1. printf is a variadic function

   printf takes a format string to interpret additional arguments. Since %d expects an integer, but none is provided, printf still tries to fetch an argument from the stack (or a register), leading to undefined behavior (UB).

2. Why does it compile?

   C does not check the number of arguments passed to printf() at compile time (unless warnings are enabled).

3. What happens at runtime?

   Since printf("%d") expects an integer but gets random data instead, the output is garbage or might cause a segmentation fault.

How to Fix it

Always provide the correct argument:

printf("%d", 42);  // Correct

Enable compiler warnings to catch such mistakes:

gcc -Wall -Wformat -o program program.c