SYSCALL

名前

書式

#include <unistd.h>
#include <sys/syscall.h>   /* SYS_xxx の定義用 */

long syscall(long number, ...);

glibc 向けの機能検査マクロの要件 (feature_test_macros(7) 参照):
 syscall():

glibc 2.19 以降:

 _DEFAULT_SOURCE

glibc 2.19 より前:

 _BSD_SOURCE || _SVID_SOURCE

説明

syscall() は、システムコールを行う前に CPU レジスターを保存し、システムコールから返った際にレジスターを復元し、システムコールが返したエラーを errno(3) に格納する。

システムコールのシンボル定数は、ヘッダーファイル <sys/syscall.h> に書かれている。  

返り値

注意

アーキテクチャー固有の要件

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.

次のシステムコールに影響がある: fadvise64_64(2), ftruncate64(2), posix_fadvise(2), pread64(2), pwrite64(2), readahead(2), sync_file_range(2), 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.  

アーキテクチャー毎の呼び出し規約

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.

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]

s390 と s390x では、 NR (システムコール番号) が 256 未満の場合 svc NR で NR が直接渡される場合がある。

[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.

2 つ目の表は、システムコールの引数を渡すのに使用されるレジスターの一覧である。

Notes:

[1]

mips/o32 のシステムコールの規約では、 ユーザースタックに引数を 5 個から 8 個渡す。

これらの表にはすべての呼び出し規約が記載されているわけではない点に注意すること --- アーキテクチャーによっては、ここに記載されていない他のレジスターが見境なく上書きされる場合もある。  

例

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

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

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Index

名前

書式

説明

返り値

注意

アーキテクチャー固有の要件

アーキテクチャー毎の呼び出し規約

例

関連項目

この文書について