In computer programming, an infinite loop is a loop that does not terminate on its own, causing the program to run indefinitely unless it is interrupted manually or by an external event.
In assembly language, an infinite loop can be created by a function calling itself directly or indirectly. This can happen when a function does not have a proper exit condition, or when the exit condition is not met due to incorrect logic or unexpected input.
To avoid infinite loops in function calls in assembly, it is important to:
- Always include a proper exit condition in your functions. The exit condition should be a Boolean expression that evaluates to false when the function should terminate.
- Use loops carefully. Loops should be used only when necessary, and they should be structured in a way that ensures that they will eventually terminate.
- Be aware of the registers that are preserved across function calls. In x86-64 assembly, the RCX and RDX registers are not preserved across calls. This means that if you use these registers in a function, you must save their values before calling another function and restore them after the call.
Here is an example of an infinite loop in assembly:
``` f: ; Function f pushq %rbp ; Save the base pointer movq %rsp, %rbp ; Set the base pointer to the stack pointer subq $16, %rsp ; Allocate space for local variables movq %rbx, (%rbp-8) ; Save RBX movl $3, %ebx f_loop: ; Loop label leaq .LC0(%rip), %rdi ; Load the address of the string ".LC0" into RDI call puts ; Call the puts function to print the string decl %ebx ; Decrement EBX jnz f_loop ; Jump to the loop label if EBX is not zero movq (%rbp-8), %rbx ; Restore RBX movq %rbp, %rsp ; Restore the stack pointer popq %rbp ; Restore the base pointer ret ; Return from the function ```This function will print the string ".LC0" three times, and then it will loop forever because the exit condition (jnz f_loop) is always true.
To fix this infinite loop, we can add a proper exit condition to the function:
``` f: ; Function f pushq %rbp ; Save the base pointer movq %rsp, %rbp ; Set the base pointer to the stack pointer subq $16, %rsp ; Allocate space for local variables movq %rbx, (%rbp-8) ; Save RBX movl $3, %ebx f_loop: ; Loop label leaq .LC0(%rip), %rdi ; Load the address of the string ".LC0" into RDI call puts ; Call the puts function to print the string decl %ebx ; Decrement EBX cmpl $0, %ebx ; Compare EBX to 0 jne f_loop ; Jump to the loop label if EBX is not equal to 0 movq (%rbp-8), %rbx ; Restore RBX movq %rbp, %rsp ; Restore the stack pointer popq %rbp ; Restore the base pointer ret ; Return from the function ```Now the function will print the string ".LC0" three times and then return. The infinite loop has been avoided by adding the exit condition (cmpl $0, %ebx).