SYSCALL

Section: Linux Programmer's Manual (2)
Updated: 2020-06-09
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NAME

syscall - indirect system call  

SYNOPSIS

#include <unistd.h>
#include <sys/syscall.h>   /* For SYS_xxx definitions */

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  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  - -  

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, the trap number controls the maximum number of arguments passed. A trap #0x10 can be used with only 0-argument system calls, a trap #0x11 can be used with 0- or 1-argument system calls, and so on up to trap #0x17 for 7-argument system calls.
[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/ABIarg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes
alphaa0  a1  a2  a3  a4  a5  -  
arcr0  r1  r2  r3  r4  r5  -  
arm/OABIr0  r1  r2  r3  r4  r5  r6  
arm/EABIr0  r1  r2  r3  r4  r5  r6  
arm64x0  x1  x2  x3  x4  x5  -  
blackfinR0  R1  R2  R3  R4  R5  -  
i386ebx  ecx  edx  esi  edi  ebp  -  
ia64out0  out1  out2  out3  out4  out5  -  
m68kd1  d2  d3  d4  d5  a0  -  
microblazer5  r6  r7  r8  r9  r10  -  
mips/o32a0  a1  a2  a3  -  -  -  1
mips/n32,64a0  a1  a2  a3  a4  a5  -  
nios2r4  r5  r6  r7  r8  r9  -  
pariscr26  r25  r24  r23  r22  r21  -  
powerpcr3  r4  r5  r6  r7  r8  r9  
powerpc64r3  r4  r5  r6  r7  r8  -  
riscva0  a1  a2  a3  a4  a5  -  
s390r2  r3  r4  r5  r6  r7  -  
s390xr2  r3  r4  r5  r6  r7  -  
superhr4  r5  r6  r7  r0  r1  r2  
sparc/32o0  o1  o2  o3  o4  o5  -  
sparc/64o0  o1  o2  o3  o4  o5  -  
tileR00  R01  R02  R03  R04  R05  -  
x86-64rdi  rsi  rdx  r10  r8  r9  -  
x32rdi  rsi  rdx  r10  r8  r9  -  
xtensaa6  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 5.10 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/.

 

Index

NAME
SYNOPSIS
DESCRIPTION
RETURN VALUE
NOTES
Architecture-specific requirements
Architecture calling conventions
EXAMPLES
SEE ALSO
COLOPHON