read — read from a file descriptor
#include <unistd.h>
ssize_t read( |
int | fd, |
void * | buf, | |
size_t | count) ; |
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.
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.
Non-blocking I/O has been selected using
O_NONBLOCK
and no data
was immediately available for reading.
fd
is not a
valid file descriptor or is not open for reading.
buf
is
outside your accessible address space.
The call was interrupted by a signal before any data was read.
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.
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
. 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.
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.
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2), readv(2), select(2), write(2), fread(3)
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