"a+" の挙動確認

a.c

#include <stdio.h>
int main() {
	FILE *fp = fopen("a-test.txt", "a+");
	char buf[4];
	printf("%ld\n", ftell(fp));
	if (fgets(buf, sizeof(buf), fp))
		printf("%s\n", buf);
	fputs("test", fp);
	printf("%ld\n", ftell(fp));
	if (!fgets(buf, sizeof(buf), fp))
		printf("fgets failed\n");
	fclose(fp);
	return 0;
}