2. System calls (functions provided by the kernel)

Enter a Linux command to search for:


Section: Linux Programmer's Manual (2)
Updated: 2015-03-29
Index  |  Return to Main Contents


syscall - indirect system call  


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

long syscall(long number, ...);


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>.  


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.  


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:

syscall(SYS_readahead, fd, 0,
        (unsigned int) (offset >> 32),
        (unsigned int) (offset & 0xFFFFFFFF),

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).

Similar issues can occur on MIPS with the O32 ABI, on PowerPC with the 32-bit ABI, and on Xtensa.

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).  

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, and the register used to return the system call result.

arch/ABIinstruction syscall #retval Notes

arm/OABIswi NR -a1 NR is syscall #
arm/EABIswi 0x0 r7r0 
arm64svc #0 x8x0 
blackfinexcpt 0x0 P0R0 
i386int $0x80 eaxeax 
ia64break 0x100000 r15r8 See below
mipssyscall v0v0 See below
pariscble 0x100(%sr2, %r0) r20r28 
s390svc 0 r1r2 See below
s390xsvc 0 r1r2 See below
sparc/32t 0x10 g1o0 
sparc/64t 0x6d g1o0 
x86_64syscall raxrax See below
x32syscall raxrax See below

For s390 and s390x, NR (the system call number) may be passed directly with "svc NR" if it is less than 256.

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.

On a few architectures, a register is used to indicate simple boolean failure of the system call: ia64 uses r10 for this purpose, and mips uses a3.

The second table shows the registers used to pass the system call arguments.

arch/ABIarg1  arg2  arg3  arg4  arg5  arg6  arg7  Notes

arm/OABIa1  a2  a3  a4  v1  v2  v3  
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  -  
mips/o32a0  a1  a2  a3  -  -  -  See below
mips/n32,64a0  a1  a2  a3  a4  a5  -  
pariscr26  r25  r24  r23  r22  r21  -  
s390r2  r3  r4  r5  r6  r7  -  
s390xr2  r3  r4  r5  r6  r7  -  
sparc/32o0  o1  o2  o3  o4  o5  -  
sparc/64o0  o1  o2  o3  o4  o5  -  
x86_64rdi  rsi  rdx  r10  r8  r9  -  
x32rdi  rsi  rdx  r10  r8  r9  -  

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.  


#define _GNU_SOURCE
#include <unistd.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <signal.h>

main(int argc, char *argv[])
    pid_t tid;

    tid = syscall(SYS_gettid);
    tid = syscall(SYS_tgkill, getpid(), tid, SIGHUP);


_syscall(2), intro(2), syscalls(2), errno(3), vdso(7)  


This page is part of release 4.04 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 http://www.kernel.org/doc/man-pages/.



Architecture-specific requirements
Architecture calling conventions

Return to Main Contents