpc64, the summary overflow bit
.RI ( SO )
in field 0 of the condition register
.RI ( cr0 )
is used.
.IP [2]
.I NR
is the system call number.
.IP [3]
For s390 and s390x,
.I NR
(the system call number) may be passed directly with
.I "svc\ NR"
if it is less than 256.
.IP [4]
On SuperH, the trap number controls the maximum number of arguments passed.
A
.IR "trap\ #0x10"
can be used with only 0-argument system calls, a
.IR "trap\ #0x11"
can be used with 0- or 1-argument system calls,
and so on up to
.IR "trap #0x17"
for 7-argument system calls.
.IP [5]
The x32 ABI shares syscall table with x86-64 ABI, but there are some
nuances:
.RS
.IP \(bu 3
In order to indicate that a system call is called under the x32 ABI,
an additional bit,
.BR __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.
.IP \(bu
Since x32 has different sizes for
.I long
and pointer types, layouts of some (but not all;
.I struct timeval
or
.I 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
.BR __X32_SYSCALL_BIT ).
For example,
.B __NR_readv
is defined as 19 for the x86-64 ABI and as
.IR __X32_SYSCALL_BIT " | " \fB515\fP
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
.BR preadv2 (2),
which uses
.I struct iovec
entities with 4-byte pointers and sizes ("compat_iovec" in kernel terms),
but passes an 8-byte
.I pos
argument in a single register and not two, as is done in every other ABI.
.RE
.IP [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
.BR pipe (2)
system call;
Alpha uses this technique in the architecture-specific
.BR getxpid (2),
.BR getxuid (2),
and
.BR 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.
.if t \{\
.in
.ft P
\}
.PP
The second table shows the registers used to pass the system call arguments.
.if t \{\
.ft CW
\}
.TS
l       l2      l2      l2      l2      l2      l2      l2      l.
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      -
.TE
.PP
Notes:
.IP [1] 4
The mips/o32 system call convention passes
arguments 5 through 8 on the user stack.
.if t \{\
.in
.ft P
\}
.PP
Note that these tables don't cover the entire calling convention\(emsome
architectures may indiscriminately clobber other registers not listed here.
.SH EXAMPLE
.EX
#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);
}
.EE
.SH SEE ALSO
.BR _syscall (2),
.BR intro (2),
.BR syscalls (2),
.BR errno (3),
.BR vdso (7)