posix: device_io: implement fileno()

Implement fileno() as required by the POSIX_DEVICE_IO Option
Group.

Signed-off-by: Chris Friedt <cfriedt@tenstorrent.com>

lib/os/fdtable.c

@@ -414,6 +414,16 @@ FILE *zvfs_fdopen(int fd, const char *mode)
 	return (FILE *)&fdtable[fd];
 }
 
+int zvfs_fileno(FILE *file)
+{
+	if (!IS_ARRAY_ELEMENT(fdtable, file)) {
+		errno = EBADF;
+		return -1;
+	}
+
+	return (struct fd_entry *)file - fdtable;
+}
+
 int zvfs_fstat(int fd, struct stat *buf)
 {
 	if (_check_fd(fd) < 0) {

lib/posix/options/device_io.c

@@ -15,6 +15,7 @@
 /* prototypes for external, not-yet-public, functions in fdtable.c or fs.c */
 int zvfs_close(int fd);
 FILE *zvfs_fdopen(int fd, const char *mode);
+int zvfs_fileno(FILE *file);
 int zvfs_open(const char *name, int flags);
 ssize_t zvfs_read(int fd, void *buf, size_t sz, size_t *from_offset);
 ssize_t zvfs_write(int fd, const void *buf, size_t sz, size_t *from_offset);
@@ -52,6 +53,11 @@ FILE *fdopen(int fd, const char *mode)
 	return zvfs_fdopen(fd, mode);
 }
 
+int fileno(FILE *file)
+{
+	return zvfs_fileno(file);
+}
+
 int open(const char *name, int flags, ...)
 {
 	/* FIXME: necessarily need to check for O_CREAT and unpack ... if set */