ARM assembly segmentation fault

Question

So i've just run my code on a raspberry pi (for the ARM processor), and its throwing a segmentation fault when i run the executable. Here's how i compile it:

gcc -o cw2 cw2.s

I can't figure out why! It takes an input file for a stream of characters (just text).

Here is my ARM assembly code:

.global main
.global printf
.global fopen
.global fclose
.global getc

main:
    ; Open File
    PUSH {R1} ; Push address of R1 onto stack
    LDR R0, [R1, #0x04] ; Get argv[1] from stack to R0
    LDR R1, =r ; Load address of file open format (read)
    BL fopen ; Open file
    LDR R1, =fin ; Load address of file in to R1
    STR R0, [R1] ; Store contents of R1 into R0

    ; Setup array
    LDR R4, =ch_array ; Array address
    MOV R3, #0 ; Array index

    BL loopFile
    BL printArray

@ Loop through the file
loopFile:
    LDR R1, =fin ; Load R1 with address of file in
    LDR R0, [R1] ; Load R0 with pointer? of R1
    BL getc ; Get next character from file into R0?
    CMP R0, #-1 ; Check for end of file
    BEQ endl ; Close file

    ; get index from character (getc)
    /*
        Assuming the base address is in R4 (my array), the ascii is in R0
        from the file input into R0: increment the word in memory at
        the address 'R4 + (R0 * 4)'. *4 because each word is 4 bytes.
    */
    LSL R0, R0, #2
    LDR R1, [R4, R0]
    ADD R1, R1, #1
    STR R1, [R4, R0]

    B loopFile ; Run loop again

@ Close the file
endl:
    LDR R1, =fin ; Load R1 with address of file in
    LDR R0, [R1] ; Load R0 with pointer? of R1
    BL fclose ; Close the file

printArray:
    CMP R3, #ARRAY_MAX ; while (i < ARRAY_MAX)
    BEQ _exit ; Exit if max

    ; Check if value of the array at index i where
    ; (i = character number) is 0, if so then skip it
    ; else print it

    B printArray ; Loop next iteration of array

_exit:
    MOV R7, #1
    MOV R0, #0
    SWI 0

.data
    .equ ARRAY_MAX, 255
    ch_array:
        .rept ARRAY_MAX ;For all elements in array, repeat:
        .word 0x00 ;Initialize to 0
        .endr ;End repetition
    fin:   .word 0x00
    r:     .asciz "r"
    space: .word ' '
    nl:    .word '\n'

I also don't know how to test to print out data to see if it is actually reading anything in.

Use a debugger. PS: you should use fgetc not getc, because the latter may be a macro. But if your code compiled, then it isn't and should be fine as is. — Jester
@Jester how would I go about iterating through my array to see if the values are stored? — madcrazydrumma

juansolsona juansolsona · Accepted Answer · 2016-02-23T18:58:33

Just taking an outlook, seems like if you are not checking if the fopen function can open the file. If it cannot, the return will be NULL and it may lead to an exception in getc.

Hope it helps.

ARM assembly segmentation fault

1 Answers