GreenMamba GreenMamba - 1 year ago 67
Linux Question

compare number and variable in assemblyx86

This code reads data from an inputfile character by character and it writes to another file.
It should stop to read and write once it finds a character with value equal to 3.
I said should because the program doesn't stop once it finds a value equal to 3, instead it continues to read until the end of file.

Inputfile is like:

2 4 5 3 1 8

My code is:

.section .data

varInputHandle: .long 100
varOutputHandle: .long 100
varExitCode: .long 1
cont: .long 1
.section .bss
.lcomm varBuffer, 1

.section .text # declaring our .text segment
.globl _start # telling where program execution should start


popl %eax # Get the number of arguments
popl %ebx # Get the program name

# open input file first
popl %ebx # Get the first actual argument - file to read
movl $5, %eax # open
movl $0, %ecx # read-only mode
int $0x80
movl %eax, varInputHandle #store input file handle to memory

#open output file, make it writable, create if not exists
popl %ebx # Get the second actual argument - file to write
movl $5, %eax # open
movl $0101, %ecx # create flag + write only access (if google is telling me truth)
movl $0666, %edx #permissions for out file as rw-rw-rw-
int $0x80
movl %eax, varOutputHandle #store output file handle to memory

movl $0, cont
incb cont
#read single char to varBuffer
movl $3, %eax
movl varInputHandle, %ebx
movl $varBuffer, %ecx
movl $1, %edx
int $0x80

#if no char was read (EOF?), jmp finishProcessing
cmpl $0, %eax
jz finishProcessing # looks like total success, finish cleanly

cmpl $3, varBuffer // this instruction is never true, don't know why
je exitToOs
#write it
movl $4, %eax
movl varOutputHandle, %ebx # file_descriptor
movl $varBuffer, %ecx
movl $1, %edx
int $0x80

# done, go for the next char
jmp processingLoop

movl $0, varExitCode #everything went OK, set exit code to 0

movl varOutputHandle, %ebx # file_descriptor
movl varInputHandle, %ebx
movl $1, %eax
movl varExitCode, %ebx
int $0x80

cmpl $-1, %ebx
movl $6, %eax #sys_close
int $0x80

cmpl $3, varBuffer
seems to never be true therefore I can't jump to exitToOs.

Answer Source

In your code you have:

 cmpl $3, varBuffer

This can never be true because you do not have binary data in your file.

When you read character by character, you are reading ASCII values. In order to properly make this comparison you must do one of two things:

  1. Convert the character read from ASCII to decimal
  2. Compare the read value to an ASCII value

Since you are relying on $0 to represent the end of file, I would suggest that you take the (easier) approach of checking for the ASCII value that you would like to find. In your case this would be:

 cmpl $33, varBuffer  ; Compare character to 0x33 / 51 / "3"
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