Fix #539 crash on Apple M1 by casting 0 to (char *) explicitly

Currently, when the NULL-terminated variadic function
filter_create_ext() is invoked, the value "0" is passed as
the last argument to act as a terminator. However, this is
an integer value, which is incompatible with the pointer
data type expected by filter_create_ext().

This is undefined behavior in C, correct operation is not
guaranteed. In fact, it causes flex to crash on Apple M1
when GCC is used - the loop is not terminated when it should,
instead, it keeps running, corrupting the argument list for
invoking m4. As a result, it creates the following error:

> flex: fatal internal error, exec of gm4 failed

This commit fixes the problem by explicitly casting the value 0 to
the correct pointer type (char *).

Signed-off-by: Yifeng Li <tomli@tomli.me>

1 files changed, 1 insertions, 1 deletions

diff --git a/src/main.c b/src/main.c
index c4e6849..eb2c09f 100644
--- a/src/main.c
+++ b/src/main.c
@@ -295,7 +295,7 @@ void initialize_output_filters(void)
 	if ( !(m4 = getenv("M4"))) {
 		m4 = M4;
 	}
-	filter_create_ext(output_chain, m4, "-P", 0);
+	filter_create_ext(output_chain, m4, "-P", (char *) 0);
 	filter_create_int(output_chain, filter_fix_linedirs, NULL);
 
 	/* For debugging, only run the requested number of filters. */