CODEWITHSUNDEEP
assemblyarmgnu
InsaneCoder
InsaneCoder

Reputation: 8288

Segmentation fault when STR to a memory location pointed by label in ARM assembly

Its just the second day of my learning ARM assembly and I am stuck at a problem where I get segmentation fault at STR.

@P = Q+R+S
@Q=2, R=4, S=5

.global main
main:
    adrl r4,vals
    ldr r1,[r4,#Q] @load Q=2 into r1
    ldr r2,[r4,#R]
    ldr r3,[r4,#S]
    add r0,r1,r2
    add r0,r0,r3
    str r0,[r4,#P]
    mov r7,#1
    svc 0

.equ P,0
.equ Q,4
.equ R,8
.equ S,12

vals:   .space 4
    .word 2
    .word 4
    .word 5
    .align
    .end

can someone please help as why its crashing?

EDIT

I add the variables to the data section.

@P = Q+R+S
@Q=2, R=4, S=5
.data
vals:   .space 4
    .word 2
    .word 4
    .word 5
    .align

.text
.global main
main:
    adrl r4,vals
    ldr r1,[r4,#Q] @load Q=2 into r1
    ldr r2,[r4,#R]
    ldr r3,[r4,#S]
    add r0,r1,r2
    add r0,r0,r3
    @str r0,[r4,#P]
    mov r7,#1
    svc 0

.equ P,0
.equ Q,4
.equ R,8
.equ S,12
.end

Compiling and linking like below :

$ as -o main.o main.s
main.s: Assembler messages:
main.s:13: Error: symbol .data is in a different section

Upvotes: 1

Views: 465

Answers (1)

InsaneCoder
InsaneCoder

Reputation: 8288

The segmentation fault happens because the vals is by default assembled into the text section, hence becoming non-writable.

The solution is to define vals in the data section like below :

@P = Q+R+S
@Q=2, R=4, S=5
.data
vals:   .space 4
    .word 2
    .word 4
    .word 5
    .align

.text
.global main
main:
    ldr r4,=vals
    ldr r1,[r4,#Q] @load Q=2 into r1
    ldr r2,[r4,#R]
    ldr r3,[r4,#S]
    add r0,r1,r2
    add r0,r0,r3
    @str r0,[r4,#P]
    mov r7,#1
    svc 0

.equ P,0
.equ Q,4
.equ R,8
.equ S,12
.end

Also adrl r4,vals is replaced with ldr r4,=vals because adrl cannot be used to refer label in a different section, as it may break.

Upvotes: 2

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