# std::fopen

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< cpp‎ | io‎ | c

Defined in header `<cstdio>` |  |
std::FILE* fopen( const char* filename, const char* mode ); |  |

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

### 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 no effect on POSIX systems, but on Windows, for example, it disables special handling of '\n' and '\x1A'. 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. (C++17)
The behavior is undefined if the mode is not one of the strings listed above. Some implementations define additional supported modes (e.g. Windows).

### 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.

### 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 through filesystem API). On platforms that support them, `filename` may include absolute or relative filesystem path.

For portable directory and file naming, see C++ filesystem library or boost.filesystem

### Example

Run this code

    #include <cstdio>
    #include <cstdlib>

    int main()
    {
        FILE* fp = std::fopen("test.txt", "r");
        if(!fp) {
            std::perror("File opening failed");
            return EXIT_FAILURE;
        }

        int c; // note: int, not char, required to handle EOF
        while ((c = std::fgetc(fp)) != EOF) { // standard C I/O file reading loop
           std::putchar(c);
        }

        if (std::ferror(fp))
            std::puts("I/O error when reading");
        else if (std::feof(fp))
            std::puts("End of file reached successfully");

        std::fclose(fp);
    }

### See also

fclose | closes a file (function)
fflush | synchronizes an output stream with the actual file (function)
freopen | open an existing stream with a different name (function)

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