NAME

read, readv, pread - read from a 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, read() will return 0 to indicate end-of-file.

• If some process has the pipe open for writing and O_NONBLOCK is set, read() will return -1 and set errno to [EAGAIN].

• If some process has the pipe open for writing and O_NONBLOCK is clear, read() will block the calling thread until some data is written or the pipe is closed by all processes that had the pipe 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 the calling thread until some data becomes available.

• The use of the O_NONBLOCK flag has no effect if there is some data 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.

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-nondiscard 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.

Upon successful completion, readv() marks for update the st_atime field of the file.

If the Synchronized Input and Output option is supported:

If the O_DSYNC and O_RSYNC bits have been set, read I/O operations on the file descriptor complete as defined by synchronised I/O data integrity completion. If the O_SYNC and O_RSYNC bits have been set, read I/O operations on the file descriptor complete as defined by synchronised I/O file integrity completion.

If the Shared Memory Objects option is supported:

If fildes refers to a shared memory object, the result of the read() function is unspecified.

For regular files, no data transfer will occur past the offset maximum established in the open file description associated with fildes.

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 to 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(), 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]

The O_NONBLOCK flag is set for the file descriptor and the process would be delayed.

[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.

[EINTR]

The read operation was terminated due to the receipt of a signal, and no data was transferred.

[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.

[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 description associated with fildes.

The readv() function will fail if:

[EINVAL]

The sum of the iov_len values in the iov array overflowed an ssize_t.

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 or write at or beyond the offset maximum associated with the file.

[ENXIO]

A request was outside the capabilities of the device.

[ESPIPE]

fildes is associated with a pipe or FIFO.

 EXAMPLES

None.

 APPLICATION USAGE

None.

 FUTURE DIRECTIONS

None.

 SEE ALSO

fcntl(), ioctl(), lseek(), open(), pipe(), <stropts.h>, <sys/uio.h>, <unistd.h>, XBD specification, General Terminal Interface .

DERIVATION

Derived from Issue 1 of the SVID.