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read(2)                     BSD System Calls Manual                    read(2)


     pread, read, readv -- read input


     Standard C Library (libc, -lc)


     #include <sys/types.h>
     #include <sys/uio.h>
     #include <unistd.h>

     pread(int d, void *buf, size_t nbyte, off_t offset);

     read(int fildes, void *buf, size_t nbyte);

     readv(int d, const struct iovec *iov, int iovcnt);


     Read() attempts to read nbyte bytes of data from the object referenced by
     the descriptor fildes into the buffer pointed to by buf.  Readv() per-
     forms the same action, but scatters the input data into the iovcnt
     buffers specified by the members of the iov array: iov[0], iov[1], ...,
     iov[iovcnt-1].  Pread() performs the same function, but reads from the
     specified position in the file without modifying the file pointer.

     For readv(), the iovec structure is defined as:

           struct iovec {
                   char   *iov_base;  /* Base address. */
                   size_t iov_len;    /* Length. */

     Each iovec entry specifies the base address and length of an area in mem-
     ory where data should be placed.  Readv() will always fill an area com-
     pletely before proceeding to the next.

     On objects capable of seeking, the read() starts at a position given by
     the pointer associated with fildes (see lseek(2)).  Upon return from
     read(), the pointer is incremented by the number of bytes actually read.

     Objects that are not capable of seeking always read from the current
     position.  The value of the pointer associated with such an object is

     Upon successful completion, read(), readv(), and pread() return the num-
     ber of bytes actually read and placed in the buffer.  The system guaran-
     tees to read the number of bytes requested if the descriptor references a
     normal file that has that many bytes left before the end-of-file, but in
     no other case.


     If successful, the number of bytes actually read is returned.  Upon read-
     ing end-of-file, zero is returned.  Otherwise, a -1 is returned and the
     global variable errno is set to indicate the error.


     The pread(), read(), and readv() calls will succeed unless:

     [EAGAIN]           The file was marked for non-blocking I/O, and no data
                        were ready to be read.

     [EBADF]            fildes is not a valid file or socket descriptor open
                        for reading.

     [EFAULT]           Buf points outside the allocated address space.

     [EINTR]            A read from a slow device was interrupted before any
                        data arrived by the delivery of a signal.

     [EINVAL]           The pointer associated with fildes was negative.

     [EIO]              An I/O error occurred while reading from the file sys-

     [EIO]              The process group is orphaned.

     [EIO]              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 for the open file descrip-
                        tor associated with fildes.

     [EISDIR]           An attempt is made to read a directory.

     [ENOBUFS]          An attempt to allocate a memory buffer fails.

     [ENOMEM]           Insufficient memory is available.

     [ENXIO]            An action is requested of a device that does not

     [ENXIO]            A requested action cannot be performed by the device.

     The pread() call may also return the following errors:

     [EINVAL]           The specified file offset is invalid.

     [ESPIPE]           The file descriptor is associated with a pipe, socket,
                        or FIFO.

     The read() call may also return the following errors:

     [ECONNRESET]       The connection is closed by the peer during a read
                        attempt on a socket.

     [ENOTCONN]         A read is attempted on an unconnected socket.

     [ETIMEDOUT]        A transmission timeout occurs during a read attempt on
                        a socket.

     The readv() call may also return one of the following errors:

     [EFAULT]           Part of the iov points outside the process's allocated
                        address space.

     [EINVAL]           Iovcnt was less than or equal to 0, or greater than

     [EINVAL]           One of the iov_len values in the iov array was nega-

     [EINVAL]           The sum of the iov_len values in the iov array over-
                        flowed a 32-bit integer.


     #include <sys/types.h>
     #include <sys/uio.h>
     #include <unistd.h>

     The include files <sys/types.h> and <sys/uio.h> are necessary for all


     dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2),


     The read() function call is expected to conform to ISO/IEC 9945-1:1990
     (``POSIX.1'').  The readv() and pread() functions are expected to conform
     to X/Open Portability Guide Issue 4, Version 2 (``XPG4.2'').


     The pread() function call appeared in AT&T System V.4 UNIX.  The readv()
     function call appeared in 4.2BSD.  A read() function call appeared in
     Version 6 AT&T UNIX.

BSD                            February 26, 1994                           BSD

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