read(2)

read, readv -- read from file

Synopsis

   #include <unistd.h>
   
   ssize_t read(int fildes, void buf, size_t nbyte);
   

   #include <sys/types.h>
   #include <sys/uio.h>
   

   ssize_t readv(int fildes, const struct iovec iov, int iovcnt);

Description

read attempts to read nbyte bytes from the file associated with fildes into the buffer pointed to by buf. If nbyte is 0, read returns 0 and has no other results. fildes is a file descriptor obtained from a creat(2), open(2), dup(2), fcntl(2), pipe(2) or ioctl(2) system call.

On devices capable of seeking, the read starts at a position in the file given by the file pointer associated with fildes. On return from read, the file pointer is incremented by the number of bytes actually read.

Devices that are incapable of seeking always read from the current position. The value of a file pointer associated with such a file is undefined.

readv performs the same action as 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].

For readv, the iovec structure contains the following members:

   void	*iov_base;
   size_t	iov_len;

Each iovec entry specifies the base address and length of an area in memory where data should be placed. readv always fills one buffer completely before proceeding to the next.

On success, read and readv return 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 if the number of bytes left in the file is less than nbyte, or if the file is a pipe or a special file. A value of 0 is returned when an end-of-file has been reached.

read reads data previously written to a file. If any portion of an ordinary file prior to the end of file has not been written, read returns the number of bytes read as 0. For example, the lseek routine allows the file pointer to be set beyond the end of existing data in the file. If additional data is written at this point, later reads in the gap between the previous end of data and newly written data return bytes with a value of 0 until data is written into the gap.

A read or readv from a STREAMS (see intro(2)) file can operate in three different modes: byte-stream mode, message-nondiscard mode, and message-discard mode. The default is byte-stream mode. This can be changed using the I_SRDOPT ioctl(2) request and can be tested with the I_GRDOPT ioctl(2)

request. In byte-stream mode, read and readv usually retrieve data from the stream until they have retrieved nbyte bytes, or until there is no more data to be retrieved. Byte-stream mode usually ignores message boundaries.

In STREAMS message-nondiscard mode, read and readv retrieve data until they have read nbyte bytes, or until they reach a message boundary. If read or readv does not retrieve all the data in a message, the remaining data is replaced on the stream and can be retrieved by the next read or readv call. Message-discard mode also retrieves data until it has retrieved nbyte bytes, or it reaches a message boundary. However, unread data remaining in a message after the read or readv returns is discarded, and is not available for a later read, readv, or getmsg (see getmsg(2)).

When attempting to read from a regular file with mandatory file/record locking set (see chmod(2)), and there is a write lock owned by another process on the segment of the file to be read:

If O_NDELAY or O_NONBLOCK is set, read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read sleeps until the blocking record lock is removed.

When attempting to read from an empty pipe (or FIFO):

If no process has the pipe open for writing, read returns 0 to indicate end-of-file.
If some process has the pipe open for writing and O_NDELAY is set, read returns 0.
If some process has the pipe open for writing and O_NONBLOCK is set, read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data is written to the pipe or the pipe is closed by all processes that had opened the pipe for writing.

When attempting to read a file associated with a terminal that has no data currently available:

If O_NDELAY is set, read returns 0.
If O_NONBLOCK is set, read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data becomes available.

When attempting to read a file associated with a stream that is not a pipe or FIFO, or terminal, and that has no data currently available:

If O_NDELAY or O_NONBLOCK is set, read returns -1 and sets errno to EAGAIN.
If O_NDELAY and O_NONBLOCK are clear, read blocks until data becomes available.

When reading from a STREAMS file, handling of zero-byte 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. read then returns the number of bytes read, and places the zero-byte message back on the stream to be retrieved by the next read or getmsg (see getmsg(2)). In the two other modes, a zero-byte message returns a value of 0 and the message is removed from the stream. When a zero-byte message is read as the first message on a stream, a value of 0 is returned regardless of the read mode.

A read or readv 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.

Normally, a read from a STREAMS file can only process messages with data and without control information. The read fails if a message containing control information 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(2). In control-data mode, control messages are converted to data messages by read. In control-discard mode, control messages are discarded by read, but any data associated with the control messages is returned to the user.

Return values

On success, read and readv return a non-negative integer indicating the number of bytes actually read. On failure, read and readv return -1 and set errno to identify the error.

A read from a STREAMS file also fails if an error message is received at the stream head. In this case, errno is set to the value returned in the error message. 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.

Errors

In the following conditions, read and readv fail and set errno to:

EACCES: fildes is open to a dynamic device and read permission is denied.
EAGAIN: Mandatory file/record locking was set, O_NDELAY or O_NONBLOCK was set, and there was a blocking record lock.
EAGAIN: Total amount of system memory available when reading via raw I/O is temporarily insufficient.
EAGAIN: No data is waiting to be read on a file associated with a tty device and O_NONBLOCK was set.
EAGAIN: No message is waiting to be read on a stream and O_NDELAY or O_NONBLOCK was set.
EBADF: fildes is not a valid file descriptor open for reading.
EBADMSG: Message waiting to be read on a stream is not a data message.
EDEADLK: The read was going to go to sleep and cause a deadlock to occur.
EFAULT: buf points outside the allocated address space.
EINTR: A signal was caught during the read or readv system call.
EINVAL: Attempted to read from a stream linked to a multiplexor.
EIO: A physical I/O error has occurred, or the process is in a background process group and is attempting to read from its controlling terminal, and either the process is ignoring or blocking the SIGTTIN signal or the process group of the process is orphaned.
EIO: fildes is open to a device that is in the process of closing.
ENOLCK: The system record lock table was full, so the read or readv could not go to sleep until the blocking record lock was removed.
ENOLINK: fildes is on a remote machine and the link to that machine is no longer active.
EOVERFLOW: The file is a regular file, nbyte is greater than 0, the starting position is before the end-of-file and the starting position is greater than or equal to the offset maximum established in the open file descriptor associated with fildes. There is no data transfer.

In addition, readv may return one of the following errors:

EFAULT: iov points outside the allocated address space.
EINVAL: iovcnt was less than or equal to 0 or greater than 16.
EINVAL: The sum of the iov_len values in the iov array overflowed a 32-bit integer.

References

creat(2), dup(2), fcntl(2), getmsg(2), intro(2), ioctl(2), open(2), pipe(2), pread(2), streamio(7), termio(7), types(5), write(2)

Notices

read updates the time of last access (see stat(2)) of the file.

If fildes refers to a socket, read is equivalent to recv(3sock) with no flags set.

Considerations for threads programming

Open file descriptors are a process resource and available to any sibling thread; if used concurrently, actions by one thread can interfere with those of a sibling.

While one thread is blocked, siblings might still be executing.