Check for NULL pointer in ARM Assembler-CodePudding

I am using the fgets() function call in my arm assembler program and looping through the contents of a file and printing it. All of this works, but I want to exit the program when fgets() returns NULL. I can't actually figure out what I'm supposed to be comparing it too. Here is what I have:

read_file:
    mov r2, r5       //file pointer is stored in R5
    ldr r0, =buffer
    mov r1, #254
    bl fgets
    bl printf
    cmp r0,#0
    bne read_file
    bl exit

I'm assuming I should be checking the buffer which is getting the string, but I might need to be checking another register for a NULL return?

CodePudding user response：

The problem was that I was comparing the wrong return value. I had worked out correctly that the return value should be stored in R0, but what I am actually comparing here with this code is the return value of the printf() not the return value of fgets() which has already been overwritten at the point I am doing a comparison.

    bl fgets
    bl printf
    cmp r0,#0

After moving the comparison higher up in the code and directly after the fgets() it works.

read_file:
        mov r2, r5
        ldr r0, =buffer
        mov r1, #254
        bl fgets
        cmp r0, #0
        beq exit
        bl printf
        bl read_file
        bl exit