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This section will go through making the add-year.s program from Chapter 6 a little more robust.
Since this is a pretty simple program, we will limit ourselves to a single recovery point that covers the whole program. The only thing we will do to recover is to print the error and exit. The code to do that is pretty simple:
.include "linux.s" .equ ST_ERROR_CODE, 8 .equ ST_ERROR_MSG, 12 .globl error_exit .type error_exit, @function error_exit: pushl %ebp movl %esp, %ebp #Write out error code movl ST_ERROR_CODE(%ebp), %ecx pushl %ecx call count_chars popl %ecx movl %eax, %edx movl $STDERR, %ebx movl $SYS_WRITE, %eax int $LINUX_SYSCALL #Write out error message movl ST_ERROR_MSG(%ebp), %ecx pushl %ecx call count_chars popl %ecx movl %eax, %edx movl $STDERR, %ebx movl $SYS_WRITE, %eax int $LINUX_SYSCALL pushl $STDERR call write_newline #Exit with status 1 movl $SYS_EXIT, %eax movl $1, %ebx int $LINUX_SYSCALL
Enter it in a file called error-exit.s. To call it, you just need to push the address of an error message, and then an error code onto the stack, and call the function.
Now let's look for potential error spots in our add-year program. First of all, we don't check to see if either of our open system calls actually complete properly.
Linux returns its status code in %eax, so we need to check and see if there is an error.
#Open file for reading movl $SYS_OPEN, %eax movl $input_file_name, %ebx movl $0, %ecx movl $0666, %edx int $LINUX_SYSCALL movl %eax, INPUT_DESCRIPTOR(%ebp) #This will test and see if %eax is #negative. If it is not negative, it #will jump to continue_processing. #Otherwise it will handle the error #condition that the negative number #represents. cmpl $0, %eax jl continue_processing #Send the error .section .data no_open_file_code: .ascii "0001: \0" no_open_file_msg: .ascii "Can't Open Input File\0" .section .text pushl $no_open_file_msg pushl $no_open_file_code call error_exit continue_processing: #Rest of program
So, after calling the system call, we check and see if we have an error by checking to see if the result of the system call is less than zero. If so, we call our error reporting and exit routine.
After every system call, function call, or instruction which can have erroneous results you should add error checking and handling code.
To assemble and link the files, do:
as add-year.s -o add-year.o as error-exit.s -o error-exit.o ld add-year.o write-newline.o error-exit.o read-record.o write- record.o count-chars.o -o add-year
Now try to run it without the necessary files. It now exits cleanly and gracefully!
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