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, 9 insertions, 1 deletions

diff --git a/txr.1 b/txr.1
index 9765dc2f..6f3f2258 100644
--- a/txr.1
+++ b/txr.1
@@ -48925,7 +48925,7 @@ grammar. Note that it permits no whitespace characters:
 .mono
 .mets < mode-string := [ < mode ] [ < options ]
 .mets < mode := { < selector [ + ] | + }
-.mets < selector := { r | w | a }
+.mets < selector := { r | w | a | m }
 .mets < options := { b | l | u | < digit | < redirection }
 .mets < digit := { 0 | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 }
 .onom
@@ -48968,6 +48968,14 @@ to zero length. If it doesn't exist, it is created.
 .coIP w+
 The file is opened for reading and writing. If it exists, it
 is truncated to zero length. If it doesn't exist, it is created.
+.coIP m
+The file is opened for modification. This is the same as
+.code w
+except that the file is not truncated if it exists.
+.coIP m+
+The file is opened for reading and modification. This is the same as
+.code w+
+except that the file is not truncated if it exists.
 .coIP a
 The file is opened for writing. If it doesn't exist, it is
 created. If it exists, the current position is advanced to