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### 4.17 `fopen`—open a file

Synopsis

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

    FILE *_fopen_r(struct _reent *reent,
        const char *file, const char *mode);

Description
`fopen` initializes the data structures needed to read or write a file. Specify the file’s name as the string at file, and the kind of access you need to the file with the string at mode.

The alternate function `_fopen_r` is a reentrant version. The extra argument reent is a pointer to a reentrancy structure.

Three fundamental kinds of access are available: read, write, and append. `*mode` must begin with one of the three characters ‘`r`’, ‘`w`’, or ‘`a`’, to select one of these:

`r`

  Open the file for reading; the operation will fail if the file does not exist, or if the host system does not permit you to read it.

`w`

  Open the file for writing from the beginning of the file: effectively, this always creates a new file. If the file whose name you specified already existed, its old contents are discarded.

`a`

  Open the file for appending data, that is writing from the end of file. When you open a file this way, all data always goes to the current end of file; you cannot change this using `fseek`.

Some host systems distinguish between “binary” and “text” files. Such systems may perform data transformations on data written to, or read from, files opened as “text”. If your system is one of these, then you can append a ‘`b`’ to any of the three modes above, to specify that you are opening the file as a binary file (the default is to open the file as a text file).

‘`rb`’, then, means “read binary”; ‘`wb`’, “write binary”; and ‘`ab`’, “append binary”.

To make C programs more portable, the ‘`b`’ is accepted on all systems, whether or not it makes a difference.

Finally, you might need to both read and write from the same file. You can also append a ‘`+`’ to any of the three modes, to permit this. (If you want to append both ‘`b`’ and ‘`+`’, you can do it in either order: for example, `"rb+"` means the same thing as `"r+b"` when used as a mode string.)

Use `"r+"` (or `"rb+"`) to permit reading and writing anywhere in an existing file, without discarding any data; `"w+"` (or `"wb+"`) to create a new file (or begin by discarding all data from an old one) that permits reading and writing anywhere in it; and `"a+"` (or `"ab+"`) to permit reading anywhere in an existing file, but writing only at the end.

Returns
`fopen` returns a file pointer which you can use for other file operations, unless the file you requested could not be opened; in that situation, the result is `NULL`. If the reason for failure was an invalid string at mode, `errno` is set to `EINVAL`.

Portability
`fopen` is required by ANSI C.

Supporting OS subroutines required: `close`, `fstat`, `isatty`, `lseek`, `open`, `read`, `sbrk`, `write`.

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