The Open Group Base Specifications Issue 6
IEEE Std 1003.1, 2004 Edition
Copyright © 2001-2004 The IEEE and The Open Group, All Rights reserved.
NAME
 
    pread, read - read from a file
 
SYNOPSIS
 
    #include <unistd.h>
 
    [XSI] [Option Start] ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset); [Option End]
 
    ssize_t read(int fildes, void *buf, size_t nbyte);
 
DESCRIPTION
 
    The read() function shall attempt to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf. The behavior of multiple concurrent reads on the same pipe, FIFO, or terminal device is unspecified.
 
    Before any action described below is taken, and if nbyte is zero, the read() function may detect and return errors as described below. In the absence of errors, or if error detection is not performed, the read() function shall return zero and have no other results.
 
    On files that support seeking (for example, a regular file), the read() shall start at a position in the file given by the file offset associated with fildes. The file offset shall be 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 shall occur past the current end-of-file. If the starting position is at or after the end-of-file, 0 shall be returned. If the file refers to a device special file, the result of subsequent read() requests is implementation-defined.
 
    If the value of nbyte is greater than {SSIZE_MAX}, the result is implementation-defined.
 
    When attempting to read from an empty pipe or FIFO:
 
        *
 
          If no process has the pipe open for writing, read() shall return 0 to indicate end-of-file.
        *
 
          If some process has the pipe open for writing and O_NONBLOCK is set, read() shall return -1 and set errno to [EAGAIN].
        *
 
          If some process has the pipe open for writing and O_NONBLOCK is clear, read() shall 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() shall return -1 and set errno to [EAGAIN].
        *
 
          If O_NONBLOCK is clear, read() shall 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() shall return 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 shall return bytes with value 0 until data is written into the gap.
 
    Upon successful completion, where nbyte is greater than 0, read() shall mark for update the st_atime field of the file, and shall return the number of bytes read. This number shall 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 shall return -1 with errno set to [EINTR].
 
    If a read() is interrupted by a signal after it has successfully read some data, it shall return the number of bytes read.
 
    For regular files, no data transfer shall occur past the offset maximum established in the open file description associated with fildes.
 
    If fildes refers to a socket, read() shall be equivalent to recv() with no flags set.
 
    [SIO] [Option Start] If the O_DSYNC and O_RSYNC bits have been set, read I/O operations on the file descriptor shall complete as defined by synchronized I/O data integrity completion. If the O_SYNC and O_RSYNC bits have been set, read I/O operations on the file descriptor shall complete as defined by synchronized I/O file integrity completion. [Option End]
 
    [SHM] [Option Start] If fildes refers to a shared memory object, the result of the read() function is unspecified. [Option End]
 
    [TYM] [Option Start] If fildes refers to a typed memory object, the result of the read() function is unspecified. [Option End]
 
    [XSR] [Option Start] 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 shall be byte-stream mode. This can be changed using the I_SRDOPT ioctl() request, and can be tested with I_GRDOPT ioctl(). In byte-stream mode, read() shall retrieve 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() shall retrieve 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 shall be 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 shall be discarded, and shall not be available for a subsequent read(), getmsg(), or getpmsg() call.
 
    How read() handles zero-byte STREAMS messages is determined by the current read mode setting. In byte-stream mode, read() shall accept 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 shall then return the number of bytes read, and place the zero-byte message back on the STREAM to be retrieved by the next read(), getmsg(), or getpmsg(). In message-nondiscard mode or message-discard mode, a zero-byte message shall return 0 and the message shall be removed from the STREAM. When a zero-byte message is read as the first message on a STREAM, the message shall be removed from the STREAM and 0 shall be returned, regardless of the read mode.
 
    A read() from a STREAMS file shall return 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() shall fail 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() shall convert any control part to data and pass it to the application before passing any data part originally present in the same message. In control-discard mode, read() shall discard message control parts but return to the process any data part in the message.
 
    In addition, read() shall fail if the STREAM head had processed an asynchronous error before the call. In this case, the value of errno shall not reflect the result of read(), but reflect the prior error. If a hangup occurs on the STREAM being read, read() shall continue to operate normally until the STREAM head read queue is empty. Thereafter, it shall return 0. [Option End]
 
    [XSI] [Option Start] The pread() function shall be equivalent to read(), except that it shall read 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 shall result in an error. [Option End]
 
RETURN VALUE
 
    Upon successful completion, read() [XSI] [Option Start]  and pread() [Option End] shall return a non-negative integer indicating the number of bytes actually read. Otherwise, the functions shall return -1 and set errno to indicate the error.
 
ERRORS
 
    The read() and [XSI] [Option Start] pread() [Option End] functions shall fail if:
 
    [EAGAIN]
        The O_NONBLOCK flag is set for the file descriptor and the thread would be delayed.
    [EBADF]
        The fildes argument is not a valid file descriptor open for reading.
    [EBADMSG]
        [XSR] [Option Start] The file is a STREAM file that is set to control-normal mode and the message waiting to be read includes a control part. [Option End]
    [EINTR]
        The read operation was terminated due to the receipt of a signal, and no data was transferred.
    [EINVAL]
        [XSR] [Option Start] The STREAM or multiplexer referenced by fildes is linked (directly or indirectly) downstream from a multiplexer. [Option End]
    [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-defined reasons.
    [EISDIR]
        [XSI] [Option Start] The fildes argument refers to a directory and the implementation does not allow the directory to be read using read() or pread(). The readdir() function should be used instead. [Option End]
    [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 read() function shall fail if:
 
    [EAGAIN] or [EWOULDBLOCK]
        The file descriptor is for a socket, is marked O_NONBLOCK, and no data is waiting to be received.
    [ECONNRESET]
        A read was attempted on a socket and the connection was forcibly closed by its peer.
    [ENOTCONN]
        A read was attempted on a socket that is not connected.
    [ETIMEDOUT]
        A read was attempted on a socket and a transmission timeout occurred.
 
    The read() and [XSI] [Option Start] pread() [Option End] functions may fail if:
 
    [EIO]
        A physical I/O error has occurred.
    [ENOBUFS]
        Insufficient resources were available in the system to perform the operation.
    [ENOMEM]
        Insufficient memory was available to fulfill the request.
    [ENXIO]
        A request was made of a nonexistent device, or the request was outside the capabilities of the device.
 
    The pread() function shall fail, and the file pointer shall remain unchanged, if:
 
    [EINVAL]
        [XSI] [Option Start] The offset argument is invalid. The value is negative. [Option End]
    [EOVERFLOW]
        [XSI] [Option Start] The file is a regular file and an attempt was made to read at or beyond the offset maximum associated with the file. [Option End]
    [ENXIO]
        [XSI] [Option Start] A request was outside the capabilities of the device. [Option End]
    [ESPIPE]
        [XSI] [Option Start] fildes is associated with a pipe or FIFO. [Option End]