1FOPEN(3P) POSIX Programmer's Manual FOPEN(3P)
2
6 This manual page is part of the POSIX Programmer's Manual. The Linux
7 implementation of this interface may differ (consult the corresponding
8 Linux manual page for details of Linux behavior), or the interface may
9 not be implemented on Linux.
10
12 fopen - open a stream
13
15 #include <stdio.h>
16 FILE *fopen(const char *restrict filename, const char *restrict mode);
18
21 The fopen() function shall open the file whose pathname is the string
22 pointed to by filename, and associates a stream with it.
23 The mode argument points to a string. If the string is one of the fol‐
25 lowing, the file shall be opened in the indicated mode. Otherwise, the
26 behavior is undefined.
27 r or rb
29 Open file for reading.
30 w or wb
32 Truncate to zero length or create file for writing.
33 a or ab
35 Append; open or create file for writing at end-of-file.
36 r+ or rb+ or r+b
38 Open file for update (reading and writing).
39 w+ or wb+ or w+b
41 Truncate to zero length or create file for update.
42 a+ or ab+ or a+b
44 Append; open or create file for update, writing at end-of-file.
45 The character 'b' shall have no effect, but is allowed for ISO C stan‐
48 dard conformance. Opening a file with read mode (r as the first char‐
49 acter in the mode argument) shall fail if the file does not exist or
50 cannot be read.
51 Opening a file with append mode (a as the first character in the mode
53 argument) shall cause all subsequent writes to the file to be forced to
54 the then current end-of-file, regardless of intervening calls to
55 fseek().
56 When a file is opened with update mode ( '+' as the second or third
58 character in the mode argument), both input and output may be performed
59 on the associated stream. However, the application shall ensure that
60 output is not directly followed by input without an intervening call to
61 fflush() or to a file positioning function ( fseek(), fsetpos(), or
62 rewind()), and input is not directly followed by output without an
63 intervening call to a file positioning function, unless the input oper‐
64 ation encounters end-of-file.
65 When opened, a stream is fully buffered if and only if it can be deter‐
67 mined not to refer to an interactive device. The error and end-of-file
68 indicators for the stream shall be cleared.
69 If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file
71 did not previously exist, upon successful completion, the fopen() func‐
72 tion shall mark for update the st_atime, st_ctime, and st_mtime fields
73 of the file and the st_ctime and st_mtime fields of the parent direc‐
74 tory.
75 If mode is w, wb, w+, wb+, or w+b, and the file did previously exist,
77 upon successful completion, fopen() shall mark for update the st_ctime
78 and st_mtime fields of the file. The fopen() function shall allocate a
79 file descriptor as open() does.
80 After a successful call to the fopen() function, the orientation of the
82 stream shall be cleared, the encoding rule shall be cleared, and the
83 associated mbstate_t object shall be set to describe an initial conver‐
84 sion state.
85 The largest value that can be represented correctly in an object of
87 type off_t shall be established as the offset maximum in the open file
88 description.
89
91 Upon successful completion, fopen() shall return a pointer to the
92 object controlling the stream. Otherwise, a null pointer shall be
93 returned, and errno shall be set to indicate the error.
94
96 The fopen() function shall fail if:
97 EACCES Search permission is denied on a component of the path prefix,
99 or the file exists and the permissions specified by mode are
100 denied, or the file does not exist and write permission is
101 denied for the parent directory of the file to be created.
102 EINTR A signal was caught during fopen().
104 EISDIR The named file is a directory and mode requires write access.
106 ELOOP A loop exists in symbolic links encountered during resolution of
108 the path argument.
109 EMFILE {OPEN_MAX} file descriptors are currently open in the calling
111 process.
112 ENAMETOOLONG
114 The length of the filename argument exceeds {PATH_MAX} or a
116 pathname component is longer than {NAME_MAX}.
117 ENFILE The maximum allowable number of files is currently open in the
119 system.
120 ENOENT A component of filename does not name an existing file or file‐
122 name is an empty string.
123 ENOSPC The directory or file system that would contain the new file
125 cannot be expanded, the file does not exist, and the file was to
126 be created.
127 ENOTDIR
129 A component of the path prefix is not a directory.
130 ENXIO The named file is a character special or block special file, and
132 the device associated with this special file does not exist.
133 EOVERFLOW
135 The named file is a regular file and the size of the file cannot
136 be represented correctly in an object of type off_t.
137 EROFS The named file resides on a read-only file system and mode
139 requires write access.
140 The fopen() function may fail if:
143 EINVAL The value of the mode argument is not valid.
145 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
147 resolution of the path argument.
148 EMFILE {FOPEN_MAX} streams are currently open in the calling process.
150 EMFILE {STREAM_MAX} streams are currently open in the calling process.
152 ENAMETOOLONG
154 Pathname resolution of a symbolic link produced an intermediate
156 result whose length exceeds {PATH_MAX}.
157 ENOMEM Insufficient storage space is available.
159 ETXTBSY
161 The file is a pure procedure (shared text) file that is being
162 executed and mode requires write access.
163 The following sections are informative.
166
168 Opening a File
169 The following example tries to open the file named file for reading.
170 The fopen() function returns a file pointer that is used in subsequent
171 fgets() and fclose() calls. If the program cannot open the file, it
172 just ignores it.
173 #include <stdio.h>
176 ...
177 FILE *fp;
178 ...
179 void rgrep(const char *file)
180 {
181 ...
182 if ((fp = fopen(file, "r")) == NULL)
183 return;
184 ...
185 }
186
188 None.
189
191 None.
192
194 None.
195
197 fclose(), fdopen(), freopen(), the Base Definitions volume of
198 IEEE Std 1003.1-2001, <stdio.h>
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201 Portions of this text are reprinted and reproduced in electronic form
202 from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
203 -- Portable Operating System Interface (POSIX), The Open Group Base
204 Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
205 Electrical and Electronics Engineers, Inc and The Open Group. In the
206 event of any discrepancy between this version and the original IEEE and
207 The Open Group Standard, the original IEEE and The Open Group Standard
208 is the referee document. The original Standard can be obtained online
209 at http://www.opengroup.org/unix/online.html .
210IEEE/The Open Group 2003 FOPEN(3P)