.\" Copyright (c) 1980, 1991, 1993 .\" The Regents of the University of California. All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed by the University of .\" California, Berkeley and its contributors. .\" 4. Neither the name of the University nor the names of its contributors .\" may be used to endorse or promote products derived from this software .\" without specific prior written permission. .\" .\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" @(#)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 .In sys/types.h .In sys/uio.h .In 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. .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 fildes (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. .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: .Bl -tag -width Er .\" =========== .It Bq Er EAGAIN The file was marked for non-blocking I/O, and no data were ready to be read. .\" =========== .It Bq Er EBADF .Fa fildes 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 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 fildes was negative. .\" =========== .It 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. .\" =========== .It Bq Er EIO The process group is orphaned. .\" =========== .It 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 . .\" =========== .It Bq Er EISDIR An attempt is made to read a directory. .\" =========== .It Bq Er ENOBUFS An attempt to allocate a memory buffer fails. .\" =========== .It Bq Er ENOMEM Insufficient memory is available. .\" =========== .It Bq Er ENXIO An action is requested of a device that does not exist.. .\" =========== .It Bq Er ENXIO A requested action cannot be performed by the device. .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 .Pp The .Fn read call may also return the following errors: .Bl -tag -width Er .\" =========== .It Bq Er ECONNRESET The connection is closed by the peer during a read attempt on a socket. .\" =========== .It Bq Er ENOTCONN A read is attempted on an unconnected socket. .\" =========== .It Bq Er ETIMEDOUT A transmission timeout occurs during a read attempt on a socket. .El .Pp The .Fn readv call may also return one of the following errors: .Bl -tag -width Er .\" =========== .It Bq Er EFAULT Part of the .Fa iov points outside the process's allocated address space. .\" =========== .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. .El .Sh LEGACY SYNOPSIS .Fd #include .Fd #include .Fd #include .Pp The include files .In sys/types.h and .In 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 .Bx 4.2 . A .Fn read function call appeared in .At v6 .