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 pathname, const char *restrict mode);
18
20 The functionality described on this reference page is aligned with the
21 ISO C standard. Any conflict between the requirements described here
22 and the ISO C standard is unintentional. This volume of POSIX.1‐2017
23 defers to the ISO C standard.
24 The fopen() function shall open the file whose pathname is the string
26 pointed to by pathname, and associates a stream with it.
27 The mode argument points to a string. If the string is one of the fol‐
29 lowing, the file shall be opened in the indicated mode. Otherwise, the
30 behavior is undefined.
31 r or rb Open file for reading.
33 w or wb Truncate to zero length or create file for writing.
35 a or ab Append; open or create file for writing at end-of-file.
37 r+ or rb+ or r+b
39 Open file for update (reading and writing).
40 w+ or wb+ or w+b
42 Truncate to zero length or create file for update.
43 a+ or ab+ or a+b
45 Append; open or create file for update, writing at end-
46 of-file.
47 The character 'b' shall have no effect, but is allowed for ISO C stan‐
49 dard conformance. Opening a file with read mode (r as the first char‐
50 acter in the mode argument) shall fail if the file does not exist or
51 cannot be read.
52 Opening a file with append mode (a as the first character in the mode
54 argument) shall cause all subsequent writes to the file to be forced to
55 the then current end-of-file, regardless of intervening calls to
56 fseek().
57 When a file is opened with update mode ('+' as the second or third
59 character in the mode argument), both input and output may be performed
60 on the associated stream. However, the application shall ensure that
61 output is not directly followed by input without an intervening call to
62 fflush() or to a file positioning function (fseek(), fsetpos(), or
63 rewind()), and input is not directly followed by output without an
64 intervening call to a file positioning function, unless the input oper‐
65 ation encounters end-of-file.
66 When opened, a stream is fully buffered if and only if it can be deter‐
68 mined not to refer to an interactive device. The error and end-of-file
69 indicators for the stream shall be cleared.
70 If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file
72 did not previously exist, upon successful completion, fopen() shall
73 mark for update the last data access, last data modification, and last
74 file status change timestamps of the file and the last file status
75 change and last data modification timestamps of the parent directory.
76 If mode is w, wb, a, ab, w+, wb+, w+b, a+, ab+, or a+b, and the file
78 did not previously exist, the fopen() function shall create a file as
79 if it called the creat() function with a value appropriate for the path
80 argument interpreted from pathname and a value of S_IRUSR | S_IWUSR |
81 S_IRGRP | S_IWGRP | S_IROTH | S_IWOTH for the mode argument.
82 If mode is w, wb, w+, wb+, or w+b, and the file did previously exist,
84 upon successful completion, fopen() shall mark for update the last data
85 modification and last file status change timestamps of the file.
86 After a successful call to the fopen() function, the orientation of the
88 stream shall be cleared, the encoding rule shall be cleared, and the
89 associated mbstate_t object shall be set to describe an initial conver‐
90 sion state.
91 The file descriptor associated with the opened stream shall be allo‐
93 cated and opened as if by a call to open() with the following flags:
94 ┌─────────────────┬───────────────────────────┐
96 │ fopen() Mode │ open() Flags │
97 ├─────────────────┼───────────────────────────┤
98 │r or rb │ O_RDONLY │
99 │w or wb │ O_WRONLY|O_CREAT|O_TRUNC │
100 │a or ab │ O_WRONLY|O_CREAT|O_APPEND │
101 │r+ or rb+ or r+b │ O_RDWR │
102 │w+ or wb+ or w+b │ O_RDWR|O_CREAT|O_TRUNC │
103 │a+ or ab+ or a+b │ O_RDWR|O_CREAT|O_APPEND │
104 └─────────────────┴───────────────────────────┘
106 Upon successful completion, fopen() shall return a pointer to the
107 object controlling the stream. Otherwise, a null pointer shall be
108 returned, and errno shall be set to indicate the error.
109
111 The fopen() function shall fail if:
112 EACCES Search permission is denied on a component of the path prefix,
114 or the file exists and the permissions specified by mode are
115 denied, or the file does not exist and write permission is
116 denied for the parent directory of the file to be created.
117 EINTR A signal was caught during fopen().
119 EISDIR The named file is a directory and mode requires write access.
121 ELOOP A loop exists in symbolic links encountered during resolution of
123 the path argument.
124 EMFILE All file descriptors available to the process are currently
126 open.
127 EMFILE {STREAM_MAX} streams are currently open in the calling process.
129 ENAMETOOLONG
131 The length of a pathname exceeds {PATH_MAX}, or pathname resolu‐
132 tion of a symbolic link produced an intermediate result with a
133 length that exceeds {PATH_MAX}.
134 ENFILE The maximum allowable number of files is currently open in the
136 system.
137 ENOENT The mode string begins with 'r' and a component of pathname does
139 not name an existing file, or mode begins with 'w' or 'a' and a
140 component of the path prefix of pathname does not name an exist‐
141 ing file, or pathname is an empty string.
142 ENOENT or ENOTDIR
144 The pathname argument contains at least one non-<slash> charac‐
145 ter and ends with one or more trailing <slash> characters. If
146 pathname without the trailing <slash> characters would name an
147 existing file, an [ENOENT] error shall not occur.
148 ENOSPC The directory or file system that would contain the new file
150 cannot be expanded, the file does not exist, and the file was to
151 be created.
152 ENOTDIR
154 A component of the path prefix names an existing file that is
155 neither a directory nor a symbolic link to a directory, or the
156 pathname argument contains at least one non-<slash> character
157 and ends with one or more trailing <slash> characters and the
158 last pathname component names an existing file that is neither a
159 directory nor a symbolic link to a directory.
160 ENXIO The named file is a character special or block special file, and
162 the device associated with this special file does not exist.
163 EOVERFLOW
165 The named file is a regular file and the size of the file cannot
166 be represented correctly in an object of type off_t.
167 EROFS The named file resides on a read-only file system and mode
169 requires write access.
170 The fopen() function may fail if:
172 EINVAL The value of the mode argument is not valid.
174 ELOOP More than {SYMLOOP_MAX} symbolic links were encountered during
176 resolution of the path argument.
177 EMFILE {FOPEN_MAX} streams are currently open in the calling process.
179 ENAMETOOLONG
181 The length of a component of a pathname is longer than
182 {NAME_MAX}.
183 ENOMEM Insufficient storage space is available.
185 ETXTBSY
187 The file is a pure procedure (shared text) file that is being
188 executed and mode requires write access.
189 The following sections are informative.
191
193 Opening a File
194 The following example tries to open the file named file for reading.
195 The fopen() function returns a file pointer that is used in subsequent
196 fgets() and fclose() calls. If the program cannot open the file, it
197 just ignores it.
198 #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.1‐2017, <stdio.h>
226
228 Portions of this text are reprinted and reproduced in electronic form
229 from IEEE Std 1003.1-2017, Standard for Information Technology -- Por‐
230 table Operating System Interface (POSIX), The Open Group Base Specifi‐
231 cations Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of
232 Electrical and Electronics Engineers, Inc and The Open Group. In the
233 event of any discrepancy between this version and the original IEEE and
234 The Open Group Standard, the original IEEE and The Open Group Standard
235 is the referee document. The original Standard can be obtained online
236 at http://www.opengroup.org/unix/online.html .
237 Any typographical or formatting errors that appear in this page are
239 most likely to have been introduced during the conversion of the source
240 files to man page format. To report such errors, see https://www.ker‐
241 nel.org/doc/man-pages/reporting_bugs.html .
242IEEE/The Open Group 2017 FOPEN(3P)