--- a/ssmtp.c
+++ b/ssmtp.c
@@ -55,21 +55,21 @@ bool_t use_oldauth = False;		/* use old
 
 #define ARPADATE_LENGTH 32		/* Current date in RFC format */
 char arpadate[ARPADATE_LENGTH];
-char *auth_user = (char)NULL;
-char *auth_pass = (char)NULL;
-char *auth_method = (char)NULL;		/* Mechanism for SMTP authentication */
-char *mail_domain = (char)NULL;
-char *from = (char)NULL;		/* Use this as the From: address */
+char *auth_user = NULL;
+char *auth_pass = NULL;
+char *auth_method = NULL;		/* Mechanism for SMTP authentication */
+char *mail_domain = NULL;
+char *from = NULL;		/* Use this as the From: address */
 char *hostname;
 char *mailhost = "mailhub";
-char *minus_f = (char)NULL;
-char *minus_F = (char)NULL;
+char *minus_f = NULL;
+char *minus_F = NULL;
 char *gecos;
-char *prog = (char)NULL;
+char *prog = NULL;
 char *root = NULL;
 char *tls_cert = "/etc/ssl/certs/ssmtp.pem";	/* Default Certificate */
-char *uad = (char)NULL;
-char *config_file = (char)NULL;		/* alternate configuration file */
+char *uad = NULL;
+char *config_file = NULL;		/* alternate configuration file */
 
 headers_t headers, *ht;
 
@@ -261,7 +261,7 @@ char *strip_post_ws(char *str)
 
 	p = (str + strlen(str));
 	while(isspace(*--p)) {
-		*p = (char)NULL;
+		*p = '\0';
 	}
 
 	return(p);
@@ -287,7 +287,7 @@ char *addr_parse(char *str)
 		q++;
 
 		if((p = strchr(q, '>'))) {
-			*p = (char)NULL;
+			*p = '\0';
 		}
 
 #if 0
@@ -310,7 +310,7 @@ char *addr_parse(char *str)
 	q = strip_post_ws(p);
 	if(*q == ')') {
 		while((*--q != '('));
-		*q = (char)NULL;
+		*q = '\0';
 	}
 	(void)strip_post_ws(p);
 
@@ -363,7 +363,7 @@ bool_t standardise(char *str, bool_t *li
 	*linestart = False;
 
 	if((p = strchr(str, '\n'))) {
-		*p = (char)NULL;
+		*p = '\0';
 		*linestart = True;
 	}
 	return(leadingdot);
@@ -384,7 +384,7 @@ void revaliases(struct passwd *pw)
 		while(fgets(buf, sizeof(buf), fp)) {
 			/* Make comments invisible */
 			if((p = strchr(buf, '#'))) {
-				*p = (char)NULL;
+				*p = '\0';
 			}
 
 			/* Ignore malformed lines and comments */
@@ -519,7 +519,7 @@ void rcpt_save(char *str)
 #endif
 
 	/* Ignore missing usernames */
-	if(*str == (char)NULL) {
+	if(*str == '\0') {
 		return;
 	}
 
@@ -576,7 +576,7 @@ void rcpt_parse(char *str)
 		}
 
 		/* End of string? */
-		if(*(q + 1) == (char)NULL) {
+		if(*(q + 1) == '\0') {
 			got_addr = True;
 		}
 
@@ -584,7 +584,7 @@ void rcpt_parse(char *str)
 		if((*q == ',') && (in_quotes == False)) {
 			got_addr = True;
 
-			*q = (char)NULL;
+			*q = '\0';
 		}
 
 		if(got_addr) {
@@ -676,7 +676,7 @@ void header_save(char *str)
 	if(strncasecmp(ht->string, "From:", 5) == 0) {
 #if 1
 		/* Hack check for NULL From: line */
-		if(*(p + 6) == (char)NULL) {
+		if(*(p + 6) == '\0') {
 			return;
 		}
 #endif
@@ -741,7 +741,7 @@ void header_parse(FILE *stream)
 	size_t size = BUF_SZ, len = 0;
 	char *p = (char *)NULL, *q;
 	bool_t in_header = True;
-	char l = (char)NULL;
+	char l = '\0';
 	int c;
 
 	while(in_header && ((c = fgetc(stream)) != EOF)) {
@@ -776,9 +776,9 @@ void header_parse(FILE *stream)
 						in_header = False;
 
 				default:
-						*q = (char)NULL;
+						*q = '\0';
 						if((q = strrchr(p, '\n'))) {
-							*q = (char)NULL;
+							*q = '\0';
 						}
 						header_save(p);
 
@@ -809,9 +809,9 @@ void header_parse(FILE *stream)
 						in_header = False;
 
 				default:
-						*q = (char)NULL;
+						*q = '\0';
 						if((q = strrchr(p, '\n'))) {
-							*q = (char)NULL;
+							*q = '\0';
 						}
 						header_save(p);
 
@@ -876,7 +876,7 @@ bool_t read_config()
 		char *rightside;
 		/* Make comments invisible */
 		if((p = strchr(buf, '#'))) {
-			*p = (char)NULL;
+			*p = '\0';
 		}
 
 		/* Ignore malformed lines and comments */
@@ -1310,7 +1310,7 @@ char *fd_gets(char *buf, int size, int f
 			buf[i++] = c;
 		}
 	}
-	buf[i] = (char)NULL;
+	buf[i] = '\0';
 
 	return(buf);
 }
@@ -1737,7 +1737,7 @@ char **parse_options(int argc, char *arg
 		j = 0;
 
 		add = 1;
-		while(argv[i][++j] != (char)NULL) {
+		while(argv[i][++j] != '\0') {
 			switch(argv[i][j]) {
 #ifdef INET6
 			case '6':