read - read from a file descriptor
ssize_t read(int fd, void *buf, size_t count);
() attempts to read up to count
bytes from file descriptor
into the buffer starting at buf
is zero, read
() returns zero and has no other results. If
is greater than SSIZE_MAX
, the result is unspecified.
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.
- The file descriptor fd refers to a file other than a
socket and has been marked nonblocking (O_NONBLOCK), and the read
- 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.
- fd is not a valid file descriptor or is not open for
- buf is outside your accessible address space.
- The call was interrupted by a signal before any data was
read; see signal(7).
- 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.
- 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.
- 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.
- fd refers to a directory.
Other errors may occur, depending on the object connected to fd
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
SVr4, 4.3BSD, POSIX.1-2001.
On NFS file systems, reading small amounts of data will only update the
timestamp 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 file systems and disks were considered to be fast enough that the
implementation of O_NONBLOCK
was deemed unnecessary. So,
may not be available on files and/or disks.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found