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.

3 files changed, 45 insertions, 7 deletions

diff --git a/stream.c b/stream.c
index 0ca0f799..c0362f3f 100644
--- a/stream.c
+++ b/stream.c
@@ -1044,6 +1044,28 @@ static struct strm_ops stdio_ops =
 static struct strm_ops stdio_sock_ops;
 #endif
 
+static FILE *w_fopen_mode(const wchar_t *wname, const wchar_t *mode,
+                          const struct stdio_mode m)
+{
+#if HAVE_FCNTL
+  if (m.notrunc) {
+    char *name = utf8_dup_to(wname);
+    int flags = (m.read ? O_RDWR : O_WRONLY) | O_CREAT;
+    int fd = open(name, flags, 0777);
+    free(name);
+    if (fd < 0)
+      return NULL;
+    return (fd < 0) ? NULL : w_fdopen(fd, mode);
+  }
+#else
+  if (m.notrunc)
+    uw_throwf(file_error_s,
+              lit("open-file: system doesn't support \"o\" mode"), nao);
+#endif
+  return w_fopen(wname, mode);
+}
+
+
 static void tail_calc(unsigned long *state, int *usec, int *mod)
 {
   unsigned long count = (*state)++;
@@ -1096,7 +1118,7 @@ static void tail_strategy(val stream, unsigned long *state)
 
       /* Try to open the file.
        */
-      if (!(newf = w_fopen(c_str(h->descr), c_str(mode)))) {
+      if (!(newf = w_fopen_mode(c_str(h->descr), c_str(mode), m))) {
         /* If already have the file open previously, and the name
          * does not open any more, then the file has rotated.
          * Have the caller try to read the last bit of data
@@ -1349,6 +1371,12 @@ static struct stdio_mode do_parse_mode(val mode_str, struct stdio_mode m_dfl)
     m.write = 1;
     m.append = 1;
     break;
+  case 'm':
+    ms++;
+    m.write = 1;
+    m.create = 1;
+    m.notrunc = 1;
+    break;
   default:
     break;
   }
@@ -3947,7 +3975,8 @@ val open_directory(val path)
 val open_file(val path, val mode_str)
 {
   struct stdio_mode m, m_r = stdio_mode_init_r;
-  FILE *f = w_fopen(c_str(path), c_str(normalize_mode(&m, mode_str, m_r)));
+  val norm_mode = normalize_mode(&m, mode_str, m_r);
+  FILE *f = w_fopen_mode(c_str(path), c_str(norm_mode), m);
 
   if (!f) {
     int eno = errno;
@@ -3979,7 +4008,7 @@ val open_tail(val path, val mode_str, val seek_end_p)
 {
   struct stdio_mode m, m_r = stdio_mode_init_r;
   val mode = normalize_mode(&m, mode_str, m_r);
-  FILE *f = w_fopen(c_str(path), c_str(mode));
+  FILE *f = w_fopen_mode(c_str(path), c_str(mode), m);
   struct stdio_handle *h;
   val stream;
   unsigned long state = 0;
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)))
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