Open and close files

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To open and close files, the <stdio.h> library has three functions: fopen, freopen, and fclose.

Opening Files

#include <stdio.h>
FILE *fopen(const char *filename, const char *mode);
FILE *freopen(const char *filename, const char *mode, FILE *stream);

The fopen and freopen functions open files.

The fopen function opens the file whose name is in 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           open a text file for reading
w           truncate to zero length or create a text file for writing
a           append; open or create text file for writing at end-of-file
rb          open binary file for reading
wb          truncate to zero length or create a binary file for writing
ab          append; open or create binary file for writing at end-of-file
r+          open text file for update (reading and writing)
w+          truncate to zero length or create a text file for update
a+          append; open or create text file for update
r+b or rb+  open binary file for update (reading and writing)
w+b or wb+  truncate to zero length or create a binary file for update
a+b or ab+  append; open or create binary file for update

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 the fseek function. In some implementations, opening a binary file with append mode ('b' as the second or third character in the above list of mode arguments) may initially position the file position indicator for the stream beyond the last data written, because of null character padding.

When a file is opened with update mode ('+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output may not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input may not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening (or creating) a text file with update mode may instead open (or create) a binary stream in some implementations.

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 are cleared.

The fopen function returns a pointer to the object controlling the stream. If the open operation fails, fopen returns a null pointer.

The freopen function opens the file whose name is the string pointed to by filename and associates the stream pointed to by stream with it. The mode argument is used just as in the fopen function.

The freopen function first attempts to close any file that is associated with the specified stream. Failure to close the file successfully is ignored. The error and end-of-file indicators for the stream are cleared.

The freopen function returns a null pointer if the open operation fails, or the value stream if the open operation succeeds.

Closing Files

#include <stdio.h>
int fclose(FILE *stream);

The fclose function causes the stream pointed to by stream to be flushed and the associated file to be closed. Any unwritten buffered data for the stream are delivered to the host environment to be written to the file; any unread buffered data are discarded. The stream is disassociated from the file. If the associated buffer was automatically allocated, it is deallocated. The function returns zero if the stream was successfully closed or EOF if any errors were detected.

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