user: implement mlibc as the libc, finally.

It's finally done..

Signed-off-by: kaguya <vpshinomiya@protonmail.com>

user/include/mlibc/tests/posix/fdopen.c

@@ -0,0 +1,39 @@
+#include <stdio.h>
+#include <assert.h>
+#include <string.h>
+#include <unistd.h>
+#include <fcntl.h>
+
+#define TEST_FILE "fdopen.tmp"
+
+int main() {
+	int fd = open(TEST_FILE, O_CREAT | O_RDWR, 0666);
+	assert(fd >= 0);
+
+	char *str = "mlibc fdopen test";
+	assert(write(fd, str, strlen(str)));
+
+	// Seek to the beginning, then reopen with fdopen in append mode.
+	lseek(fd, 0, SEEK_SET);
+	FILE *file = fdopen(fd, "a");
+	assert(file);
+
+	// Append and close.
+	str = " appended";
+	fwrite(str, strlen(str), 1, file);
+	fflush(file);
+	fclose(file);
+
+	// Open it again and check that the append succeeded.
+	fd = open(TEST_FILE, O_RDONLY);
+	assert(fd >= 0);
+	file = fdopen(fd, "r");
+	assert(file);
+	str = "mlibc fdopen test appended";
+	char buf[100] = {0};
+	assert(fread(buf, 1, strlen(str), file));
+	assert(!strcmp(buf, str));
+	fclose(file);
+
+	return 0;
+}