NAME
read - 在文件描述符上执行读操作
概述
#include <unistd.h> ssize_t read(int fd, void *buf, size_t count);
描述
read() 从文件描述符 fd 中读取 count 字节的数据并放入从 buf 开始的缓冲区中.
如果 count 为零,read()返回0,不执行其他任何操作. 如果 count 大于SSIZE_MAX,那么结果将不可预料.
返回值
成功时返回读取到的字节数(为零表示读到文件描述符), 此返回值受文件剩余字节数限制.当返回值小于指定的字节数时并不意味着错误;这可能是因为当前可读取的字节数小于指定的字节数(比如已经接近文件结尾,或者正在从管道或者终端读取数据,或者 read()被信号中断). 发生错误时返回-1,并置 errno 为相应值.在这种情况下无法得知文件偏移位置是否有变化.
错误代码
- EINTR
- 在读取到数据以前调用被信号所中断.
- EAGAIN
- 使用 O_NONBLOCK 标志指定了非阻塞式输入输出,但当前没有数据可读.
- EIO
- 输入输出错误.可能是正处于后台进程组进程试图读取其控制终端,但读操作无效,或者被信号SIGTTIN所阻塞, 或者其进程组是孤儿进程组.也可能执行的是读磁盘或者磁带机这样的底层输入输出错误.
- EISDIR
- fd 指向一个目录.
- EBADF
- fd 不是一个合法的文件描述符,或者不是为读操作而打开.
- EINVAL
- fd 所连接的对象不可读.
- EFAULT
- buf 超出用户可访问的地址空间.
也可能发生其他错误,具体情况和 fd 所连接的对象有关. POSIX 允许 read 在读取了一定量的数据后被信号所中断,并返回 -1(且 errno 被设置为EINTR),或者返回已读取的数据量.
兼容于
SVr4, SVID, AT&T, POSIX, X/OPEN, BSD 4.3
限制
在NFS文件系统中,读取小块数据仅更新时间标记,之后的调用不再读取服务器端的数据.这是因为客户端把数据放在缓存里. 由于大多数情况下不存在NFS服务器向客户端的读操作, 所以NFS客户必须将更新时间标记的操作放在服务器端,而数据可以放在客户端的缓存里留待以后更新.UNIX也可以禁用客户端的缓存,但那样的话大多数情况下会导致服务器性能下降.
参见
close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3)
#p#
NAME
read - read from a file descriptor
SYNOPSIS
#include <unistd.h> ssize_t read(int fd, void *buf, size_t count);
DESCRIPTION
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
If count is zero, read() returns zero and has no other results. If count is greater than SSIZE_MAX, the result is unspecified.
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. On error, -1 is returned, and errno is set appropriately. In this case it is left unspecified whether the file position (if any) changes.
ERRORS
- EINTR
- The call was interrupted by a signal before any data was read.
- EAGAIN
- Non-blocking I/O has been selected using O_NONBLOCK and no data was immediately available for reading.
- 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.
- EBADF
- fd is not a valid file descriptor or is not open for reading.
- EINVAL
- fd is attached to an object which is unsuitable for reading.
- EFAULT
- buf is outside your accessible address space.
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.
CONFORMING TO
SVr4, SVID, AT&T, POSIX, X/OPEN, BSD 4.3
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 atime updates to the server and client side reads satisfied from the client's cache will not cause 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 unneccesary. So, O_NONBLOCK may not be available on files and/or disks.
SEE ALSO
close(2), fcntl(2), ioctl(2), lseek(2), readdir(2), readlink(2), select(2), write(2), fread(3), readv(3)
