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@(#)read.2 8.4 (Berkeley) 2/26/94
$FreeBSD: src/lib/libc/sys/read.2,v 1.9.2.6 2001/12/14 18:34:01 ru Exp $
.Dd February 26, 1994 .Dt READ 2 .Os .Sh NAME .Nm pread , .Nm read , .Nm readv .Nd read input .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/types.h n sys/uio.h n unistd.h .Ft ssize_t .Fo pread .Fa "int d" .Fa "void *buf" .Fa "size_t nbyte" .Fa "off_t offset" .Fc .Ft ssize_t .Fo read .Fa "int fildes" .Fa "void *buf" .Fa "size_t nbyte" .Fc .Ft ssize_t .Fo readv .Fa "int d" .Fa "const struct iovec *iov" .Fa "int iovcnt" .Fc .Sh DESCRIPTION .Fn Read attempts to read .Fa nbyte bytes of data from the object referenced by the descriptor .Fa fildes 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.
p For .Fn readv , the .Fa iovec structure is defined as:
p d -literal -offset indent -compact struct iovec { char *iov_base; /* Base address. */ size_t iov_len; /* Length. */ }; .Ed
p 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.
p On objects capable of seeking, the .Fn read starts at a position given by the pointer associated with .Fa fildes (see .Xr lseek 2 ) . Upon return from .Fn read , the pointer is incremented by the number of bytes actually read.
p 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.
p
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.
.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
The
.Fn pread ,
.Fn read ,
and
.Fn readv
calls
will succeed unless:
l -tag -width Er ===========
t Bq Er EAGAIN The file was marked for non-blocking I/O,
and no data were ready to be read.
===========
t Bq Er EBADF .Fa fildes
is not a valid file or socket descriptor open for reading.
===========
t Bq Er EFAULT .Fa Buf
points outside the allocated address space.
===========
t Bq Er EINTR A read from a slow device was interrupted before
any data arrived by the delivery of a signal.
t Bq Er EINVAL The pointer associated with
.Fa fildes
was negative.
===========
t Bq Er EIO An I/O error occurred while reading from the file system.
===========
.It Bq Er EIO
The process is a member of a background process
attempting to read from its controlling terminal.
===========
.It Bq Er EIO
The process is ignoring or blocking the SIGTTIN signal.
===========
t Bq Er EIO The process group is orphaned.
===========
t Bq Er EIO The file is a regular file,
.Fa nbyte
is greater than 0,
the starting position is before the end-of-file,
and the starting position is greater than or equal
to the offset maximum established
for the open file descriptor associated with
.Fa fildes .
===========
t Bq Er EISDIR An attempt is made to read a directory.
===========
t Bq Er ENOBUFS An attempt to allocate a memory buffer fails.
===========
t Bq Er ENOMEM Insufficient memory is available.
===========
t Bq Er ENXIO An action is requested of a device that does not exist..
===========
t Bq Er ENXIO A requested action cannot be performed by the device.
.El
p
The
.Fn pread
call may also return the following errors:
l -tag -width Er ===========
t Bq Er EINVAL The specified file offset is invalid.
===========
t Bq Er ESPIPE The file descriptor is associated with a pipe, socket, or FIFO.
.El
p
The
.Fn read
call may also return the following errors:
l -tag -width Er ===========
t Bq Er ECONNRESET The connection is closed by the peer
during a read attempt on a socket.
===========
t Bq Er ENOTCONN A read is attempted on an unconnected socket.
===========
t Bq Er ETIMEDOUT A transmission timeout occurs
during a read attempt on a socket.
.El
p
The
.Fn readv
call may also return one of the following errors:
l -tag -width Er ===========
t Bq Er EFAULT Part of the
.Fa iov
points outside the process's allocated address space.
===========
t Bq Er EINVAL .Fa Iovcnt
was less than or equal to 0, or greater than 16.
===========
t Bq Er EINVAL One of the
.Fa iov_len
values in the
.Fa iov
array was negative.
===========
t Bq Er EINVAL The sum of the
.Fa iov_len
values in the
.Fa iov
array overflowed a 32-bit integer.
.El
.Sh LEGACY SYNOPSIS
.Fd #include <sys/types.h>
.Fd #include <sys/uio.h>
.Fd #include <unistd.h>
p The include files n sys/types.h and n sys/uio.h are necessary for all functions. .Sh SEE ALSO .Xr dup 2 , .Xr fcntl 2 , .Xr open 2 , .Xr pipe 2 , .Xr select 2 , .Xr socket 2 , .Xr socketpair 2 , .Xr compat 5 .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 x 4.2 . A .Fn read function call appeared in .At v6 .