diff --git a/c_api/index_io_c_ex.cpp b/c_api/index_io_c_ex.cpp
index ef2110dbe6..adcbe77b32 100644
--- a/c_api/index_io_c_ex.cpp
+++ b/c_api/index_io_c_ex.cpp
@@ -17,11 +17,11 @@
 using faiss::Index;
 using faiss::IndexBinary;
 
-int faiss_write_index_buf(const FaissIndex* idx, size_t* size, unsigned char** buf) {
+int faiss_write_index_buf(const FaissIndex* idx, size_t* size, uint8_t** buf) {
     try {
         faiss::VectorIOWriter writer;
         faiss::write_index(reinterpret_cast<const Index*>(idx), &writer);
-        unsigned char* tempBuf = (unsigned char*)malloc((writer.data.size()) * sizeof(uint8_t));
+        uint8_t* tempBuf = (uint8_t*)malloc((writer.data.size()) * sizeof(uint8_t));
         std::copy(writer.data.begin(), writer.data.end(), tempBuf);
         *buf = tempBuf;
         *size = writer.data.size();
diff --git a/c_api/index_io_c_ex.h b/c_api/index_io_c_ex.h
index 7c85409b8c..31f0339477 100644
--- a/c_api/index_io_c_ex.h
+++ b/c_api/index_io_c_ex.h
@@ -31,11 +31,11 @@ extern "C" {
 
 /** Write index to buffer
  */
-int faiss_write_index_buf(const FaissIndex* idx, size_t* buf_size, unsigned char** buf);
+int faiss_write_index_buf(const FaissIndex* idx, size_t* buf_size, uint8_t** buf);
 
 /** Read index from buffer
  */
-int faiss_read_index_buf(const unsigned char* buf, size_t limit, int io_flags,
+int faiss_read_index_buf(const uint8_t* buf, size_t limit, int io_flags,
         FaissIndex** p_out);
 
 #ifdef __cplusplus