New "m" file open mode: non-truncating "w".

Quite surprisingly ISO C lacks a way in fopen to open a file
for writing such that it is not truncated if it already
exists, and not opened in append mode.

(But you will be glad to know that ISO C is adding incredibly
useful features in this area, like Microsoft's fopen_s!)

Let us add modes "m" and "m+" which will be like "w" and "w+",
but without the truncation to zero length (no O_TRUNC
is passed to open).

* stream.c (w_fopen_mode): New static function.
(open_file, open_tail, tail_strategy): Use w_fopen_mode
instead of directly calling w_fopen.
(do_parse_mode): Handle 'm' and set new notrunc flag.

* stream.h (struct stdio_mode): New member, notrunc flag.
(stdio_mode_init_blank, stdio_mode_init_r,
stdio_mode_init_rpb): Initializer macros updated to include
initializer for notrunc flag.

* txr.1: Documented "m" mode.

1 files changed, 4 insertions, 3 deletions

diff --git a/stream.h b/stream.h
index 6fb245d6..2d7688c6 100644
--- a/stream.h
+++ b/stream.h
@@ -108,6 +108,7 @@ struct stdio_mode {
   unsigned create : 1;
   unsigned append : 1;
   unsigned binary : 1;
+  unsigned notrunc : 1;
   unsigned interactive : 1;
   unsigned unbuf : 1;
   unsigned linebuf : 1;
@@ -115,9 +116,9 @@ struct stdio_mode {
   int redir[STDIO_MODE_NREDIRS][2];
 };
 
-#define stdio_mode_init_blank { 0, 0, 0, 0, 0, 0, 0, 0, 0, -1 }
-#define stdio_mode_init_r     { 0, 1, 0, 0, 0, 0, 0, 0, 0, -1 }
-#define stdio_mode_init_rpb   { 0, 1, 1, 0, 0, 1, 0, 0, 0, -1 }
+#define stdio_mode_init_blank { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, -1 }
+#define stdio_mode_init_r     { 0, 1, 0, 0, 0, 0, 0, 0, 0, 0, -1 }
+#define stdio_mode_init_rpb   { 0, 1, 1, 0, 0, 1, 0, 0, 0, 0, -1 }
 
 #define std_input (deref(lookup_var_l(nil, stdin_s)))
 #define std_output (deref(lookup_var_l(nil, stdout_s)))