diff --git a/doc/langref.html.in b/doc/langref.html.in
index 465f3c56a7..495b19580c 100644
--- a/doc/langref.html.in
+++ b/doc/langref.html.in
@@ -7057,6 +7057,61 @@ const c = @cImport({
 });
       {#code_end#}
       {#see_also|@cImport|@cInclude|@cDefine|@cUndef|@import#}
+      {#header_close#}
+      {#header_open|Exporting a C Library#}
+      <p>
+      One of the primary use cases for Zig is exporting a library with the C ABI for other programming languages
+      to call into. The <code>export</code> keyword in front of functions, variables, and types causes them to
+      be part of the library API:
+      </p>
+      <p class="file">mathtest.zig</p>
+      {#code_begin|syntax#}
+export fn add(a: i32, b: i32) i32 {
+    return a + b;
+}
+      {#code_end#}
+      <p>To make a shared library:</p>
+      <pre><code class="shell">$ zig build-lib mathtest.zig
+</code></pre>
+      <p>To make a static library:</p>
+      <pre><code class="shell">$ zig build-lib mathtest.zig --static
+</code></pre>
+      <p>Here is an example with the {#link|Zig Build System#}:</p>
+      <p class="file">test.c</p>
+      <pre><code class="cpp">// This header is generated by zig from mathtest.zig
+#include "mathtest.h"
+#include &lt;assert.h&gt;
+
+int main(int argc, char **argv) {
+    assert(add(42, 1337) == 1379);
+    return 0;
+}</code></pre>
+      <p class="file">build.zig</p>
+      {#code_begin|syntax#}
+const Builder = @import("std").build.Builder;
+
+pub fn build(b: *Builder) void {
+    const lib = b.addSharedLibrary("mathtest", "mathtest.zig", b.version(1, 0, 0));
+
+    const exe = b.addCExecutable("test");
+    exe.addCompileFlags([][]const u8{"-std=c99"});
+    exe.addSourceFile("test.c");
+    exe.linkLibrary(lib);
+
+    b.default_step.dependOn(&exe.step);
+
+    const run_cmd = b.addCommand(".", b.env_map, [][]const u8{exe.getOutputPath()});
+    run_cmd.step.dependOn(&exe.step);
+
+    const test_step = b.step("test", "Test the program");
+    test_step.dependOn(&run_cmd.step);
+}
+      {#code_end#}
+      <p class="file">terminal</p>
+      <pre><code class="shell">$ zig build
+$ ./test
+$ echo $?
+0</code></pre>
       {#header_close#}
       {#header_open|Mixing Object Files#}
       <p>
diff --git a/src/ir.cpp b/src/ir.cpp
index 710456179c..3117410cd2 100644
--- a/src/ir.cpp
+++ b/src/ir.cpp
@@ -60,7 +60,7 @@ enum ConstCastResultId {
     ConstCastResultIdType,
     ConstCastResultIdUnresolvedInferredErrSet,
     ConstCastResultIdAsyncAllocatorType,
-    ConstCastResultIdNullWrapPtr,
+    ConstCastResultIdNullWrapPtr
 };
 
 struct ConstCastOnly;
@@ -8471,9 +8471,9 @@ static ConstCastOnly types_match_const_cast_only(IrAnalyze *ira, TypeTableEntry
     if (wanted_type == actual_type)
         return result;
 
-    // * and [*] can do a const-cast-only to ?* and ?[*], respectively
-    // but not if there is a mutable parent pointer
-    // and not if the pointer is zero bits
+    // *T and [*]T may const-cast-only to ?*U and ?[*]U, respectively
+    // but not if we want a mutable pointer
+    // and not if the actual pointer has zero bits
     if (!wanted_is_mutable && wanted_type->id == TypeTableEntryIdOptional &&
         wanted_type->data.maybe.child_type->id == TypeTableEntryIdPointer &&
         actual_type->id == TypeTableEntryIdPointer && type_has_bits(actual_type))
@@ -8488,6 +8488,18 @@ static ConstCastOnly types_match_const_cast_only(IrAnalyze *ira, TypeTableEntry
         return result;
     }
 
+    // *T and [*]T can always cast to *c_void
+    if (wanted_type->id == TypeTableEntryIdPointer &&
+        wanted_type->data.pointer.ptr_len == PtrLenSingle &&
+        wanted_type->data.pointer.child_type == g->builtin_types.entry_c_void &&
+        actual_type->id == TypeTableEntryIdPointer &&
+        (!actual_type->data.pointer.is_const || wanted_type->data.pointer.is_const) &&
+        (!actual_type->data.pointer.is_volatile || wanted_type->data.pointer.is_volatile))
+    {
+        assert(actual_type->data.pointer.alignment >= wanted_type->data.pointer.alignment);
+        return result;
+    }
+
     // pointer const
     if (wanted_type->id == TypeTableEntryIdPointer && actual_type->id == TypeTableEntryIdPointer) {
         ConstCastOnly child = types_match_const_cast_only(ira, wanted_type->data.pointer.child_type,
diff --git a/test/cases/cast.zig b/test/cases/cast.zig
index df37bd1dd9..cc5e4b4394 100644
--- a/test/cases/cast.zig
+++ b/test/cases/cast.zig
@@ -487,12 +487,35 @@ fn MakeType(comptime T: type) type {
 }
 
 test "implicit cast from *[N]T to ?[*]T" {
-  var x: ?[*]u16 = null;
-  var y: [4]u16 = [4]u16 {0, 1, 2, 3};
+    var x: ?[*]u16 = null;
+    var y: [4]u16 = [4]u16{ 0, 1, 2, 3 };
 
-  x = &y;
-  assert(std.mem.eql(u16, x.?[0..4], y[0..4]));
-  x.?[0] = 8;
-  y[3] = 6;
-  assert(std.mem.eql(u16, x.?[0..4], y[0..4]));
-}
\ No newline at end of file
+    x = &y;
+    assert(std.mem.eql(u16, x.?[0..4], y[0..4]));
+    x.?[0] = 8;
+    y[3] = 6;
+    assert(std.mem.eql(u16, x.?[0..4], y[0..4]));
+}
+
+test "implicit cast from *T to ?*c_void" {
+    var a: u8 = 1;
+    incrementVoidPtrValue(&a);
+    std.debug.assert(a == 2);
+}
+
+fn incrementVoidPtrValue(value: ?*c_void) void {
+    @ptrCast(*u8, value.?).* += 1;
+}
+
+test "implicit cast from [*]T to ?*c_void" {
+    var a = []u8{ 3, 2, 1 };
+    incrementVoidPtrArray(a[0..].ptr, 3);
+    assert(std.mem.eql(u8, a, []u8{ 4, 3, 2 }));
+}
+
+fn incrementVoidPtrArray(array: ?*c_void, len: usize) void {
+    var n: usize = 0;
+    while (n < len) : (n += 1) {
+        @ptrCast([*]u8, array.?)[n] += 1;
+    }
+}