read.2

<B>Digital的Unix操作系统VAX 4.2源码</B>

.\" SCCSID: @(#)read.2	2.1	3/10/87
.TH read 2
.SH Name
read, readv \- read from a file
.SH Syntax
.nf
cc = read(\fId, buf, nbytes\fP)
int cc, \fId\fP;
char *\fIbuf\fP;
int \fInbytes\fP;
.PP
#include <sys/types.h>
#include <sys/uio.h>
.PP
cc = readv(\fId, iov, iovcnt\fP)
int cc, \fId\fP;
struct iovec *\fIiov\fP;
int \fIiovcnt\fP;
.fi
.SH Arguments
.TP 10
.I d
File descriptor.
.TP 10
.I buf
Character pointer where information is stored.
.TP 10
.I nbytes
Integer that tells you how many bytes to read.
.TP 10
.I iov
Pointer to an
.I iovec
structure.
.TP 10
.I iovcnt
The number of 
.I iovec
structures to be processed.
.SH Description
.NXR "read system call"
.NXR "file" "reading"
.NXR "socket" "reading"
.NXR "readv system call"
.NXR "read system call" "write system call"
.NXR "read system call" "send system call"
The system call
.PN read
attempts to read
.I nbytes
of data from the object referenced by the descriptor
.I d
into the buffer pointed to by
.IR buf .
The
.PN readv
system call performs the same action, but scatters the input data
into the 
.I iovcnt
buffers specified by the members of the
.I iovec
following array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
.PP
For 
.PN readv ,
the 
.I iovec
structure is defined as follows:
.PP
.EX
struct iovec {
	caddr_t	iov_base;
	int	iov_len;
};
.EE
.PP
Each 
.I iovec
entry specifies the base address and length of an area
in memory where data should be placed. 
The
.PN readv
system call fills an area completely before proceeding
to the next area.
.PP
On objects that are capable of seeking, the
.PN read
starts at a position
given by the pointer associated with
.IR d .
See 
.MS lseek 2 
for more information.
Upon return from
.PN read ,
the pointer is incremented by the number of bytes actually read.
.PP
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.
.PP
When attempting to read from an empty pipe (or FIFO):
.IP \(bu 5
If no process has the pipe open for writing, 
.PN read
returns zero to indicate end-of-file.
.IP \(bu 5
If some process has the pipe open for writing and O_NDELAY or O_NONBLOCK is
set, 
.PN read
returns a \-1,
.I errno
is to [EWOULDBLOCK].  If some process has the 
pipe open for writing and O_NDELAY and O_NONBLOCK are clear, 
.PN read
blocks until data is written or the pipe is closed by all 
processes that opened the pipe for writing.
.PP
Upon successful completion,
.PN read
and
.PN readv
return the number of bytes actually read and placed in the buffer.
The system reads the number of bytes requested if
the descriptor references a file which has that many bytes left
before the end-of-file; this is not true in any other instance.
.PP
Unless the SV_INTERRUPT bit has been set for a signal, the 
.PN read 
system calls are automatically restarted when a process receives a
signal while waiting for input. See also 
.MS sigvec 2 .
.SH Return Values
If the returned value is 0, then
end-of-file has been reached.
.PP
If the read is successful, the
number of bytes actually read is returned.
Otherwise, a \-1 is returned and the global variable
.I errno
is set to indicate the error.
.SH Diagnostics
.NXR "read system call" "diagnostics"
The
.PN read
and
.PN readv
system calls fail if one or more of the following are true:
.TP 15
[EBADF]
The
.I d
argument is not a valid file or socket descriptor open for reading.
.TP 15
[EFAULT]
The
\fIbuf\fP points outside the allocated address space.
.TP 15
[EINTR]
A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
.TP 15
[EIO]
An I/O error occurred while reading from the file system.
.TP 15
[ESTALE]
The file handle given in the argument is invalid. The
file referred to by that file handle no longer exists
or has been revoked.
.TP 15
[EWOULDBLOCK]
The O_DELAY or O_NONBLOCK flag is set for the file descriptor and 
the process would be delayed in the read operation. 
.PP
In addition, 
.PN readv
may return one of the following errors:
.TP 15
[EINVAL]
The
.I iovcnt
was less than or equal to 0, or greater than 16.
.TP 15
[EINVAL]
One of the
.I iov_len
values in the
.I iov
array was negative.
.TP 15
[EINVAL]
The sum of the
.I iov_len
values in the
.I iov
array overflowed a 32-bit integer.
.TP 15
[EFAULT]
Part of the 
.I iov
points outside the process's allocated address space.
.TP
[ETIMEDOUT]
A connect request or remote file operation failed
because the connected party did not respond after a period
of time determined by the communications protocol.
.SH Environment
.NXR "read system call" "System V and"
SYSTEM_FIVE
.PP
When you use the System V environment, note the following:
.IP \(bu 5
If your program is compiled in this environment, a 
.PN read
and
.PN readv
system call 
returns 0 if the file has been set up for non-blocking
I/O and the read would block.
.PP
.IP \(bu 5
In this environment, the parameter
.I nbytes
is of type 
.I int 
instead of type
.I unsigned.
.PP
POSIX
.PP
In the POSIX environment, [EAGAIN] is returned in
.I errno
instead of [EWOULDBLOCK].
.SH See Also
dup(2), open(2), pipe(2), sigvec(2), socket(2), socketpair(2)