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The fopen() function opens the file whose pathname is the string pointed to by filename, and associates a stream with it.
The argument mode points to a string beginning with one of the following sequences:
-
r or rb
- Open file for reading.
-
w or wb
- Truncate to zero length or create file for writing.
-
a or ab
- Append; open or create file for writing at end-of-file.
-
r+ or rb+ or r+b
- Open file for update (reading and writing).
-
w+ or wb+ or w+b
- Truncate to zero length or create file for update.
-
a+ or ab+ or a+b
- Append; open or create file for update, writing at end-of-file.
The character b has no effect, but is allowed for ISO C standard conformance (see standards(5)). Opening a file with read mode (r as the first character in the mode argument) fails if the file does not exist
or cannot be read.
Opening a file with append mode (a as the first character in the mode argument) causes all subsequent writes to the file to be forced to the then current
end-of-file, regardless of intervening calls to fseek(3C). If two separate processes open
the same file for append, each process may write freely to the file without fear of destroying output being written by the other. The output from the two processes will be intermixed in the file in the
order in which it is written.
When a file is opened with update mode (+ as the second or third character in the mode argument), both input and output may be performed on the associated
stream. However, output must not be directly followed by input without an intervening call to fflush(3C)
or to a file positioning function ( fseek(3C), fsetpos(3C) or rewind(3C)),
and input must not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.
When opened, a stream is fully buffered if and only if it can be determined not to refer to an interactive device. The error and end-of-file indicators for the stream are cleared.
If mode is w, a, w+ or a+ and the file did not previously exist, upon successful completion, fopen() function will mark for update the st_atime, st_ctime and st_mtime fields of the file and the st_ctime and st_mtime fields of the parent directory.
If mode is w or w+ and the file did previously exist, upon successful completion, fopen() will mark for update the st_ctime and st_mtime fields of the file. The fopen() function will allocate a file descriptor as open(2) does.
The largest value that can be represented correctly in an object of type off_t will be established as the offset maximum in the open file description.
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