From c34590c4bf067d4e749e5298a8ce9f90072d6332 Mon Sep 17 00:00:00 2001
From: Will Estes <westes575@gmail.com>
Date: Sat, 12 Dec 2015 11:03:05 -0500
Subject: [PATCH] Made search for m4 more explicit.

---
 src/flexdef.h |  3 +++
 src/main.c    | 42 ++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/src/flexdef.h b/src/flexdef.h
index 9c6baec..3aa63df 100644
--- a/src/flexdef.h
+++ b/src/flexdef.h
@@ -66,6 +66,9 @@
 #ifdef HAVE_SYS_PARAMS_H
 #include <sys/params.h>
 #endif
+#ifdef HAVE_SYS_STAT_H
+#include <sys/stat.h>
+#endif
 #ifdef HAVE_SYS_WAIT_H
 #include <sys/wait.h>
 #endif
diff --git a/src/main.c b/src/main.c
index 5fadfc4..c276bc4 100644
--- a/src/main.c
+++ b/src/main.c
@@ -359,8 +359,46 @@ void check_options (void)
 
     /* Setup the filter chain. */
     output_chain = filter_create_int(NULL, filter_tee_header, headerfilename);
-    if ( !(m4 = getenv("M4")))
-        m4 = M4;
+    if ( !(m4 = getenv("M4"))) {
+	    char *slash;
+		m4 = M4;
+		if (slash = strrchr(M4, '/')) {
+			m4 = slash+1;
+			/* break up $PATH */
+			const char *path = getenv("PATH");
+			if (!path) {
+				m4 = M4;
+			} else {
+				do {
+					char m4_path[PATH_MAX];
+					int length = strlen(path);
+					struct stat sbuf;
+
+					const char *endOfDir = strchr(path, ':');
+					if (!endOfDir)
+						endOfDir = path+length;
+
+					if ((endOfDir-path+2) >= sizeof(m4_path)) {
+					    path = endOfDir+1;
+						continue;
+					}
+
+					strncpy(m4_path, path, sizeof(m4_path));
+					m4_path[endOfDir-path] = '/';
+					m4_path[endOfDir-path+1] = '\0';
+					strncat(m4_path, m4, sizeof(m4_path));
+					if (stat(m4_path, &sbuf) == 0 &&
+						(S_ISREG(sbuf.st_mode)) && sbuf.st_mode & S_IXUSR) {
+						m4 = strdup(m4_path);
+						break;
+					}
+					path = endOfDir+1;
+				} while (path[0]);
+				if (!path[0])
+				    m4 = M4;
+			}
+		}
+	}
     filter_create_ext(output_chain, m4, "-P", 0);
     filter_create_int(output_chain, filter_fix_linedirs, NULL);
 
-- 
2.40.0