diff --git a/Modules/_decimal/libmpdec/mpdecimal.c b/Modules/_decimal/libmpdec/mpdecimal.c
index 287a77ed..5c8797a 100644
--- a/Modules/_decimal/libmpdec/mpdecimal.c
+++ b/Modules/_decimal/libmpdec/mpdecimal.c
@@ -392,42 +392,42 @@
 
 /* Dynamic decimal */
 ALWAYS_INLINE int
-mpd_isdynamic(mpd_t *dec)
+mpd_isdynamic(const mpd_t *dec)
 {
     return !(dec->flags & MPD_STATIC);
 }
 
 /* Static decimal */
 ALWAYS_INLINE int
-mpd_isstatic(mpd_t *dec)
+mpd_isstatic(const mpd_t *dec)
 {
     return dec->flags & MPD_STATIC;
 }
 
 /* Data of decimal is dynamic */
 ALWAYS_INLINE int
-mpd_isdynamic_data(mpd_t *dec)
+mpd_isdynamic_data(const mpd_t *dec)
 {
     return !(dec->flags & MPD_DATAFLAGS);
 }
 
 /* Data of decimal is static */
 ALWAYS_INLINE int
-mpd_isstatic_data(mpd_t *dec)
+mpd_isstatic_data(const mpd_t *dec)
 {
     return dec->flags & MPD_STATIC_DATA;
 }
 
 /* Data of decimal is shared */
 ALWAYS_INLINE int
-mpd_isshared_data(mpd_t *dec)
+mpd_isshared_data(const mpd_t *dec)
 {
     return dec->flags & MPD_SHARED_DATA;
 }
 
 /* Data of decimal is const */
 ALWAYS_INLINE int
-mpd_isconst_data(mpd_t *dec)
+mpd_isconst_data(const mpd_t *dec)
 {
     return dec->flags & MPD_CONST_DATA;
 }
@@ -597,7 +597,7 @@
 
 /* Copy sign from another decimal */
 ALWAYS_INLINE void
-mpd_signcpy(mpd_t *result, mpd_t *a)
+mpd_signcpy(mpd_t *result, const mpd_t *a)
 {
     uint8_t sign = a->flags&MPD_NEG;
 
diff --git a/Modules/_decimal/libmpdec/mpdecimal.h b/Modules/_decimal/libmpdec/mpdecimal.h
index 0f31733..676c72d 100644
--- a/Modules/_decimal/libmpdec/mpdecimal.h
+++ b/Modules/_decimal/libmpdec/mpdecimal.h
@@ -752,12 +752,12 @@
 /* 1 if dec is positive, -1 if dec is negative */
 EXTINLINE int mpd_arith_sign(const mpd_t *dec);
 EXTINLINE long mpd_radix(void);
-EXTINLINE int mpd_isdynamic(mpd_t *dec);
-EXTINLINE int mpd_isstatic(mpd_t *dec);
-EXTINLINE int mpd_isdynamic_data(mpd_t *dec);
-EXTINLINE int mpd_isstatic_data(mpd_t *dec);
-EXTINLINE int mpd_isshared_data(mpd_t *dec);
-EXTINLINE int mpd_isconst_data(mpd_t *dec);
+EXTINLINE int mpd_isdynamic(const mpd_t *dec);
+EXTINLINE int mpd_isstatic(const mpd_t *dec);
+EXTINLINE int mpd_isdynamic_data(const mpd_t *dec);
+EXTINLINE int mpd_isstatic_data(const mpd_t *dec);
+EXTINLINE int mpd_isshared_data(const mpd_t *dec);
+EXTINLINE int mpd_isconst_data(const mpd_t *dec);
 EXTINLINE mpd_ssize_t mpd_trail_zeros(const mpd_t *dec);
 
 
@@ -769,7 +769,7 @@
 EXTINLINE void mpd_setdigits(mpd_t *result);
 EXTINLINE void mpd_set_sign(mpd_t *result, uint8_t sign);
 /* copy sign from another decimal */
-EXTINLINE void mpd_signcpy(mpd_t *result, mpd_t *a);
+EXTINLINE void mpd_signcpy(mpd_t *result, const mpd_t *a);
 EXTINLINE void mpd_set_infinity(mpd_t *result);
 EXTINLINE void mpd_set_qnan(mpd_t *result);
 EXTINLINE void mpd_set_snan(mpd_t *result);