BIO_s_file()  This is the FILE pointer BIO object.

Uses FILE *; the file pointer is actually kept in BIO->ptr.

This source/sink is in theory bi-directional but it depends on the
FILE * and how it was opened.
Non-blocking IO is not supported.

A file pointer needs to be assigned before any IO operations should be made.
So normally:

bio=BIO_new(BIO_s_file());
BIO_set_fp(bio,stderr,BIO_NOCLOSE);
or
bio=BIO_new(BIO_s_file());
BIO_read_filename(bio,"infile");

BIO_read and BIO_write do not implement the 'pending' operations
used for non-blocking IO.

BIO_gets does a fgets.
BIO_puts does a fwrite().

BIO_CTRL_RESET does a fseek() back to the start of the file.

BIO_CTRL_EOF does a feof().

BIO_CTRL_INFO does a ftell().

BIO_CTRL_SET assigned the FILE * and close flag.

BIO_CTRL_GET returns the FILE * via the ptr parameter (FILE **).

BIO_CTRL_GET_CLOSE/BIO_CTRL_SET_CLOSE returns and sets the close on
free flag.

BIO_CTRL_PUSH returns 0, does nothing.
BIO_CTRL_POP returns 0, does nothing.

BIO_CTRL_PENDING unimplemented, returns 0.

BIO_CTRL_FLUSH fflush()es the FILE *.

BIO_CTRL_SHOULD_RETRY/BIO_CTRL_RETRY_TYPE unimplemented, returns 0.

BIO_CTRL_SET_FILENAME is a special case.  The file name is opened and
used.  This is a special case.

.