file: bind freopen and feof as File methods

Author: rrthomas

vlib/os/file.c.v

@@ -1,6 +1,7 @@
 module os
 
 pub struct File {
+mut:
 	cfile voidptr // Using void* instead of FILE*
 pub:
 	fd int
@@ -19,6 +20,16 @@ fn C._fseeki64(&C.FILE, u64, int) int
 
 fn C.getc(&C.FILE) int
 
+fn C.freopen(&char, &char, &C.FILE) &C.FILE
+
+fn fix_windows_path(path string) string {
+	mut p := path
+	$if windows {
+		p = path.replace('/', '\\')
+	}
+	return p
+}
+
 // open_file can be used to open or create a file with custom flags and permissions and returns a `File` object.
 pub fn open_file(path string, mode string, options ...int) ?File {
 	mut flags := 0
@@ -55,10 +66,7 @@ pub fn open_file(path string, mode string, options ...int) ?File {
 			permission = 0x0100 | 0x0080
 		}
 	}
-	mut p := path
-	$if windows {
-		p = path.replace('/', '\\')
-	}
+	p := fix_windows_path(path)
 	fd := C.open(&char(p.str), flags, permission)
 	if fd == -1 {
 		return error(posix_get_error_msg(C.errno))
@@ -160,6 +168,20 @@ pub fn stderr() File {
 	}
 }
 
+pub fn (f &File) eof() bool {
+	return C.feof(f.cfile) != 0
+}
+
+// reopen allows a `File` to be reused. It is mostly useful for reopening standard input and output.
+pub fn (mut f File) reopen(path string, mode string) ? {
+	p := fix_windows_path(path)
+	cfile := C.freopen(&char(p.str), &char(mode.str), f.cfile)
+	if isnil(cfile) {
+		return error('Failed to reopen file "$path"')
+	}
+	f.cfile = cfile
+}
+
 // read implements the Reader interface.
 pub fn (f &File) read(mut buf []u8) ?int {
 	if buf.len == 0 {

vlib/os/file_test.v

@@ -370,3 +370,26 @@ fn test_tell() ? {
 		assert pos == size - 5
 	}
 }
+
+fn test_reopen() ? {
+	mut f := os.open_file(tfile, 'w')?
+	f.write_string('Hello World!\nGood\r morning.')?
+	f.close()
+
+	mut stdin := os.stdin()
+	stdin.reopen(tfile, 'r')?
+	line := os.get_line()
+	assert line == 'Hello World!'
+}
+
+fn test_eof() ? {
+	mut f := os.open_file(tfile, 'w')?
+	f.write_string('Hello World!\n')?
+	f.close()
+
+	f = os.open(tfile)?
+	f.read_bytes(10)
+	assert !f.eof()
+	f.read_bytes(100)
+	assert f.eof()
+}