diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.c b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.c
index aaa5d3dc74..b4a9adfefa 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.c
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.c
@@ -14,7 +14,7 @@
 **/
 #include "OnigurumaUefiPort.h"
 
-int sprintf_s(char *str, size_t sizeOfBuffer, char const *fmt, ...)
+int EFIAPI sprintf_s(char *str, size_t sizeOfBuffer, char const *fmt, ...)
 {
   VA_LIST Marker;
   int   NumberOfPrinted;
diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.h b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.h
index ca478de68e..d24ac1d880 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.h
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/OnigurumaUefiPort.h
@@ -69,7 +69,7 @@ typedef UINTN size_t;
 
 int OnigStrCmp (char* Str1, char* Str2);
 
-int sprintf_s (char *str, size_t sizeOfBuffer, char const *fmt, ...);
+int EFIAPI sprintf_s (char *str, size_t sizeOfBuffer, char const *fmt, ...);
 
 #define exit(n) ASSERT(FALSE);
 
diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/oniguruma.h b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/oniguruma.h
index 034a2ccff1..462565aeca 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/oniguruma.h
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/oniguruma.h
@@ -719,7 +719,7 @@ typedef struct {
 ONIG_EXTERN
 int onig_init P_((void));
 ONIG_EXTERN
-int onig_error_code_to_str PV_((OnigUChar* s, int err_code, ...));
+int EFIAPI onig_error_code_to_str PV_((OnigUChar* s, int err_code, ...));
 ONIG_EXTERN
 void onig_set_warn_func P_((OnigWarnFunc f));
 ONIG_EXTERN
diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regerror.c b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regerror.c
index fbc764aa42..1544e23f10 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regerror.c
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regerror.c
@@ -252,6 +252,7 @@ static int to_ascii(OnigEncoding enc, UChar *s, UChar *end,
 #define MAX_ERROR_PAR_LEN   30
 
 extern int
+EFIAPI
 #ifdef HAVE_STDARG_PROTOTYPES
 onig_error_code_to_str(UChar* s, int code, ...)
 #else
@@ -324,6 +325,7 @@ onig_error_code_to_str(s, code, va_alist)
 
 
 void
+EFIAPI
 #ifdef HAVE_STDARG_PROTOTYPES
 onig_snprintf_with_pattern(UChar buf[], int bufsize, OnigEncoding enc,
                            UChar* pat, UChar* pat_end, const UChar *fmt, ...)
diff --git a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regint.h b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regint.h
index f48c55764c..d5b30898a3 100644
--- a/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regint.h
+++ b/MdeModulePkg/Universal/RegularExpressionDxe/Oniguruma/regint.h
@@ -781,7 +781,7 @@ extern void onig_print_statistics P_((FILE* f));
 #endif
 
 extern UChar* onig_error_code_to_format P_((int code));
-extern void  onig_snprintf_with_pattern PV_((UChar buf[], int bufsize, OnigEncoding enc, UChar* pat, UChar* pat_end, const UChar *fmt, ...));
+extern void EFIAPI onig_snprintf_with_pattern PV_((UChar buf[], int bufsize, OnigEncoding enc, UChar* pat, UChar* pat_end, const UChar *fmt, ...));
 extern int  onig_bbuf_init P_((BBuf* buf, int size));
 extern int  onig_compile P_((regex_t* reg, const UChar* pattern, const UChar* pattern_end, OnigErrorInfo* einfo));
 extern void onig_chain_reduce P_((regex_t* reg));