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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(3SOCKET)
       with no flags set.


       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.


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

           o      If some 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 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(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 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(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
       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 contain‐
       ing 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  pro‐
       cessed  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:

       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.


USAGE
       The pread() function has a transitional interface for 64-bit file  off‐
       sets.  See lf64(5).

ATTRIBUTES
       See attributes(5) 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-Levelread()
       is Async-Signal-Safe _ StandardSee standards(5).


SEE ALSO
       Intro(2),  chmod(2),  creat(2),  dup(2), fcntl(2), getmsg(2), ioctl(2),
       lseek(2),  open(2),  pipe(2),  recv(3SOCKET),  attributes(5),  lf64(5),
       standards(5), streamio(7I), termio(7I)



SunOS 5.11                        24 Mar 2011                          read(2)
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