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### 4.29 `funopen`, `fropen`, `fwopen`—open a stream with custom callbacks

Synopsis

         #include <stdio.h>
         FILE *funopen(const void *cookie,
             int (*readfn) (void *cookie, char *buf, int n),
             int (*writefn) (void *cookie, const char *buf, int n),
             fpos_t (*seekfn) (void *cookie, fpos_t off, int whence),
             int (*closefn) (void *cookie));
         FILE *fropen(const void *cookie,
             int (*readfn) (void *cookie, char *buf, int n));
         FILE *fwopen(const void *cookie,
             int (*writefn) (void *cookie, const char *buf, int n));

Description
`funopen` creates a `FILE` stream where I/O is performed using custom callbacks. At least one of readfn and writefn must be provided, which determines whether the stream behaves with mode <"r">, <"w">, or <"r+">.

readfn should return -1 on failure, or else the number of bytes read (0 on EOF). It is similar to `read`, except that <int> rather than <size_t> bounds a transaction size, and cookie will be passed as the first argument. A NULL readfn makes attempts to read the stream fail.

writefn should return -1 on failure, or else the number of bytes written. It is similar to `write`, except that <int> rather than <size_t> bounds a transaction size, and cookie will be passed as the first argument. A NULL writefn makes attempts to write the stream fail.

seekfn should return (fpos_t)-1 on failure, or else the current file position. It is similar to `lseek`, except that cookie will be passed as the first argument. A NULL seekfn makes the stream behave similarly to a pipe in relation to stdio functions that require positioning. This implementation assumes fpos_t and off_t are the same type.

closefn should return -1 on failure, or 0 on success. It is similar to `close`, except that cookie will be passed as the first argument. A NULL closefn merely flushes all data then lets `fclose` succeed. A failed close will still invalidate the stream.

Read and write I/O functions are allowed to change the underlying buffer on fully buffered or line buffered streams by calling `setvbuf`. They are also not required to completely fill or empty the buffer. They are not, however, allowed to change streams from unbuffered to buffered or to change the state of the line buffering flag. They must also be prepared to have read or write calls occur on buffers other than the one most recently specified.

The functions `fropen` and `fwopen` are convenience macros around `funopen` that only use the specified callback.

Returns
The return value is an open FILE pointer on success. On error, `NULL` is returned, and `errno` will be set to EINVAL if a function pointer is missing, ENOMEM if the stream cannot be created, or EMFILE if too many streams are already open.

Portability
This function is a newlib extension, copying the prototype from BSD. It is not portable. See also the `fopencookie` interface from Linux.

Supporting OS subroutines required: `sbrk`.