| READ(2) |
AerieBSD 1.0 Refernce Manual |
READ(2) |
NAME
read —
readv,
pread,
preadv
read input
SYNOPSIS
#include <sys/types.h>
#include <unistd.h>
ssize_t
read(int d, void *buf, size_t nbytes);
ssize_t
pread(int d, void *buf, size_t nbytes, off_t offset);
#include <sys/types.h>
#include <sys/uio.h>
#include <unistd.h>
ssize_t
readv(int d, const struct iovec *iov, int iovcnt);
ssize_t
preadv(int d, const struct iovec *iov, int iovcnt, off_t offset);
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 specified by the members of the
iov
array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
pread();
and
preadv();
perform the same functions, but read from the specified position in
the file without modifying the file pointer.
For
readv();
and
preadv();,
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 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
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();,
pread();,
and
preadv();
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.
Note that
readv();
and
preadv();
will fail if the value of
iovcnt
exceeds the constant
IOV_MAX.
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
read();,
readv();,
pread();,
and
preadv();
will succeed unless:
- [EBADF]
-
d
is not a valid file or socket descriptor open for reading.
- [EFAULT]
-
Part of
iov
or
buf
points outside the process's allocated address space.
- [EIO]
-
An I/O error occurred while reading from the file system,
or the process is a member of a background process attempting to read
from its controlling terminal, the process is ignoring or blocking
the SIGTTIN signal or the process group is orphaned.
- [EINTR]
-
A read from a slow device
(i.e. one that might block for an arbitrary amount of time)
was interrupted by the delivery of a signal
before any data arrived.
- [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,
read();
and
pread();
may return the following error:
- [EINVAL]
-
nbytes
was larger than
SSIZE_MAX.
pread();
and
preadv();
may return the following error:
- [ESPIPE]
-
d
is associated with a pipe, socket, or FIFO.
readv();
and
preadv();
may return one of the following errors:
- [EINVAL]
-
iovcnt
was less than or equal to 0, or greater than
IOV_MAX.
- [EINVAL]
-
The sum of the
iov_len
values in the
iov
array overflowed an
ssize_t.
SEE ALSO
dup(2),
fcntl(2),
open(2),
pipe(2),
poll(2),
select(2),
socket(2),
socketpair(2)
STANDARDS
The
read();
function conforms to
The
readv();
and
pread();
functions conform to
HISTORY
The
preadv();
function first appeared in
OpenBSD 2.7.
The
pread();
function appeared in
System V.4 AT&T UNIX.
The
readv();
function call appeared in
4.2BSD.
The
read();
function call appeared in
Version 2 AT&T UNIX.
CAVEATS
Error checks should explicitly test for \-1.
Code such as
while ((nr = read(fd, buf, sizeof(buf))) > 0)
is not maximally portable, as some platforms allow for
nbytes
to range between
SSIZE_MAX
and
SIZE_MAX
\- 2, in which case the return value of an error-free
read();
may appear as a negative number distinct from \-1.
Proper loops should use
while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0)
| AerieBSD 1.0 Reference Manual |
May 14 2010 |
READ(2) |