Too big a chunk of refactoring and portability fix.

Makefile

@@ -1,5 +1,16 @@
-libtermux-exec.so: termux-exec.c
-	$(CC) $(CFLAGS) -Wall -Wextra -Oz termux-exec.c -shared -fPIC -o libtermux-exec.so
+UNAME_S := $(shell uname -s)
+
+ifeq ($(UNAME_S),Darwin)
+	CFLAGS += -Oz
+else
+	CFLAGS += -Os
+endif
+
+libtermux-exec.so: termux-exec.c termux_rewrite_executable.c
+	$(CC) $(CFLAGS) $^ -shared -fPIC -o $@
+
+termux-rewrite-exe: termux-rewrite-exe.c termux_rewrite_executable.c
+	$(CC) $(CFLAGS) $^ -o $@
 
 install: libtermux-exec.so
 	install libtermux-exec.so $(PREFIX)/lib/libtermux-exec.so
@@ -11,6 +22,6 @@ test: libtermux-exec.so
 	@LD_PRELOAD=${CURDIR}/libtermux-exec.so ./run-tests.sh
 
 clean:
-	rm -f libtermux-exec.so tests/*-actual
+	rm -f libtermux-exec.so termux-rewrite-exe tests/*-actual
 
 .PHONY: clean install test uninstall

termux-exec.c

@@ -1,30 +1,28 @@
+#define _GNU_SOURCE
+// for RTLD_NEXT
+// ref: http://comments.gmane.org/gmane.linux.man/5208
+
+#include "termux_rewrite_executable.h"
+
 #include <dlfcn.h>
+
 #include <libgen.h>
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
 #include <unistd.h>
 
-static const char* termux_rewrite_executable(const char* filename, char* buffer, int buffer_len)
-{
-	strcpy(buffer, "/data/data/com.termux/files/usr/bin/");
-	char* bin_match = strstr(filename, "/bin/");
-	if (bin_match == filename || bin_match == (filename + 4)) {
-		// We have either found "/bin/" at the start of the string or at
-		// "/xxx/bin/". Take the path after that.
-		strncpy(buffer + 36, bin_match + 5, buffer_len - 37);
-		filename = buffer;
-	}
-	return filename;
-}
+#include <sys/types.h>
+#include <sys/stat.h>
+#include <fcntl.h>
 
 int execve(const char* filename, char* const* argv, char *const envp[])
 {
 	int fd = -1;
 	const char** new_argv = NULL;
 
 	char filename_buffer[512];
-	filename = termux_rewrite_executable(filename, filename_buffer, sizeof(filename_buffer));
+	filename = termux_rewrite_executable(filename_buffer, sizeof(filename_buffer), filename);
 
 	// Error out if the file is not executable:
 	if (access(filename, X_OK) != 0) goto final;
@@ -69,7 +67,8 @@ int execve(const char* filename, char* const* argv, char *const envp[])
 	}
 
 	char interp_buf[512];
-	const char* new_interpreter = termux_rewrite_executable(interpreter, interp_buf, sizeof(interp_buf));
+	const char* new_interpreter =
+		termux_rewrite_executable(interp_buf, sizeof(interp_buf), interpreter);
 	if (new_interpreter == interpreter) goto final;
 
 	int orig_argv_count = 0;

termux-rewrite-exe.c

@@ -0,0 +1,49 @@
+/*
+  command line test driver
+
+	## command ##
+  $ ./termux-rewrite-exe /bin/sh /usr/bin/env
+	## output ##
+  /data/data/com.termux/files/usr/bin/sh
+  /data/data/com.termux/files/usr/bin/env
+*/
+
+#include "termux_rewrite_executable.h"
+
+#include <stdio.h>
+#include <stddef.h>
+#include <string.h>
+
+int main(int argc, char** argv) {
+	int verbose = 0;
+	size_t idx = 1;
+	for ( ; idx < argc; ++idx ) {
+		if (strcmp("-v", argv[idx]) == 0) {
+		  verbose = 1;
+			continue;
+		}
+		if (strcmp("--", argv[idx]) == 0) {
+			++idx;
+			break;
+		}
+		if (strncmp("-", argv[idx], 1) == 0) {
+			fprintf(stderr, "unrecognized option '%s'", argv[idx]);
+			return 1;
+		}
+
+		break;
+	}
+	const char* fmt[]={
+		"%.0s%s\n",
+		"'%s' -> '%s'\n"
+	};
+
+	char rewritten[512];
+	for( ; idx < argc; ++idx ) {
+		printf(
+			fmt[verbose],
+			argv[idx],
+			termux_rewrite_executable(rewritten, sizeof(rewritten), argv[idx]));
+	}
+	return 0;
+}

termux_rewrite_executable.c

@@ -0,0 +1,20 @@
+#include "termux_rewrite_executable.h"
+
+#include <string.h>
+#include <stdio.h>
+
+const char* termux_rewrite_executable(
+	char* rewritten, size_t rewritten_size, const char* original)
+{
+	const char* bin_match = strstr(original, "/bin/");
+	// We have either found "/bin/" at the start of the string or at
+	// "/xxx/bin/". Take the path after that.
+	if (bin_match != original && bin_match != (original + 4)) {
+		snprintf(rewritten, rewritten_size, "%s", original);
+		return rewritten;
+	}
+
+	const char termux_usr_bin[]="/data/data/com.termux/files/usr/bin";
+	snprintf(rewritten, rewritten_size, "%s/%s", termux_usr_bin, bin_match + 5);
+	return rewritten;
+}

termux_rewrite_executable.h

@@ -0,0 +1,11 @@
+#ifndef TERMUX_REWRITE_EXECUTABLE__INCLUDED
+#define TERMUX_REWRITE_EXECUTABLE__INCLUDED
+
+#include <stddef.h>
+
+const char* termux_rewrite_executable(
+  char* rewritten,
+  size_t rewritten_size,
+  const char* original);
+
+#endif // TERMUX_REWRITE_EXECUTABLE__INCLUDED