By adding the flag -fstack-protector, you instruct the compiler to emit extra code to check for buffer overflows, such as stack-smashing attacks. This is done by adding a guard variable to functions with vulnerable objects. This includes functions that call alloca, and functions with buffers larger than or equal to 8 bytes. The guards are initialized when a function is entered and then checked when the function exits. If a guard check fails, an error handling function is called. The error handling function can be made to indicate the error in some way and exit the program. Only variables that are actually allocated on the stack are considered, optimized away variables or variables allocated in registers are not considered.