SYSCALL

Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
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NAME

syscall - indirect system call  

SYNOPSIS

#define _GNU_SOURCE         /* See feature_test_macros(7) */
#include <unistd.h>
#include <sys/syscall.h>   /* For SYS_xxx definitions */

long syscall(long number, ...);

 

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 code returned by the system call in errno(3) if an error occurs.

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 code 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() 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 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 userspace.

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 used to return the system call result, and the register used to signal an error.

arch/ABI	instruction	syscall #	retval	error	Notes
alpha	callsys	v0	a0	a3	[1]
arc	trap0	r8	r0	-
arm/OABI	swi NR	-	a1	-	[2]
arm/EABI	swi 0x0	r7	r0	-
arm64	svc #0	x8	x0	-
blackfin	excpt 0x0	P0	R0	-
i386	int $0x80	eax	eax	-
ia64	break 0x100000	r15	r8	r10	[1]
m68k	trap #0	d0	d0	-
microblaze	brki r14,8	r12	r3	-
mips	syscall	v0	v0	a3	[1]
nios2	trap	r2	r2	r7
parisc	ble 0x100(%sr2, %r0)	r20	r28	-
powerpc	sc	r0	r3	r0	[1]
s390	svc 0	r1	r2	-	[3]
s390x	svc 0	r1	r2	-	[3]
superh	trap #0x17	r3	r0	-	[4]
sparc/32	t 0x10	g1	o0	psr/csr	[1]
sparc/64	t 0x6d	g1	o0	psr/csr	[1]
tile	swint1	R10	R00	R01	[1]
x86_64	syscall	rax	rax	-	[5]
x32	syscall	rax	rax	-	[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.

[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 uses the same instruction as the x86_64 ABI and is used on the same processors. To differentiate between them, the bit mask __X32_SYSCALL_BIT is bitwise-ORed into the system call number for system calls under the x32 ABI. Both system call tables are available though, so setting the bit is not a hard requirement.

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	a1	a2	a3	a4	v1	v2	v3
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
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.  

EXAMPLE

#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 4.13 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/.

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Index

NAME

SYNOPSIS

DESCRIPTION

RETURN VALUE

NOTES

Architecture-specific requirements

Architecture calling conventions

EXAMPLE

SEE ALSO

COLOPHON