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READ(2)                     BSD System Calls Manual                    READ(2)

NAME
     pread, read, readv -- read input

LIBRARY
     Standard C Library (libc, -lc)

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

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

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

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

DESCRIPTION
     Read() attempts to read nbyte bytes of data from the object referenced by the descriptor fildes into
     the buffer pointed to by buf.  Readv() performs 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 memory where data should be
     placed.  Readv() will always fill an area completely 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 undefined.

     Upon successful completion, read(), readv(), and pread() return the number of bytes actually read and
     placed in the buffer.  The system guarantees 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.

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

ERRORS
     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 system.

     [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 descriptor 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 exist..

     [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 16.

     [EINVAL]           One of the iov_len values in the iov array was negative.

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

LEGACY SYNOPSIS
     #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 functions.

SEE ALSO
     dup(2), fcntl(2), open(2), pipe(2), select(2), socket(2), socketpair(2), compat(5)

STANDARDS
     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'').

HISTORY
     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