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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.

       If count is zero, read() returns zero and has no other results.  If
       count is greater than SSIZE_MAX, the result is unspecified.

       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.  On error, -1 is returned, and errno is set
       appropriately. In this case it is left unspecified whether the file
       position (if any) changes.

       EINTR  The call was interrupted by a signal before any data was read.

       EAGAIN Non-blocking I/O has been selected using O_NONBLOCK and no data
              was immediately available for reading.

       EIO    I/O error. This will happen for example when the process is in a
              background process group, tries to read from its controlling
              tty, 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.

       EISDIR fd refers to a directory.

       EBADF  fd is not a valid file descriptor or is not open for reading.

       EINVAL fd is attached to an object which is unsuitable for reading.

       EFAULT buf is outside your accessible address space.

       Other errors may occur, depending on the object connected to fd.  POSIX
       allows a read that is interrupted after reading some data to return -1
       (with errno set to EINTR) or to return the number of bytes already

       SVr4, SVID, AT&T, POSIX, X/OPEN, BSD 4.3

       On NFS file systems, reading small amounts of data will only update the
       time stamp 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 atime updates to the server and client side reads
       satisfied from the client's cache will not cause 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

       Many filesystems and disks were considered to be fast enough that the
       implementation of O_NONBLOCK was deemed unneccesary. So, O_NONBLOCK may
       not be available on files and/or disks.

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

Linux 2.0.32                      1997-07-12                           READ(2)


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