READ(2) Linux Programmer's Manual READ(2)
read - read from a file descriptor
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
On files that support seeking, the read operation commences at the current file offset,
and the file offset is incremented by the number of bytes read. If the current file off-
set is at or past the end of file, no bytes are read, and read() returns zero.
If count is zero, read() may detect the errors described below. In the absence of any
errors, or if read() does not check for errors, a read() with a count of 0 returns zero
and has no other effects.
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 sig-
nal. On error, -1 is returned, and errno is set appropriately. In this case it is left
unspecified whether the file position (if any) changes.
EAGAIN The file descriptor fd refers to a file other than a socket and has been marked
nonblocking (O_NONBLOCK), and the read would block.
EAGAIN or EWOULDBLOCK
The file descriptor fd refers to a socket and has been marked nonblocking (O_NON-
BLOCK), 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.
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; see signal(7).
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 speci-
fied in count, or the current file offset is not suitably aligned.
EINVAL 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.
EIO I/O error. This will happen for example when the process is in a background
process group, tries to read from its controlling terminal, and either it is ignor-
ing 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.
SVr4, 4.3BSD, POSIX.1-2001.
On NFS file systems, reading small amounts of data will update the timestamp only 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.
close(2), fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2), readlink(2),
readv(2), select(2), write(2), fread(3)
This page is part of release 3.53 of the Linux man-pages project. A description of the
project, and information about reporting bugs, can be found at
Linux 2013-02-12 READ(2)