| 1 | .\" Copyright (c) 1980 Regents of the University of California. |
| 2 | .\" All rights reserved. The Berkeley software License Agreement |
| 3 | .\" specifies the terms and conditions for redistribution. |
| 4 | .\" |
| 5 | .\" @(#)fopen.3 6.3 (Berkeley) %G% |
| 6 | .\" |
| 7 | .TH FOPEN 3S "" |
| 8 | .UC 4 |
| 9 | .SH NAME |
| 10 | fopen, freopen, fdopen \- open a stream |
| 11 | .SH SYNOPSIS |
| 12 | .B #include <stdio.h> |
| 13 | .PP |
| 14 | .SM |
| 15 | .B FILE |
| 16 | .B *fopen(filename, type) |
| 17 | .br |
| 18 | .B char *filename, *type; |
| 19 | .PP |
| 20 | .SM |
| 21 | .B FILE |
| 22 | .B *freopen(filename, type, stream) |
| 23 | .br |
| 24 | .B char *filename, *type; |
| 25 | .br |
| 26 | .SM |
| 27 | .B FILE |
| 28 | .B *stream; |
| 29 | .PP |
| 30 | .SM |
| 31 | .B FILE |
| 32 | .B *fdopen(fildes, type) |
| 33 | .br |
| 34 | .B char *type; |
| 35 | .SH DESCRIPTION |
| 36 | .I Fopen |
| 37 | opens the file named by |
| 38 | .I filename |
| 39 | and associates a stream with it. |
| 40 | .I Fopen |
| 41 | returns a pointer to be used to identify the stream in subsequent operations. |
| 42 | .PP |
| 43 | .I Type |
| 44 | is a character string having one of the following values: |
| 45 | .TP 5 |
| 46 | "r" |
| 47 | open for reading |
| 48 | .ns |
| 49 | .TP 5 |
| 50 | "w" |
| 51 | create for writing |
| 52 | .ns |
| 53 | .TP 5 |
| 54 | "a" |
| 55 | append: open for writing at end of file, or create for writing |
| 56 | .PP |
| 57 | In addition, each |
| 58 | .I type |
| 59 | may be followed by a "+" to have the file opened for reading and writing. |
| 60 | "r+" positions the stream at the beginning of the file, "w+" creates |
| 61 | or truncates it, and "a+" positions it at the end. Both reads and writes |
| 62 | may be used on read/write streams, with the limitation that an |
| 63 | .I fseek, rewind, |
| 64 | or reading an end-of-file must be used between a read and a write or vice-versa. |
| 65 | .PP |
| 66 | .I Freopen |
| 67 | substitutes the named file in place of the open |
| 68 | .IR stream . |
| 69 | It returns the original value of |
| 70 | .IR stream . |
| 71 | The original stream is closed. |
| 72 | .PP |
| 73 | .I Freopen |
| 74 | is typically used to attach the preopened constant names, |
| 75 | .B stdin, stdout, stderr, |
| 76 | to specified files. |
| 77 | .PP |
| 78 | .I Fdopen |
| 79 | associates a stream with a file descriptor obtained from |
| 80 | .I open, dup, creat, |
| 81 | or |
| 82 | .IR pipe (2). |
| 83 | The |
| 84 | .I type |
| 85 | of the stream must agree with the mode of the open file. |
| 86 | .SH "SEE ALSO" |
| 87 | open(2), |
| 88 | fclose(3) |
| 89 | .SH DIAGNOSTICS |
| 90 | .I Fopen |
| 91 | and |
| 92 | .I freopen |
| 93 | return the pointer |
| 94 | .SM |
| 95 | .B NULL |
| 96 | if |
| 97 | .I filename |
| 98 | cannot be accessed, |
| 99 | if too many files are already open, |
| 100 | or if other resources needed cannot be allocated. |
| 101 | .SH BUGS |
| 102 | .I Fdopen |
| 103 | is not portable to systems other than UNIX. |
| 104 | .PP |
| 105 | The read/write |
| 106 | .I types |
| 107 | do not exist on all systems. Those systems without |
| 108 | read/write modes will probably treat the |
| 109 | .I type |
| 110 | as if the "+" was not present. These are unreliable in any event. |
| 111 | .PP |
| 112 | In order to support the same number of open files as does the system, |
| 113 | .I fopen |
| 114 | must allocate additional memory for data structures using |
| 115 | .I calloc |
| 116 | after 20 files have been opened. |
| 117 | This confuses some programs which use their own memory allocators. |
| 118 | An undocumented routine, |
| 119 | .IR f_prealloc , |
| 120 | may be called to force immediate allocation of all internal memory |
| 121 | except for buffers. |