ARMv7 Assembly - Read from file and command line

Question

I am having a problem reading some arguments from the command line when I call the program.

I need to call it using ./try number string, for example: ./try 0 dfjjg

When I run it I get a bus error. Can anybody help me? I'm new to assembly and it's pretty hard

thanks a lot!

.data

.balign 4
file_operation: .asciz "r\000"

.balign 4
file_name: .asciz "data.txt\000"

.balign 4
end_line: .asciz "%[^\012]\000"

.balign 4
output: .asciz "data from the file: %s\000"

.balign 4
string: .asciz "ARGV: %s\n"

    .global main
    main:
    PUSH {fp, lr}
    ADD fp, sp, #4
    SUB sp, sp, #56

    @OPEN FILE
    LDR r1, addr_file_operation
    LDR r0, addr_file_name
    BL fopen
    MOV r3, r0
    STR r3, [fp, #-8]

    @READ FROM FILE
    SUB r3, fp, #60
    MOV r2, r3
    LDR r1, addr_end_line
    LDR r0, [fp, #-8]
    BL fscanf

    @READ FROM COMMAND LINE
    PUSH {ip, lr}
    LDR r1, [r1, #4]
    LDR r0, addr_string
    BL printf
    POP {ip, pc}

    @PRINT TO SCREEN
    SUB r3, fp, #60
    MOV r1, r3
    LDR r0, addr_output
    BL printf

    @CLOSE FILE
    LDR r0, [fp, #-8]
    BL fclose

    MOV r3, #0
    MOV r0, r3
    SUB sp, fp, #4
    POP {fp, lr}
    BX lr


 @DEFINE ADDRESSES
 addr_file_operation: .word file_operation
 addr_file_name: .word file_name
 addr_end_line: .word end_line
 addr_output: .word output
 addr_string: .word string

.global fscanf
.global fopen
.global fclose
.global printf

It was working fine reading from a file and printing to console but when I added the part "READ FROM COMMAND LINE" it says "BUS ERROR".

score 0 · Accepted Answer · answered Jan 11 '18 at 12:25

Function calls overwrite the registers r0, r1, r2, and r3, so whatever you expect to be in r1 when you say

LDR r1, [r1, #4]

is no longer there. Whatever is in r1 at that point is probably not a valid pointer, so your program crashes. To fix this issue, save the original r1 to the stack or a register in the range r4 to r11 at the beginning of your function, as these registers are not overwritten by other functions.

ARMv7 Assembly - Read from file and command line

1 Answers1