.\" $OpenBSD: read.2,v 1.28 2009/12/30 09:46:23 fgsch Exp $ .\" $NetBSD: read.2,v 1.6 1995/02/27 12:35:47 cgd Exp $ .\" .\" 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. 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 .\" .Dd $Mdocdate: December 30 2009 $ .Dt READ 2 .Os .Sh NAME .Nm read , .Nm readv , .Nm pread , .Nm preadv .Nd read input .Sh SYNOPSIS .Fd #include .Fd #include .Ft ssize_t .Fn read "int d" "void *buf" "size_t nbytes" .Ft ssize_t .Fn pread "int d" "void *buf" "size_t nbytes" "off_t offset" .Pp .Fd #include .Fd #include .Fd #include .Ft ssize_t .Fn readv "int d" "const struct iovec *iov" "int iovcnt" .Ft ssize_t .Fn preadv "int d" "const struct iovec *iov" "int iovcnt" "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 and .Fn preadv perform the same functions, but read from the specified position in the file without modifying the file pointer. .Pp For .Fn readv and .Fn preadv , the .Fa iovec structure is defined as: .Bd -literal -offset indent struct iovec { void *iov_base; size_t iov_len; }; .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 , .Fn pread , and .Fn preadv 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 Note that .Fn readv and .Fn preadv will fail if the value of .Fa iovcnt exceeds the constant .Dv IOV_MAX . .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 , .Fn pread , and .Fn preadv 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 Part of .Fa iov or .Fa buf points outside the process's allocated address space. .It Bq Er EIO An I/O error occurred while reading from the file system, or the process is a member of a background process attempting to read from its controlling terminal, the process is ignoring or blocking the SIGTTIN signal or the process group is orphaned. .It Bq Er EINTR A read from a slow device (i.e. one that might block for an arbitrary amount of time) was interrupted by the delivery of a signal before any data arrived. .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 read and .Fn pread may return the following error: .Bl -tag -width Er .It Bq Er EINVAL .Fa nbytes was larger than .Dv SSIZE_MAX . .El .Pp .Fn pread and .Fn preadv may return the following error: .Bl -tag -width Er .It Bq Er ESPIPE .Fa d is associated with a pipe, socket, or FIFO. .El .Pp .Fn readv and .Fn preadv 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 .Dv IOV_MAX . .It Bq Er EINVAL The sum of the .Fa iov_len values in the .Fa iov array overflowed an .Em ssize_t . .El .Sh SEE ALSO .Xr dup 2 , .Xr fcntl 2 , .Xr open 2 , .Xr pipe 2 , .Xr poll 2 , .Xr select 2 , .Xr socket 2 , .Xr socketpair 2 .Sh STANDARDS The .Fn read function conforms to .St -p1003.1-90 . The .Fn readv and .Fn pread functions conform to .St -xpg4.2 . .Sh HISTORY The .Fn preadv function first appeared in .Ox 2.7 . The .Fn pread function appeared in .At V.4 . The .Fn readv function call appeared in .Bx 4.2 . The .Fn read function call appeared in .At v2 . .Sh CAVEATS Error checks should explicitly test for \-1. Code such as .Bd -literal while ((nr = read(fd, buf, sizeof(buf))) > 0) .Ed .Pp is not maximally portable, as some platforms allow for .Va nbytes to range between .Dv SSIZE_MAX and .Dv SIZE_MAX \- 2, in which case the return value of an error-free .Fn read may appear as a negative number distinct from \-1. Proper loops should use .Bd -literal while ((nr = read(fd, buf, sizeof(buf))) != -1 && nr != 0) .Ed