< c‎ | io

Defined in header <stdio.h>
FILE *fopen( const char          *filename, const char          *mode );
(until C99)
FILE *fopen( const char *restrict filename, const char *restrict mode );
(since C99)

Opens a file indicated by filename and returns a file stream associated with that file. mode is used to determine the file access mode.


[edit] Parameters

filename - file name to associate the file stream to
mode - null-terminated character string determining file access mode
File access
mode string
Meaning Explanation Action if file
already exists
Action if file
does not exist
"r" read Open a file for reading read from start failure to open
"w" write Create a file for writing destroy contents create new
"a" append Append to a file write to end create new
"r+" read extended Open a file for read/write read from start error
"w+" write extended Create a file for read/write destroy contents create new
"a+" append extended Open a file for read/write write to end create new
File access mode flag "b" can optionally be specified to open a file in binary mode. This flag has effect only on Windows systems.
On the append file access modes, data is written to the end of the file regardless of the current position of the file position indicator.
File access mode flag "x" can optionally be appended to "w" or "w+" specifiers. This flag forces the function to fail if the file exists, instead of overwriting it. (C11)

[edit] Return value

If successful, returns a pointer to the object that controls the opened file stream, with both eof and error bits cleared. The stream is fully buffered unless filename refers to an interactive device.

On error, returns a null pointer. POSIX requires that errno is set in this case.

[edit] Notes

The format of filename is implementation-defined, and does not necessarily refer to a file (e.g. it may be the console or another device accessible though filesystem API). On platforms that support them, filename may include absolute or relative filesystem path.

[edit] Example

fopen with error checking. Code opens a file for writing data.

#include <stdio.h>
#include <stdlib.h>
int main(void)
    FILE *fp = fopen("data.txt","w");
    if (fp == NULL)
       fprintf(stderr,"fopen() failed in file %s at line # %d\n", __FILE__,__LINE__-4);
       return EXIT_FAILURE;
    /* Normal processing continues here. */
    return EXIT_SUCCESS;

[edit] See also

closes a file
synchronizes an output stream with the actual file
open an existing stream with a different name
C++ documentation for fopen