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

NAME
     read, readv — read input

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

     ssize_t
     read(int d, void *buf, size_t nbytes);

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

DESCRIPTION
     Read() attempts to read nbytes of data from the object referenced by the
     descriptor d into the buffer pointed to by buf.  Readv() performs the
     same action, but scatters the input data into the iovcnt buffers speci‐
     fied by the members of the iov array: iov[0], iov[1], ..., iov[iovcnt-1].

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

	   struct iovec {
		   void *iov_base;
		   size_t iov_len;
	   };

     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 d (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() and readv() 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 read‐
     ing end-of-file, zero is returned.	 Otherwise, a -1 is returned and the
     global variable errno is set to indicate the error.

ERRORS
     Read() and readv() will succeed unless:

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

     [EFAULT]		Buf points outside the allocated address space.

     [EIO]		An I/O error occurred while reading from the file sys‐
			tem.

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

     [EINVAL]		The pointer associated with d was negative.

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

     In addition, readv() may return one of the following errors:

     [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 nega‐
			tive.

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

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

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

STANDARDS
     Read() is expected to conform to IEEE Std 1003.1-1988 (“POSIX”).

HISTORY
     The readv() function call appeared in 4.2BSD.  A read function call
     appeared in Version 6 AT&T UNIX.

4th Berkeley Distribution      February 26, 1994     4th Berkeley Distribution
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