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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() per-forms performs
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 memory
ory where data should be placed. Readv() will always fill an area com-pletely completely
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
undefined.
Upon successful completion, read(), readv(), and pread() return the num-ber number
ber of bytes actually read and placed in the buffer. The system guaran-tees guarantees
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 reading
ing 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 sys-tem. system.
tem.
[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 descriptor
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
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 nega-tive. negative.
tive.
[EINVAL] The sum of the iov_len values in the iov array over-flowed overflowed
flowed 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
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