#127: bin/base64: fix compilation on MinGW

As of v0.5.1, the `base64' demo program no longer compiled under MinGW
because MinGW does not emulate the `writev(2)' system call.

Fix this by detecting MinGW at compile time and optionally emulating
`writev(2)' as a series of `write(2)' calls.

Resolves #127.

Makefile

@@ -1,4 +1,4 @@
-CFLAGS += -std=c99 -O3 -Wall -Wextra -pedantic
+CFLAGS += -std=c99 -O3 -Wall -Wextra -pedantic -DBASE64_STATIC_DEFINE
 
 # Set OBJCOPY if not defined by environment:
 OBJCOPY ?= objcopy
@@ -56,6 +56,7 @@ ifdef OPENMP
   CFLAGS += -fopenmp
 endif
 
+TARGET := $(shell $(CC) -dumpmachine)
 
 .PHONY: all analyze clean
 
@@ -64,9 +65,17 @@ all: bin/base64 lib/libbase64.o
 bin/base64: bin/base64.o lib/libbase64.o
 	$(CC) $(CFLAGS) -o $@ $^
 
-lib/libbase64.o: $(OBJS)
-	$(LD) -r -o $@ $^
-	$(OBJCOPY) --keep-global-symbols=lib/exports.txt $@
+# Workaround: mangle exported function names on MinGW32.
+lib/exports.build.txt: lib/exports.txt
+ifeq (i686-w64-mingw32, $(TARGET))
+	sed -e 's/^/_/' $< > $@
+else
+	cp -f $< $@
+endif
+
+lib/libbase64.o: lib/exports.build.txt $(OBJS)
+	$(LD) -r -o $@ $(OBJS)
+	$(OBJCOPY) --keep-global-symbols=$< $@
 
 lib/config.h:
 	@echo "#define HAVE_AVX512 $(HAVE_AVX512)"  > $@
@@ -97,4 +106,4 @@ analyze: clean
 	scan-build --use-analyzer=`which clang` --status-bugs make
 
 clean:
-	rm -f bin/base64 bin/base64.o lib/libbase64.o lib/config.h $(OBJS)
+	rm -f bin/base64 bin/base64.o lib/libbase64.o lib/config.h lib/exports.build.txt $(OBJS)

bin/base64.c

@@ -1,4 +1,19 @@
-#define _XOPEN_SOURCE		// IOV_MAX
+// Test for MinGW.
+#if defined(__MINGW32__) || defined(__MINGW64__)
+#  define MINGW
+#endif
+
+// Decide if the writev(2) system call needs to be emulated as a series of
+// write(2) calls. At least MinGW does not support writev(2).
+#ifdef MINGW
+#  define EMULATE_WRITEV
+#endif
+
+// Include the necessary system header when using the system's writev(2).
+#ifndef EMULATE_WRITEV
+#  define _XOPEN_SOURCE		// Unlock IOV_MAX
+#  include <sys/uio.h>
+#endif
 
 #include <stdbool.h>
 #include <stdlib.h>
@@ -8,7 +23,7 @@
 #include <getopt.h>
 #include <errno.h>
 #include <limits.h>
-#include <sys/uio.h>
+
 #include "../include/libbase64.h"
 
 // Size of the buffer for the "raw" (not base64-encoded) data in bytes.
@@ -50,6 +65,59 @@ struct buffer {
 	char *enc;
 };
 
+// Optionally emulate writev(2) as a series of write calls.
+#ifdef EMULATE_WRITEV
+
+// Quick and dirty definition of IOV_MAX as it is probably not defined.
+#ifndef IOV_MAX
+#  define IOV_MAX 1024
+#endif
+
+// Quick and dirty definition of this system struct, for local use only.
+struct iovec {
+
+	// Opaque data pointer.
+	void *iov_base;
+
+	// Length of the data in bytes.
+	size_t iov_len;
+};
+
+static ssize_t
+writev (const int fd, const struct iovec *iov, int iovcnt)
+{
+	ssize_t r, nwrite = 0;
+
+	// Reset the error marker.
+	errno = 0;
+
+	while (iovcnt-- > 0) {
+
+		// Write the vector; propagate errors back to the caller. Note
+		// that this loses information about how much vectors have been
+		// successfully written, but that also seems to be the case
+		// with the real function. The API is somewhat flawed.
+		if ((r = write(fd, iov->iov_base, iov->iov_len)) < 0) {
+			return r;
+		}
+
+		// Update the total write count.
+		nwrite += r;
+
+		// Return early after a partial write; the caller should retry.
+		if ((size_t) r != iov->iov_len) {
+			break;
+		}
+
+		// Move to the next vector.
+		iov++;
+	}
+
+	return nwrite;
+}
+
+#endif	// EMULATE_WRITEV
+
 static bool
 buffer_alloc (const struct config *config, struct buffer *buf)
 {

test/Makefile

@@ -1,11 +1,13 @@
-CFLAGS += -std=c99 -O3 -Wall -Wextra -pedantic
+CFLAGS += -std=c99 -O3 -Wall -Wextra -pedantic -DBASE64_STATIC_DEFINE
 ifdef OPENMP
   CFLAGS += -fopenmp
 endif
 
 TARGET := $(shell $(CC) -dumpmachine)
 ifneq (, $(findstring darwin, $(TARGET)))
   BENCH_LDFLAGS=
+else ifneq (, $(findstring mingw, $(TARGET)))
+  BENCH_LDFLAGS=
 else
   # default to linux, -lrt needed
   BENCH_LDFLAGS=-lrt