write() error code
 

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

Re: write() error code
 

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.

Re: write() error code
 

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.

Re: write() error code
 

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