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# fopen

From cppreference.com
< 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.

Run this code

    #include <stdio.h>
    #include <stdlib.h>

    int main(void)
    {
        FILE *fp = fopen("data.txt","w");
        if (fp == NULL)
        {
           perror("fopen()");
           fprintf(stderr,"fopen() failed in file %s at line # %d\n", __FILE__,__LINE__-4);
           return EXIT_FAILURE;
        }

        /* Normal processing continues here. */

        fclose(fp);
        return EXIT_SUCCESS;
    }

### [edit] See also

fclose | closes a file (function) [edit]
fflush | synchronizes an output stream with the actual file (function) [edit]
freopen | open an existing stream with a different name (function) [edit]
C++ documentation for fopen

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