1.\" $NetBSD: fread.3,v 1.15 2011/09/11 04:55:48 jruoho Exp $ 2.\" 3.\" Copyright (c) 1990, 1991, 1993 4.\" The Regents of the University of California. All rights reserved. 5.\" 6.\" This code is derived from software contributed to Berkeley by 7.\" Chris Torek and the American National Standards Committee X3, 8.\" on Information Processing Systems. 9.\" 10.\" Redistribution and use in source and binary forms, with or without 11.\" modification, are permitted provided that the following conditions 12.\" are met: 13.\" 1. Redistributions of source code must retain the above copyright 14.\" notice, this list of conditions and the following disclaimer. 15.\" 2. Redistributions in binary form must reproduce the above copyright 16.\" notice, this list of conditions and the following disclaimer in the 17.\" documentation and/or other materials provided with the distribution. 18.\" 3. Neither the name of the University nor the names of its contributors 19.\" may be used to endorse or promote products derived from this software 20.\" without specific prior written permission. 21.\" 22.\" THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND 23.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 24.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 25.\" ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE 26.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 27.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 28.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 29.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 30.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 31.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 32.\" SUCH DAMAGE. 33.\" 34.\" @(#)fread.3 8.2 (Berkeley) 3/8/94 35.\" 36.Dd September 11, 2011 37.Dt FREAD 3 38.Os 39.Sh NAME 40.Nm fread , 41.Nm fwrite 42.Nd binary stream input/output 43.Sh LIBRARY 44.Lb libc 45.Sh SYNOPSIS 46.In stdio.h 47.Ft size_t 48.Fn fread "void * restrict ptr" "size_t size" "size_t nmemb" "FILE * restrict stream" 49.Ft size_t 50.Fn fwrite "const void * restrict ptr" "size_t size" "size_t nmemb" "FILE * restrict stream" 51.Sh DESCRIPTION 52The function 53.Fn fread 54reads 55.Fa nmemb 56objects, each 57.Fa size 58bytes long, from the stream pointed to by 59.Fa stream , 60storing them at the location given by 61.Fa ptr . 62.Pp 63The function 64.Fn fwrite 65writes 66.Fa nmemb 67objects, each 68.Fa size 69bytes long, to the stream pointed to by 70.Fa stream , 71obtaining them from the location given by 72.Fa ptr . 73.Pp 74Mixing 75.Fn fread 76and 77.Fn fwrite 78calls without setting the file position explicitly using 79.Xr fsetpos 3 80between read and write or write and read operations will lead to unexpected 81results because of buffering the file pointer not being set to the 82expected position after each operation completes. 83This behavior is allowed by ANSI C for efficiency and it will not be changed. 84.Sh RETURN VALUES 85The functions 86.Fn fread 87and 88.Fn fwrite 89advance the file position indicator for the stream 90by the number of bytes read or written. 91They return the number of objects read or written. 92If 93.Fa size 94or 95.Fa nmemb 96is 0, the functions return 0 and the state of 97.Fa stream 98remains unchanged. 99If an error occurs, or the end-of-file is reached, 100the return value is a short object count (or zero). 101.Pp 102The function 103.Fn fread 104does not distinguish between end-of-file and error, and callers 105must use 106.Xr feof 3 107and 108.Xr ferror 3 109to determine which occurred. 110The function 111.Fn fwrite 112returns a value less than 113.Fa nmemb 114only if a write error has occurred. 115.Sh SEE ALSO 116.Xr read 2 , 117.Xr write 2 118.Sh STANDARDS 119The functions 120.Fn fread 121and 122.Fn fwrite 123conform to 124.St -ansiC . 125