Open a .ppm file and write some data to it. AMD64 linux Assembly Nasm - linux

Question

I am writing a program in AMD64 linux Assembly code (assembler Nasm) that does a whole bunch of stuff. Basicly, my question right now is how can I open a file, and write some data to it.
My code I have seems like it should work.
Basically I want to open a .ppm image file and write the header to it. My professor gave me some pseudo code to to help and here is that code for just the part I am trying to accomplish.
fd = open("gradient.ppm", 577, 0o644)
if fd < 0: return 1 (error)
bufsize = writeHeader(buffer, 256, 256)
status = write(fd, buffer, bufsize)
if status < 0: return 2 (error)
Here is my code. My professor has some test program written in c++ that will run my code and test to see if it works correctly, so I am not running directly from this file. (BTW, the writeheader file has been confirmed to work)
global start
extern writeRGB
extern writeHeader
section .data
filename: db "gradient.ppm",0
section .bss
buffer resb 5000
section .text
; rdi,rsi,rdx
start:
push r8
push r9
push r10
push r11
push r12
push r13
push r14
push r15
; open file
; sys_open: rax=2, rdi=char filename, rsi=int flags, rdx=int mode
mov rax, 2 ; 2 is system call number for sys_open
mov rdi, filename ; filname is in data section
mov rsi, 577 ; flag that is just given to me
mov rdx, 0o644 ; Octol number of the mode that is just given to me
syscall ; execute the sys_open system call
mov r9, rax ; r9 will hold file handle (fd)
; check for error
; compare 0 and data returned to rax from opening file.
; if data in rax < 0, store 1 in r11 and jump .error which will return the 1
mov r11, 1
cmp r9, 0
jl .error
; call writeheader
; writeHeader(rdi = buffer, rsi = 256, rdx = 256)
mov rdi, buffer
mov rsi, 256
mov rdx, 256
call writeHeader
mov r8, rax ; store the buffer size (bufsize) in r8
; status = write(fd, buffer, bufsize)
; sys_write: rax=1, rdi=fd, rsi=buffer, rdx=bufsize)
mov rax, 1 ; 1 is the system call number for sys_write
mov rdi, r9 ; the file handle (fd) is stored in r9
mov rsi, buffer ; the buffer is in the .bss section
mov rdx, r8 ; r8 holds the buffer size (bufsize)
syscall ; execute the sys_write system call
mov r10, rax ; status will be stored in r10
; check for error
; compare 0 and data returned to rax from opening file.
mov r11, 2
cmp r9, 0
jl .error
pop r15
pop r14
pop r13
pop r12
pop r11
pop r10
pop r9
pop r8
mov rax, 0
ret
.error:
; mov error code in r11 into rax to indicate error, and return it
mov rax, r11 ; rll holds error code
ret
If my code should work, then there is probably something wrong in which the way the test file is accessing it, if thats the case just let me know so that I can focus my resources on fixing that problem rather than fixing my code that already works.

Answer 1

cmp r9, 0
jb .error ; <-- This will never happen!
When testing for a negative number don't use jb. That's reserved to work with unsigned numbers. Use jl.
cmp r9, 0
jl .error

Answer 2

;open file give it a name in section .data file: db "......", 0
mov rsi, 577
mov rdx, 0o644
mov rdi, file
mov rax, sys_open
syscall
mov r13, rax ;save file descriptor
cmp rax, 0 ;return error if negative
jl .error

Answer 3

Copy/paste related error in your code!
; status = write(fd, buffer, bufsize)
; sys_write: rax=1, rdi=fd, rsi=buffer, rdx=bufsize)
mov rax, 1 ; 1 is the system call number for sys_write
mov rdi, r9 ; the file handle (fd) is stored in r9
mov rsi, buffer ; the buffer is in the .bss section
mov rdx, r8 ; r8 holds the buffer size (bufsize)
syscall ; execute the sys_write system call
mov r10, rax ; status will be stored in r10
; check for error
; compare 0 and data returned to rax from opening file.
mov r11, 2
cmp r9, 0
jl .error
You have put the status in R10 but are comparing the value in R9.