syscall (2)

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syscall - indirect system call


#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:
Before glibc 2.19:


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.


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),

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.

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