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.
