читать из файлового дескриптора (read from a file descriptor)
Имя (Name)
read - read from a file descriptor
Синопсис (Synopsis)
#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);
Описание (Description)
read() attempts to read up to count bytes from file descriptor fd
into the buffer starting at buf.
On files that support seeking, the read operation commences at
the file offset, and the file offset is incremented by the number
of bytes read. If the file offset is at or past the end of file,
no bytes are read, and read() returns zero.
If count is zero, read() may detect the errors described below.
In the absence of any errors, or if read() does not check for
errors, a read() with a count of 0 returns zero and has no other
effects.
According to POSIX.1, if count is greater than SSIZE_MAX, the
result is implementation-defined; see NOTES for the upper limit
on Linux.
Возвращаемое значение (Return value)
On success, the number of bytes read is returned (zero indicates
end of file), and the file position is advanced by this number.
It is not an error if this number is smaller than the number of
bytes requested; this may happen for example because fewer bytes
are actually available right now (maybe because we were close to
end-of-file, or because we are reading from a pipe, or from a
terminal), or because read() was interrupted by a signal. See
also NOTES.
On error, -1 is returned, and errno is set to indicate the error.
In this case, it is left unspecified whether the file position
(if any) changes.
Ошибки (Error)
EAGAIN The file descriptor fd refers to a file other than a
socket and has been marked nonblocking (O_NONBLOCK), and
the read would block. See open(2) for further details on
the O_NONBLOCK flag.
EAGAIN or EWOULDBLOCK
The file descriptor fd refers to a socket and has been
marked nonblocking (O_NONBLOCK), and the read would block.
POSIX.1-2001 allows either error to be returned for this
case, and does not require these constants to have the
same value, so a portable application should check for
both possibilities.
EBADF fd is not a valid file descriptor or is not open for
reading.
EFAULT buf is outside your accessible address space.
EINTR The call was interrupted by a signal before any data was
read; see signal(7).
EINVAL fd is attached to an object which is unsuitable for
reading; or the file was opened with the O_DIRECT flag,
and either the address specified in buf, the value
specified in count, or the file offset is not suitably
aligned.
EINVAL fd was created via a call to timerfd_create(2) and the
wrong size buffer was given to read(); see
timerfd_create(2) for further information.
EIO I/O error. This will happen for example when the process
is in a background process group, tries to read from its
controlling terminal, and either it is ignoring or
blocking SIGTTIN or its process group is orphaned. It may
also occur when there is a low-level I/O error while
reading from a disk or tape. A further possible cause of
EIO on networked filesystems is when an advisory lock had
been taken out on the file descriptor and this lock has
been lost. See the Lost locks section of fcntl(2) for
further details.
EISDIR fd refers to a directory.
Other errors may occur, depending on the object connected to fd.
Стандарты (Conforming to)
SVr4, 4.3BSD, POSIX.1-2001.
Примечание (Note)
The types size_t and ssize_t are, respectively, unsigned and
signed integer data types specified by POSIX.1.
On Linux, read() (and similar system calls) will transfer at most
0x7ffff000 (2,147,479,552) bytes, returning the number of bytes
actually transferred. (This is true on both 32-bit and 64-bit
systems.)
On NFS filesystems, reading small amounts of data will update the
timestamp only the first time, subsequent calls may not do so.
This is caused by client side attribute caching, because most if
not all NFS clients leave st_atime (last file access time)
updates to the server, and client side reads satisfied from the
client's cache will not cause st_atime updates on the server as
there are no server-side reads. UNIX semantics can be obtained
by disabling client-side attribute caching, but in most
situations this will substantially increase server load and
decrease performance.
Ошибки (баги) (Bugs)
According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread
Interactions with Regular File Operations"):
All of the following functions shall be atomic with respect
to each other in the effects specified in POSIX.1-2008 when
they operate on regular files or symbolic links: ...
Among the APIs subsequently listed are read() and readv(2). And
among the effects that should be atomic across threads (and
processes) are updates of the file offset. However, on Linux
before version 3.14, this was not the case: if two processes that
share an open file description (see open(2)) perform a read() (or
readv(2)) at the same time, then the I/O operations were not
atomic with respect updating the file offset, with the result
that the reads in the two processes might (incorrectly) overlap
in the blocks of data that they obtained. This problem was fixed
in Linux 3.14.
Смотри также (See also)
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2),
readdir(2), readlink(2), readv(2), select(2), write(2), fread(3)
