Memory transfer intel assembly AT&T - memory

Question

I have a problem moving a string bytewise from one memory adress to another. Been at this for hours and tried some different strategies. Im new to Intel assemby so I need some tips and insight to help me solve the problem.
The getText routine is supposed to transfer n (found in %rsi) bytes from ibuf to the adress in %rdi. counterI is the offset used to indicate where to start the transfer, and after the routine is over it should point to the next byte that wasn't transfered. If there isn't n bytes it should cancel the transfer and return the actual number of bytes transfered in %rax.
getText:
movq $ibuf, %r10
#in rsi is the number of bytes to be transfered
#rdi contains the memory adress for the memory space to transfer to
movq $0, %r8 #start with offset 0
movq $0, %rax #zero return register
movq (counterI), %r11
cmpb $0, (%r10, %r11, 1) #check if ibuf+counterI=NULL
jne MOVE #if so call and read to ibuf
call inImage
MOVE:
cmpq $0,%rsi #if number of bytes to read is 0
je EXIT #exit
movq counterI, %r9
movq $0, %r9 #used for debugging only shold not be 0
movb (%r10, %r9, 1), %bl #loads one byte to rdi from ibuf
movb %bl, (%rdi, %r8, 1)
incq counterI #increase pointer offset
decq %rsi #dec number of bytes to read
incq %r8 #inc offset in write buffert
movq %r8, %rax #returns number of bytes wrote to buf
movq (counterI), %r9
cmpb $0, (%r10, %r9,1) #check if ibuf+offset is NULL
je EXIT #if so exit
cmpq $0, %rsi #can be cleaned up later
jne MOVE
EXIT:
movb $0, (%rdi, %r8, 1) #move NULL to buf+%r8?
ret

Answer 1

movq counterI, %r9
movq $0, %r9 #used for debugging only shold not be 0
The second instruction makes the first useless but given the remark I understand you will remove it. Better still, you can remove both if you would change every occurence of %R9 into %R11.
movzbq (%r10, %r9, 1), %r10 #loads one byte+zeroes to rdi from ibuf
movq %r10, (%rdi, %r8, 1) #HERE IS THE PROBLEM I THINK
Here is a dangerous construct. You're first using %R10 as an address but then drop a zero extended data byte in it. Later in the code you will again use %R10 as an address but sadly that won't be in there! The solution is to move into a different register and to not bother about the zero extention.
movb (%r10, %r9, 1), %bl #loads one byte to rdi from ibuf
movb %bl, (%rdi, %r8, 1)
The following code can be shortened
cmpb $0, (%r10, %r9,1) #check if ibuf+offset is NULL
je EXIT #if so exit
cmpq $0, %rsi #can be cleaned up later
jne MOVE
EXIT:
as
cmpb $0, (%r10, %r9, 1) #check if ibuf+offset is NULL
jne MOVE
EXIT: