diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index cb214bee3..e5cfe94cb 100644
--- a/src/libstore/daemon.cc
+++ b/src/libstore/daemon.cc
@@ -165,7 +165,7 @@ struct TunnelSource : BufferedSource
     Source & from;
     BufferedSink & to;
     TunnelSource(Source & from, BufferedSink & to) : from(from), to(to) { }
-    size_t readUnbuffered(unsigned char * data, size_t len) override
+    size_t readUnbuffered(char * data, size_t len) override
     {
         to << STDERR_READ << len;
         to.flush();
diff --git a/src/libstore/local-store.cc b/src/libstore/local-store.cc
index 93d073768..348e5d0d4 100644
--- a/src/libstore/local-store.cc
+++ b/src/libstore/local-store.cc
@@ -1143,7 +1143,7 @@ StorePath LocalStore::addToStoreFromDump(Source & source0, const string & name,
         dump.resize(oldSize + want);
         auto got = 0;
         try {
-            got = source.read((uint8_t *) dump.data() + oldSize, want);
+            got = source.read(dump.data() + oldSize, want);
         } catch (EndOfFile &) {
             inMemory = true;
             break;
diff --git a/src/libstore/nar-accessor.cc b/src/libstore/nar-accessor.cc
index e35031f7a..1427a0f98 100644
--- a/src/libstore/nar-accessor.cc
+++ b/src/libstore/nar-accessor.cc
@@ -96,7 +96,7 @@ struct NarAccessor : public FSAccessor
                 NarMember{FSAccessor::Type::tSymlink, false, 0, 0, target});
         }
 
-        size_t read(unsigned char * data, size_t len) override
+        size_t read(char * data, size_t len) override
         {
             auto n = source.read(data, len);
             pos += n;
diff --git a/src/libstore/remote-fs-accessor.cc b/src/libstore/remote-fs-accessor.cc
index 2d02a181b..63bde92de 100644
--- a/src/libstore/remote-fs-accessor.cc
+++ b/src/libstore/remote-fs-accessor.cc
@@ -75,7 +75,7 @@ std::pair<ref<FSAccessor>, Path> RemoteFSAccessor::fetch(const Path & path_)
                         throw SysError("seeking in '%s'", cacheFile);
 
                     std::string buf(length, 0);
-                    readFull(fd.get(), (unsigned char *) buf.data(), length);
+                    readFull(fd.get(), buf.data(), length);
 
                     return buf;
                 });
diff --git a/src/libstore/remote-store.cc b/src/libstore/remote-store.cc
index 6e0c85237..be29f8e6f 100644
--- a/src/libstore/remote-store.cc
+++ b/src/libstore/remote-store.cc
@@ -856,7 +856,7 @@ std::exception_ptr RemoteStore::Connection::processStderr(Sink * sink, Source *
         else if (msg == STDERR_READ) {
             if (!source) throw Error("no source");
             size_t len = readNum<size_t>(from);
-            auto buf = std::make_unique<unsigned char[]>(len);
+            auto buf = std::make_unique<char[]>(len);
             writeString({(const char *) buf.get(), source->read(buf.get(), len)}, to);
             to.flush();
         }
diff --git a/src/libutil/archive.cc b/src/libutil/archive.cc
index 046ef841b..ed0eb2fb5 100644
--- a/src/libutil/archive.cc
+++ b/src/libutil/archive.cc
@@ -50,14 +50,14 @@ static void dumpContents(const Path & path, size_t size,
     AutoCloseFD fd = open(path.c_str(), O_RDONLY | O_CLOEXEC);
     if (!fd) throw SysError("opening file '%1%'", path);
 
-    std::vector<unsigned char> buf(65536);
+    std::vector<char> buf(65536);
     size_t left = size;
 
     while (left > 0) {
         auto n = std::min(left, buf.size());
         readFull(fd.get(), buf.data(), n);
         left -= n;
-        sink({(char *) buf.data(), n});
+        sink({buf.data(), n});
     }
 
     writePadding(size, sink);
@@ -155,14 +155,14 @@ static void parseContents(ParseSink & sink, Source & source, const Path & path)
     sink.preallocateContents(size);
 
     uint64_t left = size;
-    std::vector<unsigned char> buf(65536);
+    std::vector<char> buf(65536);
 
     while (left) {
         checkInterrupt();
         auto n = buf.size();
         if ((uint64_t)n > left) n = left;
         source(buf.data(), n);
-        sink.receiveContents({(char *) buf.data(), n});
+        sink.receiveContents({buf.data(), n});
         left -= n;
     }
 
diff --git a/src/libutil/serialise.cc b/src/libutil/serialise.cc
index 534ad5cbb..87c1099a1 100644
--- a/src/libutil/serialise.cc
+++ b/src/libutil/serialise.cc
@@ -84,7 +84,7 @@ bool FdSink::good()
 }
 
 
-void Source::operator () (unsigned char * data, size_t len)
+void Source::operator () (char * data, size_t len)
 {
     while (len) {
         size_t n = read(data, len);
@@ -96,12 +96,12 @@ void Source::operator () (unsigned char * data, size_t len)
 void Source::drainInto(Sink & sink)
 {
     std::string s;
-    std::vector<unsigned char> buf(8192);
+    std::vector<char> buf(8192);
     while (true) {
         size_t n;
         try {
             n = read(buf.data(), buf.size());
-            sink({(char *) buf.data(), n});
+            sink({buf.data(), n});
         } catch (EndOfFile &) {
             break;
         }
@@ -117,9 +117,9 @@ std::string Source::drain()
 }
 
 
-size_t BufferedSource::read(unsigned char * data, size_t len)
+size_t BufferedSource::read(char * data, size_t len)
 {
-    if (!buffer) buffer = decltype(buffer)(new unsigned char[bufSize]);
+    if (!buffer) buffer = decltype(buffer)(new char[bufSize]);
 
     if (!bufPosIn) bufPosIn = readUnbuffered(buffer.get(), bufSize);
 
@@ -138,12 +138,12 @@ bool BufferedSource::hasData()
 }
 
 
-size_t FdSource::readUnbuffered(unsigned char * data, size_t len)
+size_t FdSource::readUnbuffered(char * data, size_t len)
 {
     ssize_t n;
     do {
         checkInterrupt();
-        n = ::read(fd, (char *) data, len);
+        n = ::read(fd, data, len);
     } while (n == -1 && errno == EINTR);
     if (n == -1) { _good = false; throw SysError("reading from file"); }
     if (n == 0) { _good = false; throw EndOfFile("unexpected end-of-file"); }
@@ -158,10 +158,10 @@ bool FdSource::good()
 }
 
 
-size_t StringSource::read(unsigned char * data, size_t len)
+size_t StringSource::read(char * data, size_t len)
 {
     if (pos == s.size()) throw EndOfFile("end of string reached");
-    size_t n = s.copy((char *) data, len, pos);
+    size_t n = s.copy(data, len, pos);
     pos += n;
     return n;
 }
@@ -225,7 +225,7 @@ std::unique_ptr<Source> sinkToSource(
         std::string cur;
         size_t pos = 0;
 
-        size_t read(unsigned char * data, size_t len) override
+        size_t read(char * data, size_t len) override
         {
             if (!coro)
                 coro = coro_t::pull_type(VirtualStackAllocator{}, [&](coro_t::push_type & yield) {
@@ -244,7 +244,7 @@ std::unique_ptr<Source> sinkToSource(
             }
 
             auto n = std::min(cur.size() - pos, len);
-            memcpy(data, (unsigned char *) cur.data() + pos, n);
+            memcpy(data, cur.data() + pos, n);
             pos += n;
 
             return n;
@@ -258,9 +258,9 @@ std::unique_ptr<Source> sinkToSource(
 void writePadding(size_t len, Sink & sink)
 {
     if (len % 8) {
-        unsigned char zero[8];
+        char zero[8];
         memset(zero, 0, sizeof(zero));
-        sink({(char *) zero, 8 - (len % 8)});
+        sink({zero, 8 - (len % 8)});
     }
 }
 
@@ -321,7 +321,7 @@ Sink & operator << (Sink & sink, const Error & ex)
 void readPadding(size_t len, Source & source)
 {
     if (len % 8) {
-        unsigned char zero[8];
+        char zero[8];
         size_t n = 8 - (len % 8);
         source(zero, n);
         for (unsigned int i = 0; i < n; i++)
@@ -330,7 +330,7 @@ void readPadding(size_t len, Source & source)
 }
 
 
-size_t readString(unsigned char * buf, size_t max, Source & source)
+size_t readString(char * buf, size_t max, Source & source)
 {
     auto len = readNum<size_t>(source);
     if (len > max) throw SerialisationError("string is too long");
@@ -345,7 +345,7 @@ string readString(Source & source, size_t max)
     auto len = readNum<size_t>(source);
     if (len > max) throw SerialisationError("string is too long");
     std::string res(len, 0);
-    source((unsigned char*) res.data(), len);
+    source(res.data(), len);
     readPadding(len, source);
     return res;
 }
@@ -404,7 +404,7 @@ void StringSink::operator () (std::string_view data)
     s->append(data);
 }
 
-size_t ChainSource::read(unsigned char * data, size_t len)
+size_t ChainSource::read(char * data, size_t len)
 {
     if (useSecond) {
         return source2.read(data, len);
diff --git a/src/libutil/serialise.hh b/src/libutil/serialise.hh
index f9ab79997..5bbbc7ce3 100644
--- a/src/libutil/serialise.hh
+++ b/src/libutil/serialise.hh
@@ -51,12 +51,12 @@ struct Source
     /* Store exactly ‘len’ bytes in the buffer pointed to by ‘data’.
        It blocks until all the requested data is available, or throws
        an error if it is not going to be available.   */
-    void operator () (unsigned char * data, size_t len);
+    void operator () (char * data, size_t len);
 
     /* Store up to ‘len’ in the buffer pointed to by ‘data’, and
        return the number of bytes stored.  It blocks until at least
        one byte is available. */
-    virtual size_t read(unsigned char * data, size_t len) = 0;
+    virtual size_t read(char * data, size_t len) = 0;
 
     virtual bool good() { return true; }
 
@@ -71,18 +71,18 @@ struct Source
 struct BufferedSource : Source
 {
     size_t bufSize, bufPosIn, bufPosOut;
-    std::unique_ptr<unsigned char[]> buffer;
+    std::unique_ptr<char[]> buffer;
 
     BufferedSource(size_t bufSize = 32 * 1024)
         : bufSize(bufSize), bufPosIn(0), bufPosOut(0), buffer(nullptr) { }
 
-    size_t read(unsigned char * data, size_t len) override;
+    size_t read(char * data, size_t len) override;
 
     bool hasData();
 
 protected:
     /* Underlying read call, to be overridden. */
-    virtual size_t readUnbuffered(unsigned char * data, size_t len) = 0;
+    virtual size_t readUnbuffered(char * data, size_t len) = 0;
 };
 
 
@@ -138,7 +138,7 @@ struct FdSource : BufferedSource
 
     bool good() override;
 protected:
-    size_t readUnbuffered(unsigned char * data, size_t len) override;
+    size_t readUnbuffered(char * data, size_t len) override;
 private:
     bool _good = true;
 };
@@ -163,7 +163,7 @@ struct StringSource : Source
     const string & s;
     size_t pos;
     StringSource(const string & _s) : s(_s), pos(0) { }
-    size_t read(unsigned char * data, size_t len) override;
+    size_t read(char * data, size_t len) override;
 };
 
 
@@ -187,10 +187,10 @@ struct TeeSource : Source
     Sink & sink;
     TeeSource(Source & orig, Sink & sink)
         : orig(orig), sink(sink) { }
-    size_t read(unsigned char * data, size_t len)
+    size_t read(char * data, size_t len)
     {
         size_t n = orig.read(data, len);
-        sink({(char *) data, n});
+        sink({data, n});
         return n;
     }
 };
@@ -202,7 +202,7 @@ struct SizedSource : Source
     size_t remain;
     SizedSource(Source & orig, size_t size)
         : orig(orig), remain(size) { }
-    size_t read(unsigned char * data, size_t len)
+    size_t read(char * data, size_t len)
     {
         if (this->remain <= 0) {
             throw EndOfFile("sized: unexpected end-of-file");
@@ -216,7 +216,7 @@ struct SizedSource : Source
     /* Consume the original source until no remain data is left to consume. */
     size_t drainAll()
     {
-        std::vector<unsigned char> buf(8192);
+        std::vector<char> buf(8192);
         size_t sum = 0;
         while (this->remain > 0) {
             size_t n = read(buf.data(), buf.size());
@@ -256,13 +256,13 @@ struct LambdaSink : Sink
 /* Convert a function into a source. */
 struct LambdaSource : Source
 {
-    typedef std::function<size_t(unsigned char *, size_t)> lambda_t;
+    typedef std::function<size_t(char *, size_t)> lambda_t;
 
     lambda_t lambda;
 
     LambdaSource(const lambda_t & lambda) : lambda(lambda) { }
 
-    size_t read(unsigned char * data, size_t len) override
+    size_t read(char * data, size_t len) override
     {
         return lambda(data, len);
     }
@@ -278,7 +278,7 @@ struct ChainSource : Source
         : source1(s1), source2(s2)
     { }
 
-    size_t read(unsigned char * data, size_t len) override;
+    size_t read(char * data, size_t len) override;
 };
 
 
@@ -322,7 +322,7 @@ template<typename T>
 T readNum(Source & source)
 {
     unsigned char buf[8];
-    source(buf, sizeof(buf));
+    source((char *) buf, sizeof(buf));
 
     uint64_t n =
         ((uint64_t) buf[0]) |
@@ -354,7 +354,7 @@ inline uint64_t readLongLong(Source & source)
 
 
 void readPadding(size_t len, Source & source);
-size_t readString(unsigned char * buf, size_t max, Source & source);
+size_t readString(char * buf, size_t max, Source & source);
 string readString(Source & source, size_t max = std::numeric_limits<size_t>::max());
 template<class T> T readStrings(Source & source);
 
@@ -386,9 +386,9 @@ struct StreamToSourceAdapter : Source
         : istream(istream)
     { }
 
-    size_t read(unsigned char * data, size_t len) override
+    size_t read(char * data, size_t len) override
     {
-        if (!istream->read((char *) data, len)) {
+        if (!istream->read(data, len)) {
             if (istream->eof()) {
                 if (istream->gcount() == 0)
                     throw EndOfFile("end of file");
@@ -411,7 +411,7 @@ struct FramedSource : Source
 {
     Source & from;
     bool eof = false;
-    std::vector<unsigned char> pending;
+    std::vector<char> pending;
     size_t pos = 0;
 
     FramedSource(Source & from) : from(from)
@@ -423,13 +423,13 @@ struct FramedSource : Source
             while (true) {
                 auto n = readInt(from);
                 if (!n) break;
-                std::vector<unsigned char> data(n);
+                std::vector<char> data(n);
                 from(data.data(), n);
             }
         }
     }
 
-    size_t read(unsigned char * data, size_t len) override
+    size_t read(char * data, size_t len) override
     {
         if (eof) throw EndOfFile("reached end of FramedSource");
 
@@ -439,7 +439,7 @@ struct FramedSource : Source
                 eof = true;
                 return 0;
             }
-            pending = std::vector<unsigned char>(len);
+            pending = std::vector<char>(len);
             pos = 0;
             from(pending.data(), len);
         }
diff --git a/src/libutil/tarfile.cc b/src/libutil/tarfile.cc
index c4d8a4f91..2da169ba7 100644
--- a/src/libutil/tarfile.cc
+++ b/src/libutil/tarfile.cc
@@ -66,7 +66,7 @@ private:
         *buffer = self->buffer.data();
 
         try {
-            return self->source->read(self->buffer.data(), 4096);
+            return self->source->read((char *) self->buffer.data(), 4096);
         } catch (EndOfFile &) {
             return 0;
         } catch (std::exception & err) {
diff --git a/src/libutil/util.cc b/src/libutil/util.cc
index 3d4fc4c25..e6b6d287d 100644
--- a/src/libutil/util.cc
+++ b/src/libutil/util.cc
@@ -340,13 +340,13 @@ void writeFile(const Path & path, Source & source, mode_t mode)
     if (!fd)
         throw SysError("opening file '%1%'", path);
 
-    std::vector<unsigned char> buf(64 * 1024);
+    std::vector<char> buf(64 * 1024);
 
     try {
         while (true) {
             try {
                 auto n = source.read(buf.data(), buf.size());
-                writeFull(fd.get(), {(char *) buf.data(), n});
+                writeFull(fd.get(), {buf.data(), n});
             } catch (EndOfFile &) { break; }
         }
     } catch (Error & e) {
@@ -632,11 +632,11 @@ void replaceSymlink(const Path & target, const Path & link,
 }
 
 
-void readFull(int fd, unsigned char * buf, size_t count)
+void readFull(int fd, char * buf, size_t count)
 {
     while (count) {
         checkInterrupt();
-        ssize_t res = read(fd, (char *) buf, count);
+        ssize_t res = read(fd, buf, count);
         if (res == -1) {
             if (errno == EINTR) continue;
             throw SysError("reading from file");
@@ -1137,7 +1137,7 @@ void runProgram2(const RunOptions & options)
         in.readSide = -1;
         writerThread = std::thread([&]() {
             try {
-                std::vector<unsigned char> buf(8 * 1024);
+                std::vector<char> buf(8 * 1024);
                 while (true) {
                     size_t n;
                     try {
@@ -1145,7 +1145,7 @@ void runProgram2(const RunOptions & options)
                     } catch (EndOfFile &) {
                         break;
                     }
-                    writeFull(in.writeSide.get(), {(char *) buf.data(), n});
+                    writeFull(in.writeSide.get(), {buf.data(), n});
                 }
                 promise.set_value();
             } catch (...) {
diff --git a/src/libutil/util.hh b/src/libutil/util.hh
index fcb7cad39..0f82bed78 100644
--- a/src/libutil/util.hh
+++ b/src/libutil/util.hh
@@ -155,7 +155,7 @@ void replaceSymlink(const Path & target, const Path & link,
 
 /* Wrappers arount read()/write() that read/write exactly the
    requested number of bytes. */
-void readFull(int fd, unsigned char * buf, size_t count);
+void readFull(int fd, char * buf, size_t count);
 void writeFull(int fd, std::string_view s, bool allowInterrupts = true);
 
 MakeError(EndOfFile, Error);