read(2) read(2)
NAME [Toc] [Back]
read, readv, pread - read from file
SYNOPSIS [Toc] [Back]
#include <unistd.h>
ssize_t read(int fildes, void *buf, size_t nbyte);
ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
#include <sys/uio.h>
ssize_t readv(int fildes, const struct iovec *iov, int iovcnt);
DESCRIPTION [Toc] [Back]
The read() function attempts to read nbyte bytes from the file
associated with the open file descriptor, fildes, into the buffer
pointed to by buf.
If nbyte is 0, read() will return 0 and have no other results.
On files that support seeking (for example, a regular file), the
read() starts at a position in the file given by the file offset
associated with fildes. The file offset is incremented by the number
of bytes actually read.
Files that do not support seeking, for example, terminals, always read
from the current position. The value of a file offset associated with
such a file is undefined.
No data transfer will occur past the current end- of-file. If the
starting position is at or after the end-of-file, 0 will be returned.
If the file refers to a device special file, the result of subsequent
read() requests is implementation-dependent.
If the value of nbyte is greater than {SSIZE_MAX} the result is
implementation-dependent.
When attempting to read from an empty pipe (or FIFO):
+ If no process has the pipe open for writing, the read returns
a 0.
+ If some process has the pipe open for writing and O_NONBLOCK
is set, the read returns -1 and errno is set to [EAGAIN].
+ If O_NDELAY is set, the read returns a 0.
+ If some process has the pipe open for writing and O_NDELAY and
O_NONBLOCK are clear, the read blocks until data is written to
the file or the file is no longer open for writing.
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read(2) read(2)
When attempting to read a file (other than a pipe or FIFO) that
supports non-blocking reads and has no data currently available:
+ If O_NONBLOCK is set, read() will return a -1 and set errno to
EAGAIN.
+ If O_NONBLOCK is clear, read() will block until some data
becomes available.
+ The use of the O_NONBLOCK flag has no effect if there is some
data available.
When attempting to read from a regular file with enforcement-mode file
and record locking set (see chmod(2)), and the segment of the file to
be read is blocked by a write lock owned by another process, the
behavior is determined by the O_NDELAY and O_NONBLOCK file status
flags:
+ If O_NDELAY or O_NONBLOCK is set, read() returns -1 and errno
is set to [EAGAIN].
+ If O_NDELAY and O_NONBLOCK are clear, read() does not return
until the blocking write lock is removed.
When attempting to read a file associated with a tty that has no data
currently available:
+ If O_NDELAY is set, the read returns 0.
+ If O_NDELAY and O_NONBLOCK are clear, the read blocks until
data becomes available.
The read() function reads data previously written to a file. If any
portion of a regular file prior to the end-of-file has not been
written, read() returns bytes with value 0. For example, lseek()
allows the file offset to be set beyond the end of existing data in
the file. If data is later written at this point, subsequent reads in
the gap between the previous end of data and the newly written data
will return bytes with value 0 until data is written into the gap.
For ordinary files, if the O_RSYNC|O_DSYNC file status flag is set,
the calling process blocks until the data being read and all file
attributes required to retrieve the data are the same as their image
on disk. Writes pending on the data to be read are executed before
returning to the calling process. If the O_RSYNC|O_SYNC file status
flag is set, the behavior is identical to that for O_RSYNC|O_DSYNC
with this addition: all file attributes changed by the read operation
(including access time, modification time and status change time) must
also be the same as their image on disk. For block special files, if
either the O_RSYNC|O_DSYNC or O_RSYNC|O_SYNC status flag is set, the
calling process blocks until the data being read is an image of the
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read(2) read(2)
data on the disk. Writes pending on the data to be read are executed
before returning to the calling process.
Upon successful completion, where nbyte is greater than 0, read() will
mark for update the st_atime field of the file, and return the number
of bytes read. This number will never be greater than nbyte. The
value returned may be less than nbyte if the number of bytes left in
the file is less than nbyte, if the read() request was interrupted by
a signal, or if the file is a pipe or FIFO or special file and has
fewer than nbyte bytes immediately available for reading. For example,
a read() from a file associated with a terminal may return one typed
line of data.
If a read() is interrupted by a signal before it reads any data, it
will return -1 with errno set to [EINTR].
If a read() is interrupted by a signal after it has successfully read
some data, it will return the number of bytes read.
A read() from a STREAMS file can read data in three different modes:
byte-stream mode, message-ondiscard mode, and message-discard mode.
The default is byte-stream mode. This can be changed using the
I_SRDOPT ioctl() request, and can be tested with the I_GRDOPT ioctl().
In byte-stream mode, read() retrieves data from the STREAM until as
many bytes as were requested are transferred, or until there is no
more data to be retrieved. Byte-stream mode ignores message
boundaries.
In STREAMS message-nondiscard mode, read() retrieves data until as
many bytes as were requested are transferred, or until a message
boundary is reached. If read() does not retrieve all the data in a
message, the remaining data is left on the STREAM, and can be
retrieved by the next read() call. Message-discard mode also retrieves
data until as many bytes as were requested are transferred, or a
message boundary is reached. However, unread data remaining in a
message after the read() returns is discarded, and is not available
for a subsequent read(), readv(), or getmsg() call.
How read() handles zero-byte STREAMS messages is determined by the
current read mode setting. In byte-stream mode, read() accepts data
until it has read nbyte bytes, or until there is no more data to read,
or until a zero-byte message block is encountered. The read() function
then returns the number of bytes read, and places the zero-byte
message back on the STREAM to be retrieved by the next read(),
readv(), or getmsg(). In message-nondiscard mode or message-discard
mode, a zero-byte message returns 0 and the message is removed from
the STREAM. When a zero-byte message is read as the first message on
a STREAM, the message is removed from the STREAM and 0 is returned,
regardless of the read mode.
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read(2) read(2)
A read() from a STREAMS file returns the data in the message at the
front of the STREAM head read queue, regardless of the priority band
of the message.
By default, STREAMs are in control-normal mode, in which a read() from
a STREAMS file can only process messages that contain a data part but
do not contain a control part. The read() fails if a message
containing a control part is encountered at the STREAM head. This
default action can be changed by placing the STREAM in either
control-data mode or control-discard mode with the I_SRDOPT ioctl()
command. In control-data mode, read() converts any control part to
data and passes it to the application before passing any data part
originally present in the same message. In control-discard mode,
read() discards message control parts but returns to the process any
data part in the message.
In addition, read() and readv() will fail if the STREAM head had
processed an asynchronous error before the call. In this case, the
value of errno does not reflect the result of read() or readv() but
reflects the prior error. If a hangup occurs on the STREAM being read,
read() continues to operate normally until the STREAM head read queue
is empty. Thereafter, it returns 0.
The readv() function is equivalent to read(), but places the input
data into the iovcnt buffers specified by the members of the iov
array: iov[0], iov[1], ..., iov[iovcnt-1]. The iovcnt argument is
valid if greater than 0 and less than or equal to {IOV_MAX}.
Each iovec entry specifies the base address and length of an area in
memory where data should be placed. The readv() function always fills
an area completely before proceeding to the next. The iovec structure
is defined in /usr/include/sys/uio.h.
Upon successful completion, readv() marks for update the st_atime
field of the file.
The pread() function performs the same action as read(), except that
it reads from a given position in the file without changing the file
pointer. The first three arguments of pread() are the same as read()
with the addition of a fourth argument offset for the desired position
inside the file. An attempt to perform a pread() on a file that is
incapable of seeking results in an error.
RETURN VALUE [Toc] [Back]
Upon successful completion, read() returns the number of bytes
actually read and placed in the buffer; this number may be less than
nbyte if:
+ The file is associated with a communication line (see ioctl(2)
and termio(7)), or
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read(2) read(2)
+ The number of bytes left in the file is less than nbyte bytes.
+ read() was interrupted by a signal after it had successfully
read some, but not all of the data requested.
When an end-of-file is reached, a value of 0 is returned. Otherwise,
a -1 is returned and errno is set to indicate the error.
Upon successful completion, pread() and readv() return a non-negative
integer indicating the number of bytes actually read. Otherwise, the
functions return -1 and set errno to indicate the error.
ERRORS [Toc] [Back]
The read(), pread() and readv() functions will fail if:
[EAGAIN] Enforcement-mode file and record locking is set,
O_NDELAY or O_NONBLOCK is set, and there is a
blocking write lock.
[EBADF] The fildes argument is not a valid file descriptor
open for reading.
[EBADMSG] The file is a STREAM file that is set to
control-normal mode and the message waiting to be
read includes a control part.
[EDEADLK] A resource deadlock would occur as a result of
this operation (see lockf(2) and fcntl(2)).
[EFAULT] buf points outside the allocated address space.
Reliable detection of this error is implementation
dependent.
[EINTR] A signal was caught before any data was
transferred (see sigvector(2)).
[EINVAL] The starting file offset associated with fildes is
greater than the maximum supported file size.
[EINVAL] The STREAM or multiplexer referenced by fildes is
linked (directly or indirectly) downstream from a
multiplexer.
[EIO] A physical I/O error has occurred.
[EIO] The process is a member of a background process
attempting to read from its controlling terminal,
the process is ignoring or blocking the SIGTTIN
signal or the process group is orphaned. This
error may also be generated for
implementation-dependent reasons.
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read(2) read(2)
[EISDIR] The fildes argument refers to a directory and the
implementation does not allow the directory to be
read using read(), pread() or readv(). The
readdir() function should be used instead.
[ENOLCK] The system record lock table is full, preventing
the read from sleeping until the blocking write
lock is removed.
The readv() function will fail if:
[EINVAL] The sum of the iov_len values in the iov array
overflowed an ssize_t.
[EFAULT] iov_base or iov points outside of the allocated
address space. The reliable detection of this
error is implementation-dependent.
The read(), pread() and readv() functions may fail if:
[ENXIO] A request was made of a non-existent device, or
the request was outside the capabilities of the
device.
The readv() function may fail if:
[EINVAL] The iovcnt argument was less than or equal to 0,
or greater than {IOV_MAX}.
The pread() function will fail and the file pointer remains unchanged
if:
[EINVAL] The offset argument is invalid. The value is
negative.
[EOVERFLOW] The file is a regular file and an attempt was made
to read at or beyond the offset maximum associated
with the file.
[ESPIPE] fildes is associated with a pipe or FIFO.
EXAMPLES [Toc] [Back]
Assuming a process opened a file for reading, the following call to
read(2) reads BUFSIZ bytes from the file into the buffer pointed to by
mybuf:
#include <stdio.h> /* include this for BUFSIZ definition */
char mybuf[BUFSIZ];
int nbytes, fildes;
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read(2) read(2)
nbytes = read (fildes, mybuf, BUFSIZ);
WARNINGS [Toc] [Back]
Record locking might not be enforced by the system, depending on the
setting of the file's mode bits (see lockf(2)).
Character-special devices, and raw disks in particular, apply
constraints on how read() can be used. See the specific Section (7)
entries for details on particular devices.
Check all references to signal(5) for appropriateness on systems that
support sigvector(2). sigvector() can affect the behavior described
on this page.
In general, avoid using read() to get the contents of a directory; use
the readdir() library routine (see directory(3C)).
DEPENDENCIES [Toc] [Back]
NFS
When obtaining the contents of a directory on an NFS file system, the
readdir() library routine must be used (see directory(3C)). read()
returns with an error if used to read a directory using NFS.
AUTHOR [Toc] [Back]
read() was developed by HP, AT&T, and the University of California,
Berkeley.
SEE ALSO [Toc] [Back]
fcntl(2), ioctl(2), lseek(2), open(2), pipe(2), creat(2), dup(2),
lockf(2), select(2), ustat(2), directory(3C), tty(7), <stropts.h>,
<sys/uio.h>, <unistd.h>, XBD Specification, Chapter 9, General
Terminal Interface.
STANDARDS CONFORMANCE [Toc] [Back]
read(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1,
POSIX.4
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