diff --git a/include/SPERR3D_Stream_Tools.h b/include/SPERR3D_Stream_Tools.h
index 1f571ac0..db821a23 100644
--- a/include/SPERR3D_Stream_Tools.h
+++ b/include/SPERR3D_Stream_Tools.h
@@ -35,10 +35,10 @@ class SPERR3D_Stream_Tools {
   auto get_stream_header(const void*) const -> SPERR3D_Header;
 
   // Function that reads in portions of a bitstream to facilitate progressive access.
-  auto progressive_read(std::string filename, unsigned pct) -> vec8_type;
+  auto progressive_read(std::string filename, unsigned pct) const -> vec8_type;
 
   // Function that truncates a bitstream to facilitate progressive access.
-  auto progressive_truncate(const void* stream, size_t stream_len, unsigned pct) -> vec8_type;
+  auto progressive_truncate(const void* stream, size_t stream_len, unsigned pct) const -> vec8_type;
 
  private:
   const size_t m_header_magic_nchunks = 20;
diff --git a/include/SPERR_C_API.h b/include/SPERR_C_API.h
index b4ecccad..a635a0b2 100644
--- a/include/SPERR_C_API.h
+++ b/include/SPERR_C_API.h
@@ -37,11 +37,11 @@ extern "C" {
  *   mode == 3 --> fixed point-wise error (PWE)
  *
  * Return value meanings:
- * 0: success
- * 1: `dst` is not pointing to a NULL pointer!
- * 2: `mode` isn't valid
- * 3: `is_float` value not supported
- *-1: other error
+ *  0: success
+ *  1: `dst` is not pointing to a NULL pointer!
+ *  2: `mode` isn't valid
+ *  3: `is_float` value not supported
+ * -1: other error
  */
 int sperr_comp_2d(
     const void* src,  /* Input: buffer that contains a 2D slice */
@@ -57,9 +57,9 @@ int sperr_comp_2d(
  * Decompress a 2D SPERR-compressed buffer.
  *
  * Return value meanings:
- * 0: success
- * 1: `dst` not pointing to a NULL pointer!
- * 2: `output_float` value not supported
+ *  0: success
+ *  1: `dst` not pointing to a NULL pointer!
+ *  2: `output_float` value not supported
  * -1: other error
  */
 int sperr_decomp_2d(
@@ -77,11 +77,11 @@ int sperr_decomp_2d(
  *   mode == 3 --> fixed point-wise error (PWE)
  *
  * Return value meanings:
- * 0: success
- * 1: `dst` is not pointing to a NULL pointer!
- * 2: `mode` or `quality` isn't valid
- * 3: `is_float` value not supported
- *-1: other error
+ *  0: success
+ *  1: `dst` is not pointing to a NULL pointer!
+ *  2: `mode` or `quality` isn't valid
+ *  3: `is_float` value not supported
+ * -1: other error
  */
 int sperr_comp_3d(
     const void* src,  /* Input: buffer that contains a 3D volume */
@@ -102,9 +102,9 @@ int sperr_comp_3d(
  * Decompress a 3D SPERR-compressed buffer.
  *
  * Return value meanings:
- * 0: success
- * 1: `dst` not pointing to a NULL pointer!
- * 2: `output_float` value not supported
+ *  0: success
+ *  1: `dst` is not pointing to a NULL pointer!
+ *  2: `output_float` value not supported
  * -1: other error
  */
 int sperr_decomp_3d(
@@ -117,6 +117,21 @@ int sperr_decomp_3d(
     size_t* dimz,     /* Output: Z (slowest-varying) dimension */
     void** dst);      /* Output: buffer for the output 3D slice, allocated by this function */
 
+/*
+ * Truncate a 3D SPERR-compressed bitstream to a percentage of its original length.
+ *
+ * Return value meanings:
+ *  0: success
+ *  1: `dst` is not pointing to a NULL pointer!
+ * -1: other error
+ */
+int sperr_trunc_3d(
+    const void* src,  /* Input: buffer that contains a compressed bitstream */
+    size_t src_len,   /* Input: length of the input bitstream in byte */
+    unsigned pct,     /* Input: percentage of the bitstream to keep (0 <= pct <= 100) */
+    void** dst,       /* Output: buffer for the truncated bitstream, allocated by this function */
+    size_t* dst_len); /* Output: length of `dst` in byte */
+
 #ifdef __cplusplus
 } /* end of extern "C" */
 } /* end of namespace C_API */
diff --git a/src/SPERR3D_Stream_Tools.cpp b/src/SPERR3D_Stream_Tools.cpp
index 535cd612..36181b8f 100644
--- a/src/SPERR3D_Stream_Tools.cpp
+++ b/src/SPERR3D_Stream_Tools.cpp
@@ -104,14 +104,15 @@ auto sperr::SPERR3D_Stream_Tools::get_stream_header(const void* p) const -> SPER
   return header;
 }
 
-auto sperr::SPERR3D_Stream_Tools::progressive_read(std::string filename, unsigned pct) -> vec8_type
+auto sperr::SPERR3D_Stream_Tools::progressive_read(std::string filename, unsigned pct) const
+    -> vec8_type
 {
   // Read the header of this bitstream.
   auto vec20 = sperr::read_n_bytes(filename, 20);
   if (vec20.empty())
     return vec20;
   auto arr20 = std::array<uint8_t, 20>();
-  std::copy(vec20.begin(), vec20.end(), arr20.begin());
+  std::copy(vec20.cbegin(), vec20.cend(), arr20.begin());
   const auto header_len = this->get_header_len(arr20);
   const auto header_buf = sperr::read_n_bytes(filename, header_len);
   if (header_buf.empty())
@@ -132,7 +133,7 @@ auto sperr::SPERR3D_Stream_Tools::progressive_read(std::string filename, unsigne
 
 auto sperr::SPERR3D_Stream_Tools::progressive_truncate(const void* stream,
                                                        size_t stream_len,
-                                                       unsigned pct) -> vec8_type
+                                                       unsigned pct) const -> vec8_type
 {
   const auto* u8p = static_cast<const uint8_t*>(stream);
 
diff --git a/src/SPERR_C_API.cpp b/src/SPERR_C_API.cpp
index 1a1752d1..232e5373 100644
--- a/src/SPERR_C_API.cpp
+++ b/src/SPERR_C_API.cpp
@@ -6,6 +6,8 @@
 #include "SPERR3D_OMP_C.h"
 #include "SPERR3D_OMP_D.h"
 
+#include "SPERR3D_Stream_Tools.h"
+
 int C_API::sperr_comp_2d(const void* src,
                          int is_float,
                          size_t dimx,
@@ -232,3 +234,25 @@ int C_API::sperr_decomp_3d(const void* src,
 
   return 0;
 }
+
+int C_API::sperr_trunc_3d(const void* src,
+                          size_t src_len,
+                          unsigned pct,
+                          void** dst,
+                          size_t* dst_len)
+{
+  if (*dst != NULL)
+    return 1;
+
+  auto tools = sperr::SPERR3D_Stream_Tools();
+  auto trunc = tools.progressive_truncate(src, src_len, pct);
+  if (trunc.empty())
+    return -1;
+  else {
+    auto* buf = (uint8_t*)std::malloc(trunc.size());
+    std::copy(trunc.cbegin(), trunc.cend(), buf);
+    *dst = buf;
+    *dst_len = trunc.size();
+    return 0;
+  }
+}