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setbase db 'Does the set of %s contain the set of %s ',10,0 mammals db 'mammals',0 dugongs db 'dugongs',0 I haven't shown everything here for the sake of brevity-how often do you need to see the comment headers -but by now you should be catching the sense of making calls to printf The three crucial things to remember are these: Arguments are pushed onto the stack from right to left, starting with the function call as it would be written in C The base string is pushed last If you're doing anything even a little complex with printf, it helps to write the call out first in C form, and then translate it from there into assembly After the call to printf, you must add to ESP a value equal to the total size of all arguments pushed onto the stack Don't forget that for strings you're pushing the address of the string and not the data contained in the string! For most arguments this will be 4 bytes The printf function call trashes everything but the sacred registers Don't expect to keep values in other registers intact through a call to printf! (If you try to keep a counter value in ECX while executing a loop that calls printf, the call to printf will destroy the value in your counter You must save ECX on the stack before each call to a library function, and restore it after the library call returnsor use a sacred register such as ESI, EDI, or EBX) If you can't get a printf call to work in assembly, write up a simple one-liner C program containing the call, and see if it works there If it does, you're probably getting the order or number of the arguments wrong Never forget that there must be one argument for each formatting code!
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Characters In
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Reading characters from the Linux keyboard is as easy as sending characters to the screen display In fact, the C library calls for reading data from the keyboard (which is the default data source assigned to standard input) are almost the inverse of those that display data to standard output This was deliberate, even though there are times when the symmetry gets in the way, as I'll explain
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If you poke around in a C library reference (and you should there are a multitude of interesting routines there that you can call from assembly programs), you may discover the gets routine You may have wondered (if I didn't choose to tell you here) why I didn't cover it The gets routine is simplicity itself: You pass it the name of a string array in which to place characters, and then the user types characters at the keyboard, which are placed in the array When the user presses Enter, gets appends a null at the end of the entered text and returns What's not to love Well, how big is the array And how dumb is your user Here's the catch: There's no way to tell gets when to stop accepting characters If the user types in more characters than you've allocated room to accept them in an array, gets will gleefully keep accepting characters, and overwrite whatever data is sitting next to your array in memory If that something is something important, your program will crash hard That's why, if you try to use gets, gcc will warn you that gets is dangerous It's old, and much better machinery has been created in times since The designated successor to gets is fgets, which has some safety equipment built-in and some complications, too The complications stem from the fact that you must pass a file handle to fgets In general, standard C library routines whose names begin with f act on files (I explain how to work with disk files later in this chapter) You can use fgets to read text from a disk file but remember, in Unix terms, your keyboard is connected to a file, the file called standard input If we can connect fgets to standard input, we can read text from the keyboard, which is what the old and hazardous gets does automatically The bonus in using fgets is that it allows us to specify a maximum number of characters for the routine to accept from the keyboard Anything else the user types will be truncated and discarded If this maximum value is no larger than the string buffer you define to hold characters entered by the user, there's no chance that using fgets will crash your program Connecting fgets to the standard input file is easy The C library predefines three standard file handles, and these handles are linked into your program automatically The three are stdin (standard input), stdout (standard output), and stderr (standard error) For accepting input from the keyboard through fgets, we want to use stdin It's there; you simply have to declare it as extern So here's how to use the fgets routine: 1 Make sure you have declared extern fgets and extern stdin along with your other external declarations at the top of the text section 2 Declare a buffer variable large enough to hold the string data you want the user to enter Use the RESB directive in the [bss] section of your program 3 To call fgets, first push the file handle You must push the handle itself, not the handle's address! So use the form push dword [stdin] 4 Next, push the value indicating the maximum number of characters you want fgets to accept Make sure it is no larger than the buffer variable you declare in [bss]! The stack must contain the actual value don't just push the address of a variable holding the value Pushing an immediate value or the contents of a memory variable will work 5 Next, push the address of the buffer variable where fgets is to store the characters entered by the user 6 Finally, call fgets itself 7
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6 7 (And as with all library function calls, don't forget to clean up the stack!) In terms of actual code, it should look something like this: push dword [stdin] push dword 72 push dword instring call fgets add esp,12 ; ; ; ; ; Push predefined file handle for standard input Accept no more than 72 characters from keyboard Push address of buffer for entered characters Call fgets 3 args X 4 bytes = 12 for stack cleanup
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Here, the identifier instring is a memory variable defined like this: [SECTION bss] instring resb 96 ; Section containing uninitialized data ; Reserve 96 bytes for string entry buffer
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Recall that the RESB directive just sets aside space for your variable; that space is not preloaded with any particular value, with spaces, or nulls, or anything Until the user enters data through fgets, the string storage you allocate using RESB is uninitialized and could contain any garbage values at all From the user side of the screen, fgets simply accepts characters until the user presses Enter It doesn't automatically return after the user types the maximum permitted number of characters (That would prevent the user from backing over input and correcting it) However, anything the user types beyond the number of permitted characters is discarded The CHARSINASM file shown later in this chapter contains the preceding code
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