4.2BSD/usr/man/man2/read.2

.TH READ 2 "27 July 1983"
.UC 4
.SH NAME
read, readv \- read input
.SH SYNOPSIS
.nf
.ft B
cc = read(d, buf, nbytes)
int cc, d;
char *buf;
int nbytes;
.PP
.ft B
#include <sys/types.h>
#include <sys/uio.h>
.PP
.ft B
cc = readv(d, iov, iovcnt)
int cc, d;
struct iovec *iov;
int iovcnt;
.fi
.SH DESCRIPTION
.I 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 .
.I Readv
performs the same action, but scatters the input data
into the 
.I iovcnt
buffers specified by the members of the
.I iovec
array: iov[0], iov[1], ..., iov[iovcnt\|\-\|1].
.PP
For 
.IR readv ,
the 
.I iovec
structure is defined as
.PP
.nf
.RS
.DT
struct iovec {
	caddr_t	iov_base;
	int	iov_len;
};
.RE
.fi
.PP
Each 
.I iovec
entry specifies the base address and length of an area
in memory where data should be placed. 
.I Readv
will always fill an area completely before proceeding
to the next.
.PP
On objects capable of seeking, the
.I read
starts at a position
given by the pointer associated with
.IR d ,
see
.IR lseek (2).
Upon return from
.IR 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 a 
object is undefined.
.PP
Upon successful completion,
.I read
and
.I 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 file which has that many bytes left
before the end-of-file, but in no other cases.
.PP
If the returned value is 0, then
end-of-file has been reached.
.SH "RETURN VALUE
If 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 "ERRORS
.I Read
and
.I readv
will fail if one or more of the following are true:
.TP 15
[EBADF]
\fIFildes\fP is not a valid file descriptor open for reading.
.TP 15
[EFAULT]
\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.
.PP
In addition, 
.I readv
may return one of the following errors:
.TP 15
[EINVAL]
.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.
.SH "SEE ALSO"
dup(2), open(2), pipe(2), socket(2), socketpair(2)