diff --git a/config b/config
index 029d463..7d66a2d 100644
--- a/config
+++ b/config
@@ -32,4 +32,5 @@ source "scripts/paths"
 
 # Project-specific variables below.
 [ -z "${CC}" ] && CC="gcc"
+[ -z "${CXX}" ] && CXX="g++"
 [ -z "${CFLAGS}" ] && CFLAGS="-g -O2 -W -Wall"
diff --git a/src/libfir++/000_TopHeader.h b/src/libfir++/000_TopHeader.h
index ee124af..5291fec 100644
--- a/src/libfir++/000_TopHeader.h
+++ b/src/libfir++/000_TopHeader.h
@@ -9,6 +9,16 @@
 #define HEADER_libfirpp
 
 // standard includes, or includes needed for type declarations
+#include <vector>
+
+/*! \defgroup libfirpp C++ library */
+
+/*! \brief All %FIR library functions and classes.
+
+\ingroup libfirpp
+
+*/
+namespace FIR {
 
 /* options for text editors
 vim: expandtab:ts=4:sw=4:syntax=cpp.doxygen
diff --git a/src/libfir++/000_TopSource.cc b/src/libfir++/000_TopSource.cc
new file mode 100644
index 0000000..ca7c76e
--- /dev/null
+++ b/src/libfir++/000_TopSource.cc
@@ -0,0 +1,23 @@
+/* libfir/src/libfir++/000_TopSource.cc
+ *
+ *  Copyright: ©2014, Laurence Withers.
+ *  Author: Laurence Withers <l@lwithers.me.uk>
+ *  License: GPLv3
+ */
+
+// "fir.h" includes <complex.h>, which uses a preprocessor define on the
+// symbol complex. We need to include it first so that it gets its required
+// define, and then clear it up before including anything that pulls in the
+// C++ header <complex>.
+#include "fir.h"
+#undef complex
+
+#include "fir++.h"
+
+// Below are all the includes used throughout the library.
+
+namespace FIR {
+
+/* options for text editors
+vim: expandtab:ts=4:sw=4
+*/
diff --git a/src/libfir++/100_fir.h b/src/libfir++/100_fir.h
new file mode 100644
index 0000000..8189b5d
--- /dev/null
+++ b/src/libfir++/100_fir.h
@@ -0,0 +1,47 @@
+/* libfir/src/libfir++/100_fir.h
+ * 
+ *  Copyright: ©2014, Laurence Withers.
+ *  Author: Laurence Withers <l@lwithers.me.uk>
+ *  License: GPLv3
+ */
+
+
+
+/*! \defgroup libfirpp_fir Basic FIR filtering
+
+\ingroup libfirpp
+
+\todo need our own symmetry enum
+\todo forward declares
+\todo coefficient query
+
+*/
+
+
+
+/*! \brief Basic FIR filter object.
+
+\ingroup libfirpp_fir
+
+*/
+class Filter {
+public:
+    Filter(int ncoeff, double* coeff,
+        enum fir_symmetry symm = fir_symmetry_none);
+    Filter(const Filter& other);
+    Filter(const std::vector<double>& coeff,
+        enum fir_symmetry symm = fir_symmetry_none);
+    virtual ~Filter();
+
+private:
+    struct fir_filter_t* fi_;
+
+    // disallow assignment
+    Filter& operator=(const Filter& other);
+};
+
+
+
+/* options for text editors
+vim: expandtab:ts=4:sw=4:syntax=cpp.doxygen
+*/
diff --git a/src/libfir++/999_BottomHeader.h b/src/libfir++/999_BottomHeader.h
index c1e8dcc..28d9774 100644
--- a/src/libfir++/999_BottomHeader.h
+++ b/src/libfir++/999_BottomHeader.h
@@ -5,6 +5,8 @@
  *  License: GPLv3
  */
 
+} // end namespace FIR
+
 #endif
 
 /* options for text editors
diff --git a/src/libfir++/000_TopSource.cpp b/src/libfir++/999_BottomSource.cc
similarity index 59%
rename from src/libfir++/000_TopSource.cpp
rename to src/libfir++/999_BottomSource.cc
index dfa8aba..6632108 100644
--- a/src/libfir++/000_TopSource.cpp
+++ b/src/libfir++/999_BottomSource.cc
@@ -1,13 +1,11 @@
-/* libfir/src/libfir++/TopSource.cpp
+/* libfir/src/libfir++/999_BottomSource.cc
  *
  *  Copyright: ©2014, Laurence Withers.
  *  Author: Laurence Withers <l@lwithers.me.uk>
  *  License: GPLv3
  */
 
-#include "fir++.h"
-
-// Below are all the includes used throughout the library.
+} // end namespace FIR
 
 /* options for text editors
 vim: expandtab:ts=4:sw=4
diff --git a/src/libfir++/build.lib b/src/libfir++/build.lib
index 73d8739..7269bb7 100644
--- a/src/libfir++/build.lib
+++ b/src/libfir++/build.lib
@@ -6,6 +6,8 @@
 #  libfirpp_DEP_CFLAGS
 #  libfirpp_DEP_LIBS
 
+build_target libfir
+
 if [ -z ${libfirpp_BUILT} ]
 then
     libfirpp_BASE=libfir++
@@ -15,7 +17,7 @@ then
     libfirpp_DEP_CFLAGS="" # @TODO@ cflags
     libfirpp_DEP_LIBS="" # @TODO@ libs
     SO_EXTRA="${libfirpp_DEP_CFLAGS} ${libfirpp_DEP_LIBS} -lstdc++ -lc \
-        -D_GNU_SOURCE"
+        ${libfir} -D_GNU_SOURCE"
 
     echo "Building library ${libfirpp}..."
 
diff --git a/src/libfir++/build.monolithic b/src/libfir++/build.monolithic
index c9a9bac..5feaedf 100644
--- a/src/libfir++/build.monolithic
+++ b/src/libfir++/build.monolithic
@@ -11,7 +11,7 @@ then
     MONOLITHIC_SOURCE="$(find src/libfir++/ -name '*.h' | sort)"
     make_monolithic ${HDR} Ch || return 1
 
-    MONOLITHIC_SOURCE="$(find src/libfir++/ -name '*.cpp' | sort)"
+    MONOLITHIC_SOURCE="$(find src/libfir++/ -name '*.cc' | sort)"
     make_monolithic ${SRC} C || return 1
 
     libfirpp_MONOLITHIC=1
diff --git a/src/libfir/000_TopHeader.h b/src/libfir/000_TopHeader.h
index c7ef957..b0ad494 100644
--- a/src/libfir/000_TopHeader.h
+++ b/src/libfir/000_TopHeader.h
@@ -12,6 +12,8 @@
 extern "C" {
 #endif
 
+/*! \defgroup libfir C library */
+
 /* standard includes, or includes needed for type declarations */
 
 /* options for text editors
diff --git a/src/libfir/000_TopSource.c b/src/libfir/000_TopSource.c
index 2f6be6c..a1fc33c 100644
--- a/src/libfir/000_TopSource.c
+++ b/src/libfir/000_TopSource.c
@@ -9,6 +9,7 @@
 
 /* Below are all the includes used throughout the library. */
 #include <alloca.h>
+#include <errno.h>
 #include <math.h>
 #include <stdlib.h>
 #include <string.h>
diff --git a/src/libfir/100_fir.c b/src/libfir/100_fir.c
index 7ebab7c..c407761 100644
--- a/src/libfir/100_fir.c
+++ b/src/libfir/100_fir.c
@@ -48,8 +48,14 @@ symm_setup(struct fir_filter_t* fi, int delay)
 struct fir_filter_t*
 fir_filter_new(const double* coeff, int ncoeff, enum fir_symmetry symm)
 {
-    struct fir_filter_t* fi = malloc(sizeof(*fi));
+    struct fir_filter_t* fi;
 
+    if(ncoeff < 1) {
+        errno = EINVAL;
+        return 0;
+    }
+
+    fi = malloc(sizeof(*fi));
     fi->coeff = malloc(sizeof(double) * ncoeff);
     memcpy(fi->coeff, coeff, sizeof(double) * ncoeff);
     fi->ncoeff = ncoeff;
diff --git a/src/libfir/100_fir.h b/src/libfir/100_fir.h
index b3f28e8..f9513da 100644
--- a/src/libfir/100_fir.h
+++ b/src/libfir/100_fir.h
@@ -7,29 +7,112 @@
 
 
 
+/*! \defgroup libfir_fir Basic FIR filtering
+
+\ingroup libfir
+
+Routines in this section provide the basic building blocks for %FIR filter use:
+allocation/build from coefficients, cloning, and the filter operation itself.
+
+*/
+/*!@{*/
+
+
+
+/* opaque structure */
 struct fir_filter_t;
 
 
 
+/*! \brief %FIR filter symmetry.
+
+The library extensively takes advantage of the symmetry that is often present
+in %FIR filter coefficient sets.
+
+*/
 enum fir_symmetry {
+    /*! \brief No symmetry. */
     fir_symmetry_none,
+
+    /*! \brief Odd symmetry (odd total number of points). */
     fir_symmetry_odd,
+
+    /*! \brief Even symmetry (even total number of points). */
     fir_symmetry_even,
 };
 
 
 
+/*! \brief Allocate %FIR filter instance.
+
+\param coeff Pointer to array of coefficients.
+\param ncoeff Number of coefficients in \a coeff (must be ≥ 1).
+\param symm Whether there is any symmetry.
+\returns Newly-allocated filter object.
+\retval 0 on error (and see \a errno).
+
+Allocates a new %FIR filter based on the given set of coefficients \a coeff.
+If the coefficient set is known to have odd symmetry of \a N points, then
+pass \a ncoeff as \a (N+1)/2 (and this is the number of entries in \a coeff
+that are read). If the coefficient set is known to have even symmetry of \a N
+points, then pass \a ncoeff as \a N/2.
+
+The contents of \a coeff are copied so it is not necessary to keep the original
+array in order to run the filter.
+
+*/
 struct fir_filter_t* fir_filter_new(const double* coeff, int ncoeff,
     enum fir_symmetry symm);
 
+
+
+/*! \brief Free %FIR filter instance.
+
+\param fi Filter object (may be 0).
+\returns Newly-allocated filter object.
+
+Frees all memory associated with the filter object \a fi. Does nothing if
+passed a null pointer.
+
+*/
 void fir_filter_free(struct fir_filter_t* fi);
 
+
+
+/*! \brief Clone %FIR filter instance.
+
+\param fi Filter object to clone.
+
+Clones the %FIR filter object \a fi, returning a new %FIR filter object that
+is ready to use. The new copy will have its own copy of the required
+coefficients and does not hold any reference to the original.
+
+Note that filter state (i.e. previous samples) is not copied; the new filter
+will be fresh.
+
+*/
 struct fir_filter_t* fir_filter_clone(const struct fir_filter_t* fi);
 
+
+
+/*! \brief Run %FIR filter.
+
+\param fi Filter object.
+\param x Input sample.
+\returns Output sample.
+
+This function runs the filter for the given input sample \a x, returning an
+output sample \a y. If this is the very first sample after creation (either via
+\ref fir_filter_new() or \ref fir_filter_clone()), then this sample will be
+considered as the steady-state value and the previous-sample values stored in
+\a fi will be initialised to \a x.
+
+*/
 double fir_filter(struct fir_filter_t* fi, double x);
 
 
 
+/*!@}*/
 /* options for text editors
-vim: expandtab:ts=4:sw=4
+vim: expandtab:ts=4:sw=4:syntax=ch.doxygen
 */
diff --git a/src/libfir/200_common_filters.c b/src/libfir/200_common_filters.c
index 9916aba..de9546d 100644
--- a/src/libfir/200_common_filters.c
+++ b/src/libfir/200_common_filters.c
@@ -8,11 +8,16 @@
 
 
 struct fir_filter_t*
-fir_sinc_lowpass(int npoints, double corner)
+fir_sinc_lowpass(int npoints, double corner, double gain)
 {
-    double* coeff, t;
+    double* coeff, t, sum = 0;
     int i, spoints;
 
+    if(npoints < 1) {
+        errno = EINVAL;
+        return 0;
+    }
+
     /* find number of points after taking symmetry into account */
     spoints = (npoints + 1) / 2;
     coeff = alloca(sizeof(double) * spoints);
@@ -21,13 +26,21 @@ fir_sinc_lowpass(int npoints, double corner)
     for(i = 0; i < spoints; ++i) {
         if((i * 2 + 1) == npoints) {
             coeff[i] = 1;
+            sum += 1; /* not 2, as we don't repeat this coeff */
         } else {
             t = i + 0.5 - npoints * 0.5;
             t *= corner * 2;
             coeff[i] = sin(t) / t;
+            sum += coeff[i] * 2;
         }
     }
 
+    /* normalise gain */
+    gain /= sum;
+    for(i = 0; i < spoints; ++i) {
+        coeff[i] *= gain;
+    }
+
     return fir_filter_new(coeff, spoints,
         (npoints & 1) ? fir_symmetry_odd : fir_symmetry_even);
 }
diff --git a/src/libfir/200_common_filters.h b/src/libfir/200_common_filters.h
index cfd996e..3adfc2c 100644
--- a/src/libfir/200_common_filters.h
+++ b/src/libfir/200_common_filters.h
@@ -7,10 +7,41 @@
 
 
 
-struct fir_filter_t* fir_sinc_lowpass(int npoints, double corner);
+/*! \defgroup libfir_common Common filter types
 
+\ingroup libfir
 
+Routines in this section instantiate %FIR filters using algorithms to build
+commonly-used types of filter, such as brickwall low-pass etc.
 
-/* options for text editors
-vim: expandtab:ts=4:sw=4
+*/
+/*!@{*/
+
+
+
+/*! \brief Brickwall low-pass linear phase: sinc filter.
+
+\param npoints Number of points required (≥1).
+\param corner Corner frequency as fraction of sample rate.
+\param gain Gain of filter.
+\returns Newly-allocated %FIR filter.
+
+Builds a truncated-sinc filter which has a low-pass, brickwall effect (cutoff
+at \a corner, which is a fraction of sampling rate, i.e. 0 &lt; \a corner &lt;
+0.5).
+
+The number of points \a npoints is a trade-off between accuracy of filter (an
+ideal sinc filter would have an infinite number of points) and computation time.
+\a npoints may be odd or even; symmetry will be exploited in either case.
+
+The DC gain of the filter is normalised to \a gain, which is often 1.0.
+
+*/
+struct fir_filter_t* fir_sinc_lowpass(int npoints, double corner, double gain);
+
+
+
+/*!@}*/
+/* options for text editors
+vim: expandtab:ts=4:sw=4:syntax=ch.doxygen
 */