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READ(2) Linux Programmer's Manual READ(2)


     read - read from a file descriptor


     #include <unistd.h>
     ssize_t read(int fd, void *buf, size_t count);


     read()  attempts to read up to count bytes from file descriptor fd into
     the buffer starting at buf.
     On files that support seeking, the read operation commences at the file
     offset, and the file offset is incremented by the number of bytes read.
     If the file offset is at or past the end of file, no  bytes  are  read,
     and read() returns zero.
     If count is zero, read() may detect the errors described below.  In the
     absence of any errors, or if read() does not check for errors, a read()
     with a count of 0 returns zero and has no other effects.
     According to POSIX.1, if count is greater than SSIZE_MAX, the result is
     implementation-defined; see NOTES for the upper limit on Linux.


     On success, the number of bytes read is returned (zero indicates end of
     file),  and the file position is advanced by this number.  It is not an
     error if this number is smaller than the  number  of  bytes  requested;
     this  may happen for example because fewer bytes are actually available
     right now (maybe because we were close to end-of-file,  or  because  we
     are  reading  from  a  pipe, or from a terminal), or because read() was
     interrupted by a signal.  See also NOTES.
     On error, -1 is returned, and errno  is  set  appropriately.   In  this
     case,  it  is  left  unspecified  whether  the  file  position (if any)


     EAGAIN The file descriptor fd refers to a file other than a socket  and
            has  been  marked  nonblocking  (O_NONBLOCK), and the read would
            block.  See open(2) for further details on the O_NONBLOCK  flag.
            The  file  descriptor  fd refers to a socket and has been marked
            nonblocking   (O_NONBLOCK),   and   the   read   would    block.
            POSIX.1-2001  allows  either error to be returned for this case,
            and does not require these constants to have the same value,  so
            a portable application should check for both possibilities.
     EBADF  fd is not a valid file descriptor or is not open for reading.
     EFAULT buf is outside your accessible address space.
     EINTR  The  call  was interrupted by a signal before any data was read;
            see signal(7).
     EINVAL fd is attached to an object which is unsuitable for reading;  or
            the  file  was  opened  with  the  O_DIRECT flag, and either the
            address specified in buf, the value specified in count,  or  the
            file offset is not suitably aligned.
     EINVAL fd  was  created  via  a call to timerfd_create(2) and the wrong
            size buffer was given to read(); see timerfd_create(2) for  fur-
            ther information.
     EIO    I/O  error.  This will happen for example when the process is in
            a background process group, tries to read from  its  controlling
            terminal,  and  either it is ignoring or blocking SIGTTIN or its
            process group is orphaned.  It may also occur when  there  is  a
            low-level  I/O  error while reading from a disk or tape.  A fur-
            ther possible cause of EIO on networked filesystems is  when  an
            advisory lock had been taken out on the file descriptor and this
            lock has been lost.  See the Lost locks section of fcntl(2)  for
            further details.
     EISDIR fd refers to a directory.
     Other errors may occur, depending on the object connected to fd.


     SVr4, 4.3BSD, POSIX.1-2001.


     The  types  size_t  and  ssize_t are, respectively, unsigned and signed
     integer data types specified by POSIX.1.
     On Linux, read() (and similar  system  calls)  will  transfer  at  most
     0x7ffff000  (2,147,479,552)  bytes, returning the number of bytes actu-
     ally transferred.  (This is true on both 32-bit and 64-bit systems.)
     On NFS filesystems, reading small amounts of data will update the time-
     stamp  only  the  first  time, subsequent calls may not do so.  This is
     caused by client side attribute caching, because most if  not  all  NFS
     clients  leave  st_atime (last file access time) updates to the server,
     and client side reads satisfied from the client's cache will not  cause
     st_atime updates on the server as there are no server-side reads.  UNIX
     semantics can be obtained by disabling client-side  attribute  caching,
     but in most situations this will substantially increase server load and
     decrease performance.


     According to POSIX.1-2008/SUSv4 Section XSI 2.9.7 ("Thread Interactions
     with Regular File Operations"):
         All of the following functions shall be atomic with respect to each
         other in the effects specified in POSIX.1-2008 when they operate on
         regular files or symbolic links: ...
     Among  the APIs subsequently listed are read() and readv(2).  And among
     the effects that should be atomic across threads  (and  processes)  are
     updates  of  the  file  offset.  However, on Linux before version 3.14,
     this was not the case:  if  two  processes  that  share  an  open  file
     description  (see  open(2))  perform a read() (or readv(2)) at the same
     time, then the I/O operations were not atomic with respect updating the
     file  offset, with the result that the reads in the two processes might
     (incorrectly) overlap in the blocks of data that they  obtained.   This
     problem was fixed in Linux 3.14.


     close(2),  fcntl(2), ioctl(2), lseek(2), open(2), pread(2), readdir(2),
     readlink(2), readv(2), select(2), write(2), fread(3)


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2018-02-02 READ(2)

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