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
13 fopen — open a stream
14
16 #include <stdio.h>
17 FILE *fopen(const char *restrict pathname, const char *restrict mode);
19
21 The functionality described on this reference page is aligned with the
22 ISO C standard. Any conflict between the requirements described here
23 and the ISO C standard is unintentional. This volume of POSIX.1‐2008
24 defers to the ISO C standard.
25 The fopen() function shall open the file whose pathname is the string
27 pointed to by pathname, and associates a stream with it.
28 The mode argument points to a string. If the string is one of the fol‐
30 lowing, the file shall be opened in the indicated mode. Otherwise, the
31 behavior is undefined.
32 r or rb Open file for reading.
34 w or wb Truncate to zero length or create file for writing.
36 a or ab Append; open or create file for writing at end-of-file.
38 r+ or rb+ or r+b
40 Open file for update (reading and writing).
41 w+ or wb+ or w+b
43 Truncate to zero length or create file for update.
44 a+ or ab+ or a+b
46 Append; open or create file for update, writing at end-
47 of-file.
48 The character 'b' shall have no effect, but is allowed for ISO C stan‐
50 dard conformance. Opening a file with read mode (r as the first char‐
51 acter in the mode argument) shall fail if the file does not exist or
52 cannot be read.
53 Opening a file with append mode (a as the first character in the mode
55 argument) shall cause all subsequent writes to the file to be forced to
56 the then current end-of-file, regardless of intervening calls to
57 fseek().
58 When a file is opened with update mode ('+' as the second or third
60 character in the mode argument), both input and output may be performed
61 on the associated stream. However, the application shall ensure that
62 output is not directly followed by input without an intervening call to
63 fflush() or to a file positioning function (fseek(), fsetpos(), or
64 rewind()), and input is not directly followed by output without an
65 intervening call to a file positioning function, unless the input oper‐
66 ation encounters end-of-file.
67 When opened, a stream is fully buffered if and only if it can be deter‐
69 mined not to refer to an interactive device. The error and end-of-file
70 indicators for the stream shall be cleared.
71 If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file
73 did not previously exist, upon successful completion, fopen() shall
74 mark for update the last data access, last data modification, and last
75 file status change timestamps of the file and the last file status
76 change and last data modification timestamps of the parent directory.
77 If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file
79 did not previously exist, the fopen() function shall create a file as
80 if it called the creat() function with a value appropriate for the path
81 argument interpreted from pathname and a value of S_IRUSR | S_IWUSR |
82 S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.
83 If mode is w, wb, w+, wb+, or w+b, and the file did previously exist,
85 upon successful completion, fopen() shall mark for update the last data
86 modification and last file status change timestamps of the file.
87 After a successful call to the fopen() function, the orientation of the
89 stream shall be cleared, the encoding rule shall be cleared, and the
90 associated mbstate_t object shall be set to describe an initial conver‐
91 sion state.
92 The file descriptor associated with the opened stream shall be allo‐
94 cated and opened as if by a call to open() with the following flags:
95 ┌─────────────────┬───────────────────────────┐
97 │ fopen() Mode │ open() Flags │
98 ├─────────────────┼───────────────────────────┤
99 │r or rb │ O_RDONLY │
100 │w or wb │ O_WRONLY|O_CREAT|O_TRUNC │
101 │a or ab │ O_WRONLY|O_CREAT|O_APPEND │
102 │r+ or rb+ or r+b │ O_RDWR │
103 │w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC │
104 │a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND │
105 └─────────────────┴───────────────────────────┘
107 Upon successful completion, fopen() shall return a pointer to the
108 object controlling the stream. Otherwise, a null pointer shall be
109 returned, and errno shall be set to indicate the error.
110
112 The fopen() function shall fail if:
113 EACCES Search permission is denied on a component of the path prefix,
115 or the file exists and the permissions specified by mode are
116 denied, or the file does not exist and write permission is
117 denied for the parent directory of the file to be created.
118 EINTR A signal was caught during fopen().
120 EISDIR The named file is a directory and mode requires write access.
122 ELOOP A loop exists in symbolic links encountered during resolution of
124 the path argument.
125 EMFILE All file descriptors available to the process are currently
127 open.
128 EMFILE {STREAM_MAX} streams are currently open in the calling process.
130 ENAMETOOLONG
132 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
133 tion of a symbolic link produced an intermediate result with a
134 length that exceeds {PATH_MAX}.
135 ENFILE The maximum allowable number of files is currently open in the
137 system.
138 ENOENT The mode string begins with 'r' and a component of pathname does
140 not name an existing file, or mode begins with 'w' or 'a' and a
141 component of the path prefix of pathname does not name an exist‐
142 ing file, or pathname is an empty string.
143 ENOENT or ENOTDIR
145 The pathname argument contains at least one non-<slash> charac‐
146 ter and ends with one or more trailing <slash> characters. If
147 pathname names an existing file, an [ENOENT] error shall not
148 occur.
149 ENOSPC The directory or file system that would contain the new file
151 cannot be expanded, the file does not exist, and the file was to
152 be created.
153 ENOTDIR
155 A component of the path prefix names an existing file that is
156 neither a directory nor a symbolic link to a directory, or the
157 pathname argument contains at least one non-<slash> character
158 and ends with one or more trailing <slash> characters and the
159 last pathname component names an existing file that is neither a
160 directory nor a symbolic link to a directory.
161 ENXIO The named file is a character special or block special file, and
163 the device associated with this special file does not exist.
164 EOVERFLOW
166 The named file is a regular file and the size of the file cannot
167 be represented correctly in an object of type off_t.
168 EROFS The named file resides on a read-only file system and mode
170 requires write access.
171 The fopen() function may fail if:
173 EINVAL The value of the mode argument is not valid.
175 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
177 resolution of the path argument.
178 EMFILE {FOPEN_MAX} streams are currently open in the calling process.
180 ENAMETOOLONG
182 The length of a component of a pathname is longer than
183 {NAME_MAX}.
184 ENOMEM Insufficient storage space is available.
186 ETXTBSY
188 The file is a pure procedure (shared text) file that is being
189 executed and mode requires write access.
190 The following sections are informative.
192
194 Opening a File
195 The following example tries to open the file named file for reading.
196 The fopen() function returns a file pointer that is used in subsequent
197 fgets() and fclose() calls. If the program cannot open the file, it
198 just ignores it.
199 #include <stdio.h>
201 ...
202 FILE *fp;
203 ...
204 void rgrep(const char *file)
205 {
206 ...
207 if ((fp = fopen(file, "r")) == NULL)
208 return;
209 ...
210 }
211
213 None.
214
216 None.
217
219 None.
220
222 Section 2.5, Standard I/O Streams, creat(), fclose(), fdopen(), fmemo‐
223 pen(), freopen(), open_memstream()
224 The Base Definitions volume of POSIX.1‐2008, <stdio.h>
226
228 Portions of this text are reprinted and reproduced in electronic form
229 from IEEE Std 1003.1, 2013 Edition, Standard for Information Technology
230 -- Portable Operating System Interface (POSIX), The Open Group Base
231 Specifications Issue 7, Copyright (C) 2013 by the Institute of Electri‐
232 cal and Electronics Engineers, Inc and The Open Group. (This is
233 POSIX.1-2008 with the 2013 Technical Corrigendum 1 applied.) In the
234 event of any discrepancy between this version and the original IEEE and
235 The Open Group Standard, the original IEEE and The Open Group Standard
236 is the referee document. The original Standard can be obtained online
237 at http://www.unix.org/online.html .
238 Any typographical or formatting errors that appear in this page are
240 most likely to have been introduced during the conversion of the source
241 files to man page format. To report such errors, see https://www.ker‐
242 nel.org/doc/man-pages/reporting_bugs.html .
243IEEE/The Open Group 2013 FOPEN(3P)