syscall - indirect system call  


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

int syscall(int 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 the VDSO), 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 r7r1 
blackfinexcpt 0x0 P0R0 
i386int $0x80 eaxeax 
ia64break 0x100000 r15r10/r8 
pariscble 0x100(%sr2, %r0) r20r28 
s390svc 0 r1r2 NR may be passed directly with
s390xsvc 0 r1r2 "svc NR" if NR is less than 256
sparc/32t 0x10 g1o0 
sparc/64t 0x6d g1o0 
x86_64syscall raxrax 

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



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>

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

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


_syscall(2), intro(2), syscalls(2)  


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