diff --git a/source/ExtBufStreambuf.h b/source/ExtBufStreambuf.h
new file mode 100644
index 00000000..2931170c
--- /dev/null
+++ b/source/ExtBufStreambuf.h
@@ -0,0 +1,66 @@
+#ifndef CODE_ExtBufStreambuf
+#define CODE_ExtBufStreambuf
+
+#include <streambuf>
+#include <ios>
+
+// A std::streambuf backed by a caller-owned, fixed-size char buffer (zero-copy).
+//
+// This replaces the non-portable idiom
+//     someStringStream.rdbuf()->pubsetbuf(externalBuffer, size);
+// which STAR used to read/write reads and SAM/BAM records directly from/to a
+// pre-allocated char array without copying.
+//
+// std::basic_stringbuf::setbuf() (what pubsetbuf() calls on a string stream) has
+// implementation-defined behavior: libstdc++ (GCC/Linux) adopts the external
+// buffer, but libc++ (clang/macOS) ignores it entirely. With libc++ the streams
+// therefore stayed empty, which made STAR read 0 input reads and write empty
+// SAM/BAM output. This class implements the buffer adoption explicitly so the
+// behavior is identical on every standard library.
+class ExtBufStreambuf : public std::streambuf {
+public:
+    // Use the external buffer as the get (read) area: [base, base+n).
+    void setReadBuffer(char* base, std::streamsize n) {
+        setg(base, base, base + n);
+    }
+
+    // Use the external buffer as the put (write) area: [base, base+n).
+    void setWriteBuffer(char* base, std::streamsize n) {
+        setp(base, base + n);
+    }
+
+protected:
+    pos_type seekoff(off_type off, std::ios_base::seekdir dir,
+                     std::ios_base::openmode which) override {
+        if (which & std::ios_base::in) {
+            char* base = eback();
+            char* end  = egptr();
+            char* newp = (dir == std::ios_base::beg) ? base + off
+                       : (dir == std::ios_base::cur) ? gptr() + off
+                       :                               end  + off;
+            if (newp < base) newp = base;
+            if (newp > end)  newp = end;
+            setg(base, newp, end);
+            return pos_type(newp - base);
+        }
+        if (which & std::ios_base::out) {
+            char* base = pbase();
+            char* end  = epptr();
+            char* newp = (dir == std::ios_base::beg) ? base + off
+                       : (dir == std::ios_base::cur) ? pptr() + off
+                       :                               end  + off;
+            if (newp < base) newp = base;
+            if (newp > end)  newp = end;
+            setp(base, end);
+            pbump(static_cast<int>(newp - base));
+            return pos_type(newp - base);
+        }
+        return pos_type(off_type(-1));
+    }
+
+    pos_type seekpos(pos_type sp, std::ios_base::openmode which) override {
+        return seekoff(off_type(sp), std::ios_base::beg, which);
+    }
+};
+
+#endif
diff --git a/source/ReadAlignChunk.cpp b/source/ReadAlignChunk.cpp
index 8f7f180b..5898e50a 100644
--- a/source/ReadAlignChunk.cpp
+++ b/source/ReadAlignChunk.cpp
@@ -18,13 +18,14 @@ ReadAlignChunk::ReadAlignChunk(Parameters& Pin, Genome &genomeIn, Transcriptome
     RA->iRead=0;
 
     chunkIn=new char* [P.readNends];
-    readInStream=new istringstream* [P.readNends];
-    
+    readInStream=new istream* [P.readNends];
+    readInStreamBuf=new ExtBufStreambuf [P.readNends];
+
     for (uint ii=0;ii<P.readNends;ii++) {
        chunkIn[ii]=new char[P.chunkInSizeBytesArray];//reserve more space to finish loading one read
        memset(chunkIn[ii],'\n',P.chunkInSizeBytesArray);
-       readInStream[ii] = new istringstream;
-       readInStream[ii]->rdbuf()->pubsetbuf(chunkIn[ii],P.chunkInSizeBytesArray);
+       readInStreamBuf[ii].setReadBuffer(chunkIn[ii],P.chunkInSizeBytesArray);
+       readInStream[ii] = new istream(&readInStreamBuf[ii]);
        RA->readInStream[ii]=readInStream[ii];
     };
 
@@ -32,8 +33,8 @@ ReadAlignChunk::ReadAlignChunk(Parameters& Pin, Genome &genomeIn, Transcriptome
     if (P.outSAMbool) {
         chunkOutBAM=new char [P.chunkOutBAMsizeBytes];
         RA->outBAMarray=chunkOutBAM;
-        chunkOutBAMstream=new ostringstream;
-        chunkOutBAMstream->rdbuf()->pubsetbuf(chunkOutBAM,P.chunkOutBAMsizeBytes);
+        chunkOutBAMstreamBuf.setWriteBuffer(chunkOutBAM,P.chunkOutBAMsizeBytes);
+        chunkOutBAMstream=new ostream(&chunkOutBAMstreamBuf);
         RA->outSAMstream=chunkOutBAMstream;
         RA->outSAMstream->seekp(0,ios::beg);
         chunkOutBAMtotal=0;
diff --git a/source/ReadAlignChunk.h b/source/ReadAlignChunk.h
index 50bb3d8e..8efff332 100644
--- a/source/ReadAlignChunk.h
+++ b/source/ReadAlignChunk.h
@@ -8,6 +8,7 @@
 #include "Transcriptome.h"
 #include "BAMoutput.h"
 #include "Quantifications.h"
+#include "ExtBufStreambuf.h"
 
 class ReadAlignChunk {//chunk of reads and alignments
 public:
@@ -25,8 +26,10 @@ class ReadAlignChunk {//chunk of reads and alignments
     BAMoutput *chunkOutBAMcoord, *chunkOutBAMunsorted, *chunkOutBAMquant;
     Quantifications *chunkQuants;
     
-    istringstream** readInStream;
-    ostringstream*  chunkOutBAMstream;
+    istream** readInStream;
+    ExtBufStreambuf* readInStreamBuf; //external-buffer streambufs backing readInStream (one per mate)
+    ostream*  chunkOutBAMstream;
+    ExtBufStreambuf chunkOutBAMstreamBuf; //external-buffer streambuf backing chunkOutBAMstream
     ofstream chunkOutBAMfile;
     string chunkOutBAMfileName;