Creating and Opening Files in VS .NET

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Creating and Opening Files
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By this time you should be pretty comfortable with the general mechanism for making C library calls from assembly And whether you realize it or not, you're already pretty comfortable with some of the machinery for manipulating text files You've already used printf to display formatted text to the screen by way of standard output The very same mechanism is used to write formatted text to disk-based text files-you're basically substituting a real disk file for standard output So, understanding text file I/O shouldn't be much of a conceptual leap But unlike standard output, which is predefined for you by the C library and always available, you have to create or open a disk-based text file in order to use it The fopen function is what does the job There are three general ways to open a file: for reading, for writing, and for appending When you open a file for reading, you can read text from it via such functions as fgets, but you can't write to the file When you open a file for writing, whatever may have been in the file before is thrown away, and new material is written at the beginning of the file When you open a file for appending, you may write to the file, but new material is written after any existing material, and whatever was originally in the file is retained Ordinarily, when you open a file for writing you can't read from it, but there are special modes that allow both reading from and writing to a file For text files especially (which are what we're speaking of here) that introduces some complications, so for the most part, text files are opened for either reading or for writing, but not both at once In the Unix file system, if you open a file for either writing or appending and the file does not already exist, the file is created If you don't know if a file exists and you need to find out, attempt to open it for reading and not for writing, or you'll get the file whether it exists or not! To use fopen, you must push the following parameters onto the stack before the call: 1 First onto the stack is a pointer to a code indicating which mode the file should be opened for The various available modes are listed in Table 134 The ones you'll typically use for text files are "r," "w," and "a" These should be defined as short character strings, followed by a null: writecode opencode db 'w',0 db 'r',0
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Table 134: File Access Codes for Use with fopen CODE SENSE "r" "w" "a" "r+" "w+" "a+" DESCRIPTION Opens an existing text file for reading Creates a new text file, or opens and truncates an existing file Creates a new text file, or opens an existing file so that new text is added at the end Opens an existing text file for either writing or reading Creates a new text file, or opens and truncates an existing file for both read and write access Creates a new text file, or opens an existing file for reading or for writing so that new text may be added at the end
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2 Next onto the stack is the address of the character string containing the name of the file to be opened With those two items on the stack, you make the call to fopen If the file was successfully opened, fopen returns a file handle in EAX If the open was unsuccessful, EAX will contain 0 Here's how opening a file for reading looks in code: push dword opencode ; Push pointer to open-for-read code "r" push ebx ; Pointer to name of help file is passed in ebx call fopen cmp eax,0 <jump as needed> ; Attempt to open the file for reading ; fopen returns null if attempted open failed
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The process to create a file and then write to it is identical, except that you must push the "w" code onto the stack instead of the "r" code
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