системные вызовы Linux (Linux system calls)
Примечание (Note)
Roughly speaking, the code belonging to the system call with
number __NR_xxx defined in /usr/include/asm/unistd.h can be found
in the Linux kernel source in the routine sys_xxx(). There are
many exceptions, however, mostly because older system calls were
superseded by newer ones, and this has been treated somewhat
unsystematically. On platforms with proprietary operating-system
emulation, such as sparc, sparc64, and alpha, there are many
additional system calls; mips64 also contains a full set of
32-bit system calls.
Over time, changes to the interfaces of some system calls have
been necessary. One reason for such changes was the need to
increase the size of structures or scalar values passed to the
system call. Because of these changes, certain architectures
(notably, longstanding 32-bit architectures such as i386) now
have various groups of related system calls (e.g., truncate(2)
and truncate64(2)) which perform similar tasks, but which vary in
details such as the size of their arguments. (As noted earlier,
applications are generally unaware of this: the glibc wrapper
functions do some work to ensure that the right system call is
invoked, and that ABI compatibility is preserved for old
binaries.) Examples of systems calls that exist in multiple
versions are the following:
* By now there are three different versions of stat(2):
sys_stat() (slot __NR_oldstat), sys_newstat() (slot
__NR_stat), and sys_stat64() (slot __NR_stat64), with the last
being the most current. A similar story applies for lstat(2)
and fstat(2).
* Similarly, the defines __NR_oldolduname, __NR_olduname, and
__NR_uname refer to the routines sys_olduname(), sys_uname(),
and sys_newuname().
* In Linux 2.0, a new version of vm86(2) appeared, with the old
and the new kernel routines being named sys_vm86old() and
sys_vm86().
* In Linux 2.4, a new version of getrlimit(2) appeared, with the
old and the new kernel routines being named
sys_old_getrlimit() (slot __NR_getrlimit) and sys_getrlimit()
(slot __NR_ugetrlimit).
* Linux 2.4 increased the size of user and group IDs from 16 to
32 bits. To support this change, a range of system calls were
added (e.g., chown32(2), getuid32(2), getgroups32(2),
setresuid32(2)), superseding earlier calls of the same name
without the "32" suffix.
* Linux 2.4 added support for applications on 32-bit
architectures to access large files (i.e., files for which the
sizes and file offsets can't be represented in 32 bits.) To
support this change, replacements were required for system
calls that deal with file offsets and sizes. Thus the
following system calls were added: fcntl64(2), getdents64(2),
stat64(2), statfs64(2), truncate64(2), and their analogs that
work with file descriptors or symbolic links. These system
calls supersede the older system calls which, except in the
case of the "stat" calls, have the same name without the "64"
suffix.
On newer platforms that only have 64-bit file access and
32-bit UIDs/GIDs (e.g., alpha, ia64, s390x, x86-64), there is
just a single version of the UID/GID and file access system
calls. On platforms (typically, 32-bit platforms) where the
*64 and *32 calls exist, the other versions are obsolete.
* The rt_sig* calls were added in kernel 2.2 to support the
addition of real-time signals (see signal(7)). These system
calls supersede the older system calls of the same name
without the "rt_" prefix.
* The select(2) and mmap(2) system calls use five or more
arguments, which caused problems in the way argument passing
on the i386 used to be set up. Thus, while other
architectures have sys_select() and sys_mmap() corresponding
to __NR_select and __NR_mmap, on i386 one finds old_select()
and old_mmap() (routines that use a pointer to an argument
block) instead. These days passing five arguments is not a
problem any more, and there is a __NR__newselect that
corresponds directly to sys_select() and similarly __NR_mmap2.
s390x is the only 64-bit architecture that has old_mmap().
Architecture-specific details: Alpha
* getxgid(2) returns a pair of GID and effective GID via
registers r0 and r20; it is provided instead of getgid(2) and
getegid(2).
* getxpid(2) returns a pair of PID and parent PID via registers
r0 and r20; it is provided instead of getpid(2) and
getppid(2).
* old_adjtimex(2) is a variant of adjtimex(2) that uses struct
timeval32, for compatibility with OSF/1.
* getxuid(2) returns a pair of GID and effective GID via
registers r0 and r20; it is provided instead of getuid(2) and
geteuid(2).
* sethae(2) is used for configuring the Host Address Extension
register on low-cost Alphas in order to access address space
beyond first 27 bits.
