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read(2)

read(2)                          System Calls                          read(2)



NAME
       read, readv, pread - read from 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 associ‐
       ated with the open file descriptor, fildes, into the buffer pointed  to
       by buf.


       If nbyte is 0, read() returns 0 and has 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.


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


       No data transfer will occur past the current end-of-file. If the start‐
       ing 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.


       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:

           o      If O_NDELAY or O_NONBLOCK is set, read() returns −1 and sets
                  errno to EAGAIN.


           o      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):

           o      If no process has the pipe open for writing, read()  returns
                  0 to indicate end-of-file.


           o      If  a  process has the pipe open for writing and O_NDELAY is
                  set, read() returns 0.


           o      If a process has the pipe open for writing and O_NONBLOCK is
                  set, read() returns −1 and sets errno to EAGAIN.


           o      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:

           o      If O_NDELAY is set, read() returns 0.


           o      If O_NONBLOCK is set, read() returns −1 and  sets  errno  to
                  EAGAIN.


           o      If  O_NDELAY  and  O_NONBLOCK are clear, read() blocks until
                  data become available.



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

           o      If O_NDELAY or O_NONBLOCK is set, read() returns −1 and sets
                  errno to EAGAIN.


           o      If  O_NDELAY  and  O_NONBLOCK are clear, read() blocks until
                  data becomes 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 writ‐
       ten, read() returns bytes with value 0. For  example,  lseek(2)  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.


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


       Upon  successful completion, where nbyte is greater than 0, read() will
       mark to update the st_atime field of the file, and return the number of
       bytes read. This number is never 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(2)  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  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  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  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(2) call.


       How read() handles zero-byte streams messages is determined by the cur‐
       rent  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(2). 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  mes‐
       sage  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() fails if the stream head has 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.

   readv()
       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}.


       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. 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.

   pread()
       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. pread() will read up to the maximum offset value  that
       can be represented in an off_t for regular files. An attempt to perform
       a pread() on a file that is incapable of seeking results in an error.

RETURN VALUES
       Upon successful completion, read() 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(), readv(), and pread() functions will fail if:


       EADI       The buffer pointed to by buf or a buffer  specified  in  the
                  iov array is enabled for ADI, and an ADI version mismatch is
                  detected while the system writes data  to  the  buffer.  For
                  more information, see the adi(3C) man page.


       EAGAIN     Mandatory  file/record  locking  was set, O_NDELAY or O_NON‐
                  BLOCK was set, and there was a blocking record  lock;  total
                  amount of system memory available when reading using raw I/O
                  is temporarily insufficient; no data is waiting to  be  read
                  on  a  file  associated with a tty device and O_NONBLOCK was
                  set; or no message is waiting to be read  on  a  stream  and
                  O_NDELAY or O_NONBLOCK was set.


       EBADF      The  fildes argument is not a valid file descriptor open for
                  reading.


       EBADMSG    Message waiting to be read on a stream is not  a  data  mes‐
                  sage.


       EDEADLK    The  read  was  going to go to sleep and cause a deadlock to
                  occur.


       EINTR      A signal was caught during the read operation  and  no  data
                  was transferred.


       EINVAL     An attempt was made to read from a stream linked to a multi‐
                  plexor.


       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.


       EISDIR     The fildes argument refers to a directory on a  file  system
                  type that does not support read operations on directories.


       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    The  fildes  argument is on a remote machine and the link to
                  that machine is no longer active.


       ENXIO      The device associated with fildes  is  a  block  special  or
                  character  special file and the value of the file pointer is
                  out of range.



       The read() and pread() functions will fail if:

       EFAULT    The buf argument points to an illegal address.


       EINVAL    The nbyte argument overflowed an ssize_t.



       The read() and readv() functions will fail if:

       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 associ‐
                    ated with fildes.



       The readv() function may fail if:

       EFAULT    The iov argument points outside the allocated address space.


       EINVAL    The iovcnt argument was less than or equal to  0  or  greater
                 than {IOV_MAX}. See Intro(2) for a definition of {IOV_MAX}).

                 One  of  the iov_len values in the iov array was negative, or
                 the sum of the iov_len values in the iov array overflowed  an
                 ssize_t.



       The  pread()  function  will fail and the file pointer remain unchanged
       if:

       ESPIPE    The fildes argument is associated with  a  pipe  or  FIFO  or
                 socket.


USAGE
       The  pread() function has a transitional interface for 64-bit file off‐
       sets. See lf64(7).

ATTRIBUTES
       See attributes(7) for descriptions of the following attributes:


       tab() box; cw(2.75i) |cw(2.75i) lw(2.75i) |lw(2.75i)


       ATTRIBUTE TYPEATTRIBUTE VALUE _ Interface StabilityCommitted _  MT-Lev‐
       elread() is Async-Signal-Safe _ StandardSee standards(7).


SEE ALSO
       chmod(2),  creat(2),  dup(2),  fcntl(2), getmsg(2), Intro(2), ioctl(2),
       lseek(2), open(2), pipe(2),  streamio(4I),  termio(4I),  attributes(7),
       lf64(7), standards(7)



Oracle Solaris 11.4               06 May 2016                          read(2)
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