strcmp issue

This is a discussion on strcmp issue within the C++ Programming forums, part of the General Programming Boards category; idk why but i cannot figure out what is wrong with my program. i found it crashes on strcmp but ...

  1. #1
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    strcmp issue

    idk why but i cannot figure out what is wrong with my program. i found it crashes on strcmp but as far as i can tell this is how i have always used it. i'll post what i think are the impotent parts.

    Code:
            FILE* fin;							//file pointer
    	fin = fopen("profiles.data", "r");	//open file
    
    	if(fin == NULL)
    	{
    		printf("ERROR opening file\n");
    		return -1;
    	}
    
    	char* input = "";					//input form file.
    	fscanf(fin, "%[^\n]", &input);		//read line
    
    	//initialize data liked list
    	initializeData(); //<----dont think this matters it never touches input or anything else
    
    	printf("%s ?= **\n", &input); //<----this works
    
    	//look for start of file. start of file will contain "**" on a single line by it self.
    	while(strcmp(input, "**") != 0) //<---crashes here
    	{
    		fscanf(fin, "%[^\n]", &input);
    	}
    
    //more to do after this...
    EDIT fixed =! to !=. this was right in my code just wrong here.
    Last edited by t014y; 05-20-2011 at 12:14 PM.

  2. #2
    C++ Witch laserlight's Avatar
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    =! should probably be !=, but I would expect you to get a compile error rather than a runtime crash.

    By the way, is this supposed to be C or C++?
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

  3. #3
    Registered User xentaka's Avatar
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    EDIT: Clarity.

    Beginner here so actual question:

    So what happens when you enter the while loop and there are no more "**" in the file and you reach the EOF?
    Last edited by xentaka; 05-20-2011 at 12:04 PM.

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    Where do you allocate space for the variable input? (I mean what it points to)
    I see you declare it as a pointer to char; I just see no place you allocate enough space to hold 3 bytes of data pointed to by it.

    Tim S.

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    laserlight

    =! should probably be !=, but I would expect you to get a compile error rather than a runtime crash.

    By the way, is this supposed to be C or C++?
    in my code is it "!=" idk why it is wrong here. and this is C++ code.

    Beginner here so actual question:

    So what happens when you enter the while loop and there are no more "**" in the file and you reach the EOF?
    the file is set up so that the ** indicates the start of the data in the file. if there is no ** then the data file is set up wrong and seeing as i set up the data file it would fix that.

    stahta01

    Where do you allocate space for the variable input? (I mean what it points to)
    I see you declare it as a pointer to char; I just see no place you allocate enough space to hold 3 bytes of data pointed to by it.

    Tim S.
    i do not recall needing to allocate space for scanf or fscanf. how ever i did find it strange that i had a compiler error when i tried to read in the strings with out the '&'. i was under the impression that strings were already pointers and the variable didn't need the '&'. and the printf works fine with the '&'

    this is also my first time really programming in C++ so basic differences may not be know to me.

    EDIT: more info
    Last edited by t014y; 05-20-2011 at 12:13 PM.

  6. #6
    C++ Witch laserlight's Avatar
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    Quote Originally Posted by t014y
    this is C++ code.
    Is there any specific reason that you are using C-style I/O over C++-style I/O? Note that you can work with std::string objects instead of dealing with null terminated strings.
    Quote Originally Posted by Bjarne Stroustrup (2000-10-14)
    I get maybe two dozen requests for help with some sort of programming or design problem every day. Most have more sense than to send me hundreds of lines of code. If they do, I ask them to find the smallest example that exhibits the problem and send me that. Mostly, they then find the error themselves. "Finding the smallest program that demonstrates the error" is a powerful debugging tool.
    Look up a C++ Reference and learn How To Ask Questions The Smart Way

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    Is there any specific reason that you are using C-style I/O over C++-style I/O? Note that you can work with std::string objects instead of dealing with null terminated strings.
    the reason is simply because I'm used to C-stile I/O. i didn't think it would make a difference.

  8. #8
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    Quote Originally Posted by t014y View Post
    i do not recall needing to allocate space for scanf or fscanf. how ever i did find it strange that i had a compiler error when i tried to read in the strings with out the '&'. i was under the impression that strings were already pointers and the variable didn't need the '&'. and the printf works fine with the '&'EDIT: more info
    You do need to allocate the memory. You pass a pointer to pointer, instead of pointer to buffer, which results (probably, there might be another bug) in crash. Additionaly, you cast away the const-correctness when assigning a string literal to a char* variable. (the literal has type const char*)

    Code:
    char input[256];
    fscanf(fin, "%s", input);		// I changed it to %s; I dont even know what this C-style rubbish meant
    I never put signature, but I decided to make an exception.

  9. #9
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    i changed it to be more C++ like and it seems to work now. still dont know why it didn't work before. the %[^\n] reads in a whole line where %s reads in a string that is the reason i was using that.

    now that section of code looks like this...

    Code:
    	ifstream fin;							//file pointer
    	fin.open("profiles.data");				//open file
    
    	if(!fin.is_open())
    	{
    		printf("ERROR opening file\n");
    		return -1;
    	}
    
    	string input = "";						//input form file.
    
    	getline(fin, input);
    
    	//initialize data liked list
    	initializeData();
    
    	//look for start of file. start of file will contain "**" on a single line by it self.
    	while(input.compare("**") != 0)
    	{
    		cout << input << "\n";
    		//fscanf(fin, "%[^\n]", &input);
    		getline(fin, input);
    	}
    thanx for the help. if you see any issues that might come up from this plz let me know.

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