CODEWITHSUNDEEP
gccassembly
cjw
cjw

Reputation: 354

Calling printf on two strings in GCC assembly

I am trying to call printf on the _format string but I get an error instead of printing out both of the strings. I was able to get the program to work before adding the _printStr function so I am really unsure of why it won't print out both of them. I can also print out each string separately and it works fine (with 12(%ebp) and 16(%ebp)). This is the code I have: 

            .text
            .globl      _main
_string:    .ascii      "Hello\0" 
_string2:   .ascii      " World\0" 
_format:    .ascii      "%s %s\0"


_main: // push params, call fn, clear params, return 

            pushl      $_string2 
            pushl      $_string                 
            call      _printStr
            addl       $8, %esp
            ret

//function to print a string passed to it on the stack
_printStr:
            push        %ebp               # save old frame ptr
            movl        %esp, %ebp     # set frame ptr
            pushl       8(%ebp) 
            pushl       12(%ebp) 
            pushl       _format
            call _printf
            addl        $12, %esp          # clear params from stack
            leave
            ret

Thanks for your time, I appreciate the help.

Upvotes: 1

Views: 3563

Answers (2)

Jester
Jester

Reputation: 58762

You are missing a $ sign in the pushl _format. It should read pushl $_format because you want to pass its address. Incidentally, you are also passing the two words in reverse order, it will print " World Hello". Also you are missing a line feed, but have an extra space. Finally, putting your string constants in the .text section isn't the best practice. See the fixed version in operation (note I removed some leading underscores that are not needed for ideone, but probably needed on your system)

Upvotes: 1

Neil
Neil

Reputation: 55392

Do not pass an unknown string as the first parameter to printf. Always use a known format specifier, preferably a literal if possible. If you want to use printf to print a string, do something like this:

_simpleformat:  .ascii      "%s\0"

...

                pushl       8(%ebp)
                pushl       _simpleformat
                call _printf
                addl        $8, %esp

Upvotes: 1

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