make: allow '#' to be escaped with a backslash

POSIX doesn't allow the '#' comment marker to be escaped, though
some implementations do.

As a non-POSIX extension allow '#' to be escaped with a preceding
backslash.

It isn't necessary to escape '#' in macro expansions or command
lines:  these cases are covered by an existing extension.  Commit
0aceca867 (make: comments in macro expansions and command lines)

Adds 16-32 bytes.

(pdpmake GitHub issue 38)

2 files changed, 20 insertions, 3 deletions

diff --git a/miscutils/make.c b/miscutils/make.c
index 841daa93e..87f17aad7 100644
--- a/miscutils/make.c
+++ b/miscutils/make.c
@@ -1373,9 +1373,17 @@ process_line(char *s)
 
         // Strip comment
         // don't treat '#' in macro expansion as a comment
-        if (!posix)
-                t = find_char(s, '#');
-        else
+        // nor '#' outside macro expansion preceded by backslash
+        if (!posix) {
+                char *u = s;
+                while ((t = find_char(u, '#')) && t > u && t[-1] == '\\') {
+                        for (u = t; *u; ++u) {
+                                u[-1] = u[0];
+                        }
+                        *u = '\0';
+                        u = t;
+                }
+        } else
                 t = strchr(s, '#');
         if (t)
                 *t = '\0';
diff --git a/testsuite/make.tests b/testsuite/make.tests
index 745a840f3..1fc95a8d2 100755
--- a/testsuite/make.tests
+++ b/testsuite/make.tests
@@ -459,6 +459,15 @@ target:
         : hash $(hash) hash
 '
 
+# A '#' character can be escaped with a backslash
+testing "make backslash-escaped hash isn't a comment" \
+        "make -f -" \
+        ": hash # hash\n" "" '
+hash = \#
+target:
+        : hash $(hash) hash
+'
+
 # A '#' character in a command line doesn't start a comment
 testing "make hash in command line isn't a comment" \
         "make -f -" \