read(2)

NAME

read, readv, pread — read from file

SYNOPSIS

#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

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.

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 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.

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

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
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

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 signal(5)).
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.
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

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; 

nbytes = read (fildes, mybuf, BUFSIZ); 

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 read() can be used. See the specific Section (7) entries for details on particular devices.

In general, avoid using read() to get the contents of a directory; use the readdir() library routine (see directory(3C)).

DEPENDENCIES

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

read() was developed by HP, AT&T, and the University of California, Berkeley.

STANDARDS CONFORMANCE

read(): AES, SVID2, SVID3, XPG2, XPG3, XPG4, FIPS 151-2, POSIX.1, POSIX.4