READ
Section: System Calls (2)
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BSD mandoc
BSD 4
NAME
read
readv
- read input
SYNOPSIS
Fd #include <sys/types.h>
Fd #include <sys/uio.h>
Fd #include <unistd.h>
Ft ssize_t
Fn read int d void *buf size_t nbytes
Ft ssize_t
Fn readv int d const struct iovec *iov int iovcnt
DESCRIPTION
Fn Read
attempts to read
Fa nbytes
of data from the object referenced by the descriptor
Fa d
into the buffer pointed to by
Fa buf .
Fn Readv
performs the same action, but scatters the input data
into the
Fa iovcnt
buffers specified by the members of the
Fa iov
array: iov[0], iov[1], ..., iov[iovcnt-1].
For
Fn readv ,
the
Fa iovec
structure is defined as:
struct iovec {
void *iov_base;
size_t iov_len;
};
Each
Fa iovec
entry specifies the base address and length of an area
in memory where data should be placed.
Fn Readv
will always fill an area completely before proceeding
to the next.
On objects capable of seeking, the
Fn read
starts at a position
given by the pointer associated with
Fa d
(see
lseek(2)).
Upon return from
Fn 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,
Fn read
and
Fn 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 reading end-of-file,
zero is returned.
Otherwise, a -1 is returned and the global variable
errno
is set to indicate the error.
ERRORS
Fn Read
and
Fn readv
will succeed unless:
- Bq Er EBADF
-
Fa D
is not a valid file or socket descriptor open for reading.
- Bq Er EFAULT
-
Fa Buf
points outside the allocated address space.
- Bq Er EIO
-
An I/O error occurred while reading from the file system.
- Bq Er EINTR
-
A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
- Bq Er EINVAL
-
The pointer associated with
Fa d
was negative.
- Bq Er EAGAIN
-
The file was marked for non-blocking I/O,
and no data were ready to be read.
In addition,
Fn readv
may return one of the following errors:
- Bq Er EINVAL
-
Fa Iovcnt
was less than or equal to 0, or greater than
{UIO_MAXIOV}
- Bq Er EINVAL
-
One of the
Fa iov_len
values in the
Fa iov
array was negative.
- Bq Er EINVAL
-
The sum of the
Fa iov_len
values in the
Fa iov
array overflowed a 32-bit integer.
- Bq Er EFAULT
-
Part of the
Fa 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
The
Fn read
function is expected to conform to
St -p1003.1-88 .
HISTORY
The
Fn readv
function call
appeared in
BSD 4.2
The
Fn read
function call appeared in
AT&T System
v6 .
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUES
-
- ERRORS
-
- SEE ALSO
-
- STANDARDS
-
- HISTORY
-
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Time: 19:41:53 GMT, December 25, 2022