path: root/lib/libc/sys/read.2

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.\"     @(#)read.2	8.4 (Berkeley) 2/26/94
.\" $FreeBSD$
.\"
.Dd February 26, 1994
.Dt READ 2
.Os BSD 4
.Sh NAME
.Nm read ,
.Nm readv ,
.Nm pread
.Nd read input
.Sh 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"
.Ft ssize_t
.Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset"
.Sh 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].
.Fn Pread
performs the same function, but reads from the specified position in
the file without modifying the file pointer.
.Pp
For 
.Fn readv ,
the 
.Fa iovec
structure is defined as:
.Pp
.Bd -literal -offset indent -compact
struct iovec {
	char   *iov_base;  /* Base address. */
	size_t iov_len;    /* Length. */
};
.Ed
.Pp
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.
.Pp
On objects capable of seeking, the
.Fn read
starts at a position
given by the pointer associated with
.Fa d
(see
.Xr lseek 2 ) .
Upon return from
.Fn 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
Upon successful completion,
.Fn read ,
.Fn readv ,
and
.Fn pread
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.
.Pp
.Sh IMPLEMENTATION NOTES
.Pp
In the non-threaded library
.Fn read
is implemented as the
.Va read
syscall.
.Pp
In the threaded library, the
.Va read
syscall is assembled to
.Fn _thread_sys_read
and
.Fn read
is implemented as a function which locks
.Va d
for read, then calls
.Fn _thread_sys_read .
If the call to
.Fn _thread_sys_read
would block, a context switch is performed. Before returning,
.Fn read
unlocks
.Va d .
.Pp
In the non-threaded library
.Fn readv
is implemented as the
.Va readv
syscall.
.Pp
In the threaded library, the
.Va readv
syscall is assembled to
.Fn _thread_sys_readv
and
.Fn readv
is implemented as a function which locks
.Va d
for read, then calls
.Fn _thread_sys_readv .
If the call to
.Fn _thread_sys_readv
would block, a context switch is performed. Before returning,
.Fn readv
unlocks
.Va d .
.Sh 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
.Va errno
is set to indicate the error.
.Sh ERRORS
.Fn Read ,
.Fn readv ,
and
.Fn pread
will succeed unless:
.Bl -tag -width Er
.It Bq Er EBADF
.Fa D
is not a valid file or socket descriptor open for reading.
.It Bq Er EFAULT
.Fa Buf
points outside the allocated address space.
.It Bq Er EIO
An I/O error occurred while reading from the file system.
.It Bq Er EINTR
A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
.It Bq Er EINVAL
The pointer associated with
.Fa d
was negative.
.It Bq Er EAGAIN
The file was marked for non-blocking I/O,
and no data were ready to be read.
.El
.Pp
In addition, 
.Fn readv
may return one of the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
.Fa Iovcnt
was less than or equal to 0, or greater than 16.
.It Bq Er EINVAL
One of the
.Fa iov_len
values in the
.Fa iov
array was negative.
.It Bq Er EINVAL
The sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.It Bq Er EFAULT
Part of the
.Fa iov
points outside the process's allocated address space.
.El
.Pp
The
.Fn pread
call may also return the following errors:
.Bl -tag -width Er
.It Bq Er EINVAL
The specified file offset is invalid.
.It Bq Er ESPIPE
The file descriptor is associated with a pipe, socket, or FIFO.
.El
.Sh SEE ALSO
.Xr dup 2 ,
.Xr fcntl 2 ,
.Xr open 2 ,
.Xr pipe 2 ,
.Xr select 2 ,
.Xr socket 2 ,
.Xr socketpair 2
.Sh STANDARDS
The
.Fn read
function call is expected to conform to 
.St -p1003.1-90 .
The
.Fn readv
and
.Fn pread
functions are expected to conform to
.St -xpg4.2 .
.Sh HISTORY
The
.Fn pread
function call
appeared in
.At V.4 .
The
.Fn readv
function call
appeared in
.Bx 4.2 .
A
.Fn read
function call appeared in
.At v6 .