write() error code

[calef13]

Hi,

I'm writing a small application which reads and writes to some files. MY problem is that when I write to the file in my program, I get an error, %eax is set to -14. strerror(-14) returns unknown error, and I was told on irc that posix write() doesn't return -14. As far as I can tell everything is fine with the syscall, I've triple checked everything, but it keeps failing, nothing is written to the file.

.equ SYS_WRITE, 4
.equ ST_BR_WOUT_DESC, -8
.equ LINUX_SYSCALL, 0x80

...

#write 7 to file
    movl $SYS_WRITE, %eax
    movl ST_BR_WOUT_DESC(%ebp), %ebx
    movl $7, %ecx
    #convert to ascii
    addl $48, %ecx
    movl $1, %edx
    int $LINUX_SYSCALL

Thanks in advance for any help

[wildgoose]

Can you add more then that snippet. And add comments! That 80x86 assembler you're using is camouflaging the assembly code making it harder to read due to its abstraction methodology.

[wildgoose]

Been doing some research. You're using a Linux assembler that uses the AT&T syntax?

It uses a reverse encoding and different operands.

You're passing the character and not the address of the character to the file I/O.

.equ SYS_WRITE, 4
.equ ST_BR_WOUT_DESC, -8
.equ LINUX_SYSCALL, 0x80 ... 

tmp  .long 0


#write 7 to file    
	movl $SYS_WRITE, %eax                # Linux SYSTEM - WRITE
    movl ST_BR_WOUT_DESC(%ebp), %ebx     # Set ebx file Handle on stack?
    movl $7, %edx                        # binary digit 7
        
    
#convert to ascii
    addl $48, %edx                       # char = '7' = '0'+7
    movl $tmp, %ecx                # get memory address containing character
    mov %ecx,(%edx)              # copy your character into it!


    movl $1, %edx                        # write a single byte
    
# Sys_Write( uint ebx, const char *ecx, size_t edx )
	 ecx is suppose to be the pointer to the data, not the data itself!

    int $LINUX_SYSCALL

I've used a multitude of assemblers before but never LINUX.
This is untested so hope it works.

[calef13]

yeah I'm using GAS, GNU assembler which uses AT&T syntax.

You're right, I'm passing the number instead of a pointer, man I've missed how useless the error messages are in assembly :) thanks a lot that fixed the problem.

Below is the updated code for clarity. I have a buffer, of size 1 byte, which I used earlier in the program to read in a number.

#write 7(max) to file
    movl $SYS_WRITE, %eax                           #put 4 in %eax, syscall code for write()
    movl ST_BR_WOUT_DESC(%ebp), %ebx     #file descriptor for our file, saved to the stack earlier
    movl $55, BUFFER                                    #put the ascii code for 7 into our buffer
    movl $BUFFER, %ecx                                 #put a pointer to our buffer in ecx 
    movl $BUFFER_SIZE, %edx                        #put the length of our Buffer in edx
    int $LINUX_SYSCALL