- Anonymous
- 2007 year 7 month 31 day Tuesday 23:07:50 MDT
- READ(2) Linux Programmer's Manual READ(2)
- 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
- 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.
- CONFORMING TO
- 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.
- 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)
- Linux 2.0.32 1997-07-12 READ(2)
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