read(2)read(2)NAME
read, readv, pread - read from file
SYNOPSISDESCRIPTION
The 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, will return 0 and have no other results.
On files that support seeking (for example, a regular file), the 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
requests is implementation-dependent.
If the value of nbyte is greater than the result is implementa‐
tion-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 is set, the
read returns −1 and is set to
· If is set, the read returns a 0.
· If some process has the pipe open for writing and and are
clear, the read blocks until data is written to the file or
the file is no longer open for writing.
When attempting to read a file (other than a pipe or FIFO) that sup‐
ports non-blocking reads and has no data currently available:
· If is set, will return a −1 and set to
· If is clear, will block until some data becomes available.
· The use of the 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 behav‐
ior is determined by the and file status flags:
· If or is set, returns −1 and is set to
· If and are clear, 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 is set, the read returns 0.
· If and are clear, the read blocks until data becomes avail‐
able.
The 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, returns
bytes with value 0. For example, 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 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 file status flag is set, the behavior is identical to
that for 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 or status flag is set, the calling process blocks
until the data being read is an image of the 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, 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 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 from a file
associated with a terminal may return one typed line of data.
If a is interrupted by a signal before it reads any data, it will
return −1 with set to
If a is interrupted by a signal after it has successfully read some
data, it will return the number of bytes read.
A from a 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 request, and can be
tested with the In byte-stream mode, retrieves data from the 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 message-nondiscard mode, retrieves data until as many bytes as were
requested are transferred, or until a message boundary is reached. If
does not retrieve all the data in a message, the remaining data is left
on the and can be retrieved by the next 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 returns is discarded, and is not available for a
subsequent or call.
How handles zero-byte messages is determined by the current read mode
setting. In byte-stream mode, 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 function then returns the number of
bytes read, and places the zero-byte message back on the to be
retrieved by the next or In message-nondiscard mode or message-discard
mode, a zero-byte message returns 0 and the message is removed from the
When a zero-byte message is read as the first message on a the message
is removed from the and 0 is returned, regardless of the read mode.
A from a file returns the data in the message at the front of the head
read queue, regardless of the priority band of the message.
By default, are in control-normal mode, in which a from a file can only
process messages that contain a data part but do not contain a control
part. The fails if a message containing a control part is encountered
at the head. This default action can be changed by placing the in
either control-data mode or control-discard mode with the command. In
control-data mode, 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, discards message control parts
but returns to the process any data part in the message.
In addition, and will fail if the head had processed an asynchronous
error before the call. In this case, the value of does not reflect the
result of or but reflects the prior error. If a hangup occurs on the
being read, continues to operate normally until the head read queue is
empty. Thereafter, it returns 0.
The function is equivalent to 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
Each iovec entry specifies the base address and length of an area in
memory where data should be placed. The function always fills an area
completely before proceeding to the next. The structure is defined in
Upon successful completion, marks for update the st_atime field of the
file.
The function performs the same action as except that it reads from a
given position in the file without changing the file pointer. The
first three arguments of are the same as with the addition of a fourth
argument for the desired position inside the file. An attempt to per‐
form a on a file that is incapable of seeking results in an error.
RETURN VALUE
Upon successful completion, 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
· The number of bytes left in the file is less than nbyte
bytes.
· 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 is set to indicate the error.
Upon successful completion, and return a non-negative integer indicat‐
ing the number of bytes actually read. Otherwise, the functions return
−1 and set to indicate the error.
ERRORS
The and functions will fail if:
Enforcement-mode file and record locking is set,
or is set, and there is a blocking write lock.
The fildes argument is not a valid file descriptor
open for reading.
The file is a file that is set to control-normal mode and
the message waiting to be read includes a con‐
trol part.
A resource deadlock would occur as a result of this operation
(see
lockf(2) and fcntl(2)).
buf points outside the allocated address space.
Reliable detection of this error is implemen‐
tation dependent.
A signal was caught before any data was transferred (see
signal(5)).
The starting file offset associated with
fildes is greater than the maximum supported
file size.
The or multiplexer referenced by fildes is linked
(directly or indirectly) downstream from a
multiplexer.
A physical I/O error has occurred.
The process is a member of a background process attempting to
read from its controlling terminal, the
process is ignoring or blocking the signal or
the process group is orphaned. This error may
also be generated for implementation-dependent
reasons.
The fildes argument refers to a directory and the
implementation does not allow the directory to
be read using or The function should be used
instead.
The system record lock table is full,
preventing the read from sleeping until the
blocking write lock is removed.
The function will fail if:
The sum of the iov_len values in the iov array overflowed an
ssize_t.
iov_base or iov points outside of the allocated address
space. The reliable detection of this error
is implementation-dependent.
The and functions may fail if:
A request was made of a non-existent device, or the request
was outside the capabilities of the device.
The function may fail if:
The argument was less than or equal to 0, or
greater than
The function will fail and the file pointer remains unchanged if:
The offset argument is invalid. The value is neg‐
ative.
The file is a regular file and an attempt was made to read at or
beyond
the offset maximum associated with the file.
fildes is associated with a pipe or FIFO.
EXAMPLES
Assuming a process opened a file for reading, the following call to
read(2) reads bytes from the file into the buffer pointed to by mybuf:
WARNINGS
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 con‐
straints on how can be used. See the specific Section (7) entries for
details on particular devices.
In general, avoid using to get the contents of a directory; use the
library routine (see directory(3C)).
DEPENDENCIES
NFS
When obtaining the contents of a directory on an NFS file system, the
library routine must be used (see directory(3C)). returns with an
error if used to read a directory using NFS.
AUTHOR
was developed by HP, AT&T, and the University of California, Berkeley.
SEE ALSOfcntl(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 CONFORMANCEread(2)
