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READ(3P)                  POSIX Programmer's Manual                 READ(3P)

PROLOG         top

       This manual page is part of the POSIX Programmer's Manual.  The Linux
       implementation of this interface may differ (consult the
       corresponding Linux manual page for details of Linux behavior), or
       the interface may not be implemented on Linux.

NAME         top

       pread, read — read from a file

SYNOPSIS         top

       #include <unistd.h>

       ssize_t pread(int fildes, void *buf, size_t nbyte, off_t offset);
       ssize_t read(int fildes, void *buf, size_t nbyte);

DESCRIPTION         top

       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 last data access timestamp 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.

       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.

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

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

       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.

       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.

RETURN VALUE         top

       Upon successful completion, these functions 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         top

       These functions shall fail if:

       EAGAIN The file is neither a pipe, nor a FIFO, nor a socket, the
              O_NONBLOCK flag is set for the file descriptor, and the thread
              would be delayed in the read operation.

       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    The process is a member of a background process group
              attempting to read from its controlling terminal, and either
              the calling thread is blocking SIGTTIN or the process is
              ignoring SIGTTIN or the process group of the process is
              orphaned. This error may also be generated for implementation-
              defined reasons.

       EISDIR 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.

       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 pread() function shall fail if:

       EINVAL The file is a regular file or block special file, and the
              offset argument is negative. The file pointer shall remain
              unchanged.

       ESPIPE The file is a pipe, FIFO, or socket.

       The read() function shall fail if:

       EAGAIN The file is a pipe or FIFO, the O_NONBLOCK flag is set for the
              file descriptor, and the thread would be delayed in the read
              operation.

       EAGAIN or EWOULDBLOCK
              The file is a socket, the O_NONBLOCK flag is set for the file
              descriptor, and the thread would be delayed in the read
              operation.

       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.

       These 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 following sections are informative.

EXAMPLES         top

   Reading Data into a Buffer
       The following example reads data from the file associated with the
       file descriptor fd into the buffer pointed to by buf.

           #include <sys/types.h>
           #include <unistd.h>
           ...
           char buf[20];
           size_t nbytes;
           ssize_t bytes_read;
           int fd;
           ...
           nbytes = sizeof(buf);
           bytes_read = read(fd, buf, nbytes);
           ...

APPLICATION USAGE         top

       None.

RATIONALE         top

       This volume of POSIX.1‐2008 does not specify the value of the file
       offset after an error is returned; there are too many cases. For
       programming errors, such as [EBADF], the concept is meaningless since
       no file is involved. For errors that are detected immediately, such
       as [EAGAIN], clearly the pointer should not change. After an
       interrupt or hardware error, however, an updated value would be very
       useful and is the behavior of many implementations.

       Note that a read() of zero bytes does not modify the last data access
       timestamp. A read() that requests more than zero bytes, but returns
       zero, is required to modify the last data access timestamp.

       Implementations are allowed, but not required, to perform error
       checking for read() requests of zero bytes.

   Input and Output
       The use of I/O with large byte counts has always presented problems.
       Ideas such as lread() and lwrite() (using and returning longs) were
       considered at one time. The current solution is to use abstract types
       on the ISO C standard function to read() and write().  The abstract
       types can be declared so that existing functions work, but can also
       be declared so that larger types can be represented in future
       implementations. It is presumed that whatever constraints limit the
       maximum range of size_t also limit portable I/O requests to the same
       range. This volume of POSIX.1‐2008 also limits the range further by
       requiring that the byte count be limited so that a signed return
       value remains meaningful. Since the return type is also a (signed)
       abstract type, the byte count can be defined by the implementation to
       be larger than an int can hold.

       The standard developers considered adding atomicity requirements to a
       pipe or FIFO, but recognized that due to the nature of pipes and
       FIFOs there could be no guarantee of atomicity of reads of {PIPE_BUF}
       or any other size that would be an aid to applications portability.

       This volume of POSIX.1‐2008 requires that no action be taken for
       read() or write() when nbyte is zero. This is not intended to take
       precedence over detection of errors (such as invalid buffer pointers
       or file descriptors). This is consistent with the rest of this volume
       of POSIX.1‐2008, but the phrasing here could be misread to require
       detection of the zero case before any other errors.  A value of zero
       is to be considered a correct value, for which the semantics are a
       no-op.

       I/O is intended to be atomic to ordinary files and pipes and FIFOs.
       Atomic means that all the bytes from a single operation that started
       out together end up together, without interleaving from other I/O
       operations. It is a known attribute of terminals that this is not
       honored, and terminals are explicitly (and implicitly permanently)
       excepted, making the behavior unspecified. The behavior for other
       device types is also left unspecified, but the wording is intended to
       imply that future standards might choose to specify atomicity (or
       not).

       There were recommendations to add format parameters to read() and
       write() in order to handle networked transfers among heterogeneous
       file system and base hardware types. Such a facility may be required
       for support by the OSI presentation of layer services. However, it
       was determined that this should correspond with similar C-language
       facilities, and that is beyond the scope of this volume of
       POSIX.1‐2008. The concept was suggested to the developers of the
       ISO C standard for their consideration as a possible area for future
       work.

       In 4.3 BSD, a read() or write() that is interrupted by a signal
       before transferring any data does not by default return an [EINTR]
       error, but is restarted. In 4.2 BSD, 4.3 BSD, and the Eighth Edition,
       there is an additional function, select(), whose purpose is to pause
       until specified activity (data to read, space to write, and so on) is
       detected on specified file descriptors. It is common in applications
       written for those systems for select() to be used before read() in
       situations (such as keyboard input) where interruption of I/O due to
       a signal is desired.

       The issue of which files or file types are interruptible is
       considered an implementation design issue. This is often affected
       primarily by hardware and reliability issues.

       There are no references to actions taken following an ``unrecoverable
       error''. It is considered beyond the scope of this volume of
       POSIX.1‐2008 to describe what happens in the case of hardware errors.

       Earlier versions of this standard allowed two very different
       behaviors with regard to the handling of interrupts. In order to
       minimize the resulting confusion, it was decided that POSIX.1‐2008
       should support only one of these behaviors. Historical practice on
       AT&T-derived systems was to have read() and write() return −1 and set
       errno to [EINTR] when interrupted after some, but not all, of the
       data requested had been transferred. However, the US Department of
       Commerce FIPS 151‐1 and FIPS 151‐2 require the historical BSD
       behavior, in which read() and write() return the number of bytes
       actually transferred before the interrupt.  If −1 is returned when
       any data is transferred, it is difficult to recover from the error on
       a seekable device and impossible on a non-seekable device. Most new
       implementations support this behavior.  The behavior required by
       POSIX.1‐2008 is to return the number of bytes transferred.

       POSIX.1‐2008 does not specify when an implementation that buffers
       read()s actually moves the data into the user-supplied buffer, so an
       implementation may choose to do this at the latest possible moment.
       Therefore, an interrupt arriving earlier may not cause read() to
       return a partial byte count, but rather to return −1 and set errno to
       [EINTR].

       Consideration was also given to combining the two previous options,
       and setting errno to [EINTR] while returning a short count. However,
       not only is there no existing practice that implements this, it is
       also contradictory to the idea that when errno is set, the function
       responsible shall return −1.

       This volume of POSIX.1‐2008 intentionally does not specify any
       pread() errors related to pipes, FIFOs, and sockets other than
       [ESPIPE].

FUTURE DIRECTIONS         top

       None.

SEE ALSO         top

       fcntl(3p), ioctl(3p), lseek(3p), open(3p), pipe(3p), readv(3p)

       The Base Definitions volume of POSIX.1‐2008, Chapter 11, General
       Terminal Interface, stropts.h(0p), sys_uio.h(0p), unistd.h(0p)

COPYRIGHT         top

       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1, 2013 Edition, Standard for Information
       Technology -- Portable Operating System Interface (POSIX), The Open
       Group Base Specifications Issue 7, Copyright (C) 2013 by the
       Institute of Electrical and Electronics Engineers, Inc and The Open
       Group.  (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
       applied.) In the event of any discrepancy between this version and
       the original IEEE and The Open Group Standard, the original IEEE and
       The Open Group Standard is the referee document. The original
       Standard can be obtained online at http://www.unix.org/online.html .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the
       source files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .

IEEE/The Open Group                 2013                            READ(3P)

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