read - 從文件中讀取的描述符

內容簡介

#include <unistd.h>

ssize_t read(int fd**, void *buf, size_t** count**);**

描述

read () 嘗試讀取多達數從文件描述符fd字節到緩衝區起始於 buf .

If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified.

返回值

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. On error, -1 is returned, and  errno  is set appropriately. In this case it is left unspecified whether the file position (if any) changes.

錯誤

標籤

描述

EAGAIN

Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.

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.

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 current file offset is not suitably aligned.

EIO

I/O error. This will happen for example when the process is in a background process group, tries to read from its controlling tty, 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.

EISDIR

fd refers to a directory.

Other errors may occur, depending on the object connected to  fd . POSIX allows a  read () that is interrupted after reading some data to return -1 (with  errno  set to EINTR) or to return the number of bytes already read.

遵循於

SVr4, 4.3BSD, POSIX.1-2001.

RESTRICTIONS

On NFS file systems, reading small amounts of data will only update the time stamp 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.

Many filesystems and disks were considered to be fast enough that the implementation of O_NONBLOCK was deemed unnecessary. So, O_NONBLOCK may not be available on files and/or disks.

另請參閱

• close (2)

• fcntl (2)

• ioctl (2)

• lseek (2)

• open (2)

• pread (2)

• readdir (2)

• readlink (2)

• readv (2)

• select (2)

• write (2)