diff --git a/src/readstat_bits.h b/src/readstat_bits.h
index 9cfcadd9..a89d9a2c 100644
--- a/src/readstat_bits.h
+++ b/src/readstat_bits.h
@@ -7,7 +7,7 @@
 #undef READSTAT_MACHINE_IS_TWOS_COMPLEMENT
 #define READSTAT_MACHINE_IS_TWOS_COMPLEMENT 0
 
-int machine_is_little_endian();
+int machine_is_little_endian(void);
 
 char ones_to_twos_complement1(char num);
 int16_t ones_to_twos_complement2(int16_t num);
diff --git a/src/readstat_variable.c b/src/readstat_variable.c
index 0f730f8e..6172abdc 100644
--- a/src/readstat_variable.c
+++ b/src/readstat_variable.c
@@ -2,7 +2,7 @@
 #include <stdlib.h>
 #include "readstat.h"
 
-static readstat_value_t make_blank_value();
+static readstat_value_t make_blank_value(void);
 static readstat_value_t make_double_value(double dval);
 
 static readstat_value_t make_blank_value() {
diff --git a/src/sas/ieee.c b/src/sas/ieee.c
index 44233c7a..f6b7d87f 100644
--- a/src/sas/ieee.c
+++ b/src/sas/ieee.c
@@ -16,7 +16,7 @@ static void ieee2xpt(unsigned char *ieee, unsigned char *xport);
 
 #ifndef FLOATREP
 #define FLOATREP get_native()
-int get_native();
+int get_native(void);
 #endif
 
 void memreverse(void *intp_void, int l) {
diff --git a/src/spss/readstat_por.h b/src/spss/readstat_por.h
index 07dbce09..e169d687 100644
--- a/src/spss/readstat_por.h
+++ b/src/spss/readstat_por.h
@@ -31,7 +31,7 @@ typedef struct por_ctx_s {
     ck_hash_table_t *var_dict;
 } por_ctx_t;
 
-por_ctx_t *por_ctx_init();
+por_ctx_t *por_ctx_init(void);
 void por_ctx_free(por_ctx_t *ctx);
 ssize_t por_utf8_encode(const unsigned char *input, size_t input_len, 
         char *output, size_t output_len, uint16_t lookup[256]);