diff --git a/gtp/gtp.h b/gtp/gtp.h
index e03d77d..95566fa 100644
--- a/gtp/gtp.h
+++ b/gtp/gtp.h
@@ -441,5 +441,6 @@
 extern int gsna2in_addr(struct in_addr *dst, struct ul16_t *gsna);
 extern int in_addr2gsna(struct ul16_t *gsna, struct in_addr *src);
 extern const char *imsi_gtp2str(const uint64_t *imsi);
+extern const uint64_t imsi_str2gtp(const char *imsi);
 
 #endif /* !_GTP_H */