# read

## PROLOG



           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



           pread, read — read from a file
    


## SYNOPSIS



           #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



           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 offset. 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



           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



           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 offset shall remain
                  unchanged.
    
           **ESPIPE** The file is incapable of seeking.
    
           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



       **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



           None.
    


## RATIONALE



           This volume of POSIX.1‐2017 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 offset 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
           **long** s) 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.1‐2017 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.1‐2017 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.1‐2017, 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.1‐2017. 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.1‐2017 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.1‐2017 intentionally does not specify any
           _pread_() errors related to pipes, FIFOs, and sockets other than
           **[ESPIPE]**.
    


## FUTURE DIRECTIONS



           None.
    


* * *