Reading Text from Files with fgets in .NET framework

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When fopen successfully creates or opens a file for you, it returns a file handle in EAX Keep that file handle safe somewhere-I recommend either copying it to a memory variable allocated for that purpose or putting it in one of the sacred registers If you store it in EAX, ECX, or EDX and then make a call to almost any C library function, the file handle in the register will be trashed and you'll lose it Once a file is opened for reading, you can read text lines from it sequentially with the fgets function Each time you call fgets on an opened text file, it will read one line of the file, which is defined as all the characters up to the next newline character, which in the Unix world always indicates the end of a text line Now, in any given file there's no way of knowing how many characters there will be until the next newline, so it would be dangerous to just turn fgets loose to bring back characters until it encounters a newline If you attempt to open the wrong kind of file (a binary code file is one possibility, or a compressed data file), you might bring in thousands of bytes before encountering the binary 10H value that the file system considers a newline Whatever buffer you had allocated to hold the incoming text would overflow and fgets would perhaps destroy adjacent data or crash your program For that reason, you must also pass a limit value to fgets When it begins reading a line, fgets keeps track of how many characters it has brought in from the file, and when it gets to one short of the limit value, it stops reading characters It then adds a newline to the buffer for the final character and returns Set up calls to fgets this way: 1 First, push the file handle onto the stack 2 Next, push the character count limit value This must be the actual integer value, and not a pointer to the value! 3
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2 3 Finally, push the address of the character buffer into which fgets should store the characters that it reads from the file With all that done, call fgets If fgets returns a 0 in EAX, then you've either reached the end of the file, or else a file error happened during the read Either way, there's no more data forthcoming from the file But without a 0 coming back in EAX, you can assume that valid text is present in the buffer at the address you passed to fgets on the stack I used fgets to create a simple disk-based help system for TEXTFILE ASM When the user enters no command-line arguments at all, TEXTFILE reads a short text file from disk and displays it to standard output This is a common and courteous thing to do with command-line programs, and I recommend that all utilities you build for everyday use work this way The code for the help system is relatively simple and demonstrates both fopen and fgets: diskhelp: push dword opencode push ebx call fopen cmp eax,0 jne disk call memhelp ret disk: mov ebx,eax rdln: push ebx push dword HELPLEN push dword helpline call fgets add esp,12 cmp eax,0 je done push dword helpline call printf add esp,4 jmp rdln done: push ebx call fclose add esp,4 ret
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Push pointer to open-for-read code "r" Pointer to name of help file is passed in ebx Attempt to open the file for reading fopen returns null if attempted open failed Read help info from disk, else from memory
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Save handle of opened file in ebx Push file handle on the stack Limit line length of text read Push address of help text line buffer Read a line of text from the file Clean up the stack A returned null indicates error or EOF If we get 0 in eax, close up & return Push address of help line on the stack Call printf to display help line Clean up the stack Push the handle of the file to be closed Closes the file whose handle is on the stack Clean up the stack Go home
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When subroutine diskhelp is called, the caller passes a pointer to the name of the help file to be read in EBX The file is first opened If the attempt to open the help file fails, a very short "fail safe" help message is displayed from strings stored in the [data] section of the program (This is the call to memhelp, which is another short subroutine in TEXT-FILEASM) Never leave the user staring at a mute cursor, wondering what's going on! Once the help file is opened, we start looping through a sequence that reads text lines from the opened file with fgets, and then writes those lines to standard output with printf The maximum length of the lines to be read is defined by the equate HELPLEN (As a convention, things in a program defined as macros or equates are named in uppercase letters) Pushing an equate value on the stack is no different from pushing an immediate value, and that's how the instruction is encoded But instead of being specified (perhaps differently) at several places all over your source code, the maximum length of your help file lines is defined in only one place and may be changed by changing that one equate only Equates are good Use them whenever you can Each time a line is read from the file, the address of the line is pushed onto the stack and displayed with printf When no more lines are available to be read in the help file, fgets returns a 0 in EAX, and the program branches to the function call that closes the file Note the fclose function, which is quite simple: You push the file handle of the open file onto the stack, and
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call fclose That's all it takes to close a file!
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