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pread(2)

NAME

     read, readv, pread -- read input

LIBRARY

     Standard C Library (libc, -lc)

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);

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

DESCRIPTION

     The read() system call attempts to read nbytes of data from the object
     referenced by the descriptor d into the buffer pointed to by buf.	The
     readv() system call 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].  The pread() system call 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 {
		   void   *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.  The readv() system call 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 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(), 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.

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
     [EFAULT]		The buf argument 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.

     [EISDIR]		The file descriptor is associated with a directory
			residing on a file system that does not allow regular
			read operations on directories (e.g. NFS).

     [EOPNOTSUPP]	The file descriptor is associated with a file system
			and file type that do not allow regular read opera-
			tions on it.

     [EOVERFLOW]	The file descriptor is associated with a regular file,
			nbytes is greater than 0, offset is before the end-of-
			file, and offset is greater than or equal to the off-
			set maximum established for this file system.

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

     [EINVAL]		The iovcnt argument was less than or equal to 0, or
			greater than IOV_MAX.

     [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.

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

     [EINVAL]		The offset value was negative.

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

SEE ALSO

     dup(2), fcntl(2), getdirentries(2), open(2), pipe(2), select(2),
     socket(2), socketpair(2), fread(3), readdir(3)

STANDARDS

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

HISTORY

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