File Input And Output

The standard C library provides functions for writing input and output to files.

Most of the file input/output functions are declared in header stdio.h. Add the line #include <stdio.h> to your source code to use file I/O functions.

fopen()

fopen() is used to open a file.

The function declaration changed in C99, by adding the keyword restrict. Before C99 it was:

FILE *fopen(const char *filename, const char *mode);

In C99 and above:

FILE *fopen(const char *restrict filename, const char *restrict mode);

where:

filename: Null-terminated string of the filename to associate the data to
mode: Null-terminated string that determines the file access mode

\

mode	Stands For	Description	Behaviour If File Already Exists	Behaviour If File Doesn’t Exist
“r”	read	Open a file to read from it.	Read from start.	Error.
”w”	write	Open a file to write to it.	Delete file contents.	Create new file.
”wx”	write	Write to file, but don’t overwrite if file already exists.	Error.	Create new file
”a”	append	Open a file to append data to the end.	Append new data to the end of file.	Create new file.
”r+“	read extended	Opens a file for read/write access	Read from start.	Error.
”w+“	write extended	Creates a file for read/write access	Deletes file contents.	Create new file.
”w+x”	write extended	Opens a file for read/write access, but doesn’t overwrite if file already exists.	Error.	Create new file.
”a+“	append extended	Opens a file for read/write access	Appends new data to the end of file.	Create new file.
b	binary open	Opens a file in binary mode (Windows only).	?	?

fopen() returns pointer to opened file stream on success, otherwise a NULL pointer on fail.

You have to be careful when using fopen() on a system with multiple threads, there is the possibility of creating race conditions.