ultrix man page for read

read(2) 							System Calls Manual							   read(2)

Name
       read, readv - read from a file

Syntax
       cc = read(d, buf, nbytes)
       int cc, d;
       char *buf;
       int nbytes;

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

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

Arguments
       d	 File descriptor.

       buf	 Character pointer where information is stored.

       nbytes	 Integer that tells you how many bytes to read.

       iov	 Pointer to an iovec structure.

       iovcnt	 The number of iovec structures to be processed.

Description
       The system call attempts to read nbytes of data from the object referenced by the descriptor d into the buffer pointed to by buf.  The sys-
       tem call performs the same action, but scatters the input data into the iovcnt buffers specified by the	members  of  the  iovec  following
       array: iov[0], iov[1], ..., iov[iovcnt-1].

       For the iovec structure is defined as follows:

       struct iovec {
	    caddr_t   iov_base;
	    int  iov_len;
       };

       Each  iovec  entry  specifies  the base address and length of an area in memory where data should be placed.  The system call fills an area
       completely before proceeding to the next area.

       On objects that are capable of seeking, the starts at a position given by the pointer associated with d.  See for more  information.   Upon
       return from 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.

       When attempting to read from an empty pipe (or FIFO):

       o    If no process has the pipe open for writing, returns zero to indicate end-of-file.

       o    If some process has the pipe open for writing and O_NDELAY or O_NONBLOCK is set, returns a -1, errno is  to  [EWOULDBLOCK].   If  some
	    process has the pipe open for writing and O_NDELAY and O_NONBLOCK are clear, blocks until data is written or the pipe is closed by all
	    processes that opened the pipe for writing.

       Upon successful completion, and return the number of bytes actually read and placed in the buffer.  The system reads the  number  of  bytes
       requested  if  the  descriptor  references  a  file  which  has	that many bytes left before the end-of-file; this is not true in any other
       instance.

       Unless the SV_INTERRUPT bit has been set for a signal, the system calls are automatically restarted when a process receives a signal  while
       waiting for input. See also

Return Values
       If the returned value is 0, then end-of-file has been reached.

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

Diagnostics
       The and system calls fail if one or more of the following are true:

       [EBADF]	      The d argument is not a valid file or socket descriptor open for reading.

       [EFAULT]       The 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.

       [EIO]	      An I/O error occurred while reading from the file system.

       [ESTALE]       The file handle given in the argument is invalid. The file referred to by that file handle no  longer  exists  or  has  been
		      revoked.

       [EWOULDBLOCK]  The O_DELAY or O_NONBLOCK flag is set for the file descriptor and the process would be delayed in the read operation.

       In addition, may return one of the following errors:

       [EINVAL]       The 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.

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

       [ETIMEDOUT]    A  connect request or remote file operation failed because the connected party did not respond after a period of time deter-
		      mined by the communications protocol.

Environment
       SYSTEM_FIVE

       When you use the System V environment, note the following:

       o    If your program is compiled in this environment, a and system call returns 0 if the file has been set up for non-blocking I/O and  the
	    read would block.

       o    In this environment, the parameter nbytes is of type int instead of type unsigned.

       POSIX

       In the POSIX environment, [EAGAIN] is returned in errno instead of [EWOULDBLOCK].

See Also
       dup(2), open(2), pipe(2), sigvec(2), socket(2), socketpair(2)

																	   read(2)