read(2) System Calls Manual read(2)
NAME
read, readv, pread - read from file
SYNOPSIS
DESCRIPTION
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 implementation-dependent.
When attempting to read from an empty pipe (or FIFO):
o If no process has the pipe open for writing, the read returns a 0.
o If some process has the pipe open for writing and is set, the read returns -1 and is set to
o If is set, the read returns a 0.
o 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 supports non-blocking reads and has no data currently available:
o If is set, will return a -1 and set to
o If is clear, will block until some data becomes available.
o 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 behavior is determined by the and file status flags:
o If or is set, returns -1 and is set to
o 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:
o If is set, the read returns 0.
o If and are clear, the read blocks until data becomes available.
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 call-
ing 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 con-
trol 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
perform 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:
o The file is associated with a communication line (see ioctl(2) and termio(7)), or
o The number of bytes left in the file is less than nbyte bytes.
o 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 indicating 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 control 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 implementation 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 implementa-
tion-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 negative.
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 constraints 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 ALSO
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
read(2)