help with error

This is a discussion on help with error within the C Programming forums, part of the General Programming Boards category; I keep getting an error and I can't figure out why: "Unhandled exception at 0x004115f4 in polynomials.exe: 0xC0000005: Access violation ...

  1. #1
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    help with error

    I keep getting an error and I can't figure out why:

    "Unhandled exception at 0x004115f4 in polynomials.exe: 0xC0000005: Access violation writing location 0x00339000."

    When I step through that section of code, everything works fine.
    Am I missing something or could there be something further on that corrupts my program?
    Any help is appreciated!

    Code:
    char *a, *polynomial[MAX], ch;
    
    for(k = 0; k < number; k++) {
    			for(p = 0; ; p++) {
    				ch = fgetc(text);
    				*(a+p) = ch;           //error here
    				if(ch == '\n') {
    					break;
    				}
    			}
    			if(k < number - 1) {
    				polynomial[k] = a;
    				a = calloc(1,MAXPOLY*sizeof(char));
    				if(a == NULL) {
    					printf("Error involved in allocating space. Cannot complete task.");
    					exit(EXIT_FAILURE);
    				}
    			}
    		}

  2. #2
    and the Hat of Guessing tabstop's Avatar
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    You write to a in the first for loop, but don't have anything to write to until the second.

  3. #3
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    But how would that generate an error?
    I went through both loops, and everything worked.
    Then I ran the entire program, and it stopped there.

  4. #4
    Jack of many languages Dino's Avatar
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    You never initialize your pointer "a" to anything. It has random data in it.

    You need to test for EOF after the fgetc().

    Todd
    Mac and Windows cross platform programmer. Ruby lover.

    Quote of the Day
    12/20: Mario F.:I never was, am not, and never will be, one to shut up in the face of something I think is fundamentally wrong.

    Amen brother!

  5. #5
    and the Hat of Guessing tabstop's Avatar
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    You have a char* a. That's fine, it's a pointer. Currently it's pointing to Hoboken. And then, you try to write a letter to that memory address, and that doesn't work. You need to make a point at memory you own; conveniently, this is what calloc does (you need to calloc before you even think about touching a).

  6. #6
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    I initialized a*, but I'm still getting the same problem...

    I'm trying to read a file of polynomials where the first number tells you how many polynomials are in the file. From there I have to add, multiply, and evaluate them, but I have a general idea on how to do that. At the moment, I'm having problems with creating the array of pointers to separate polynomials.

    <ex>
    Code:
    3
    26x^5 + 5x^2 + 323x^3 + 50x^2 + 14x
    8x^3 + 11x^2 + 34x + 4
    21x^6 + 3x^5 + 4x + 4
    Code:
    #define MAXPOLY 100
    #define CHAR 1000
    #define NAME 50
    
    int main(void) {
    
    	FILE *text;
    	int number, exp, p, k, go;
    	float coeff;
    	char again = 'Y', newFile = 'Y', operation, ch;
    	char fileName[NAME], *polynomial[MAXPOLY], charArray[MAXPOLY], *token, *a;
    	struct poly *first = NULL, *sum, *multiply1, *multiply2, *newLink, *pointers;
    
    
    	while(newFile == 'Y' || newFile == 'y') {
    		printf("\nEnter the file where the polynomials are found: \n");
    		fscanf(stdin, " &#37;s", &fileName);
    		text = fopen(fileName, "r");
    /* If file failed to open, print error statement and ask for a different file. */
    		if(text == NULL) {
    			printf("Error involved in opening file. Cannot open %s\n", fileName);
    			continue;
    		}
    		fscanf(text, " %d", &number);
    /* Create an array of pointers. If error involved in allocating memory, terminate program. */
    		pointers = calloc(number, sizeof(struct poly));
    		if(pointers == NULL) {
    			printf("Error involved in allocating space. Cannot complete task.");
    			break;
    		}
    
    		a = &charArray[0];
    		ch = fgetc(text);
    		for(k = 0; k < number; k++) {
    			for(p = 0; p < MAXPOLY ; p++) {
    				ch = fgetc(text);
    				*(a+p) = ch;
    				if(ch == '\n') {
    					break;
    				}
    			}
    			if(k < number - 1) {
    				polynomial[k] = a;
    				a = calloc(1,MAXPOLY*sizeof(char));
    				if(a == NULL) {
    					printf("Error involved in allocating space. Cannot complete task.");
    					exit(EXIT_FAILURE);
    				}
    			}
    		}
    	return 0;
    }
    Last edited by arrgh; 04-16-2008 at 11:15 AM.

  7. #7
    Jack of many languages Dino's Avatar
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    For a prudent programming practice, consider modifying your FOR loop:

    Code:
    for(p = 0; p < MAXPOLY ; p++) {
    Mac and Windows cross platform programmer. Ruby lover.

    Quote of the Day
    12/20: Mario F.:I never was, am not, and never will be, one to shut up in the face of something I think is fundamentally wrong.

    Amen brother!

  8. #8
    CSharpener vart's Avatar
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    Code:
    for(p = 0; ; p++) {
    				ch = fgetc(text);
    				*(a+p) = ch;
    				if(ch == '\n') {
    					break;
    				}
    use fgets
    The first 90% of a project takes 90% of the time,
    the last 10% takes the other 90% of the time.

  9. #9
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    don't you need the length of the string in order to use fgets? if so, how would you know the length of the string without reading each char?

  10. #10
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    No, fgets() needs to be told the MAXIMUM of the string you want to be able to read - and it will read at most that much, or it finds a newline, whcihever happens first.

    --
    Mats
    Compilers can produce warnings - make the compiler programmers happy: Use them!
    Please don't PM me for help - and no, I don't do help over instant messengers.

  11. #11
    C++まいる!Cをこわせ! Elysia's Avatar
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    Code:
    	char *fileName[NAME], *polynomial[MAXPOLY], charArray[MAXPOLY], *token, *a;
    	struct poly *first = NULL, *sum, *multiply1, *multiply2, *newLink, *pointers;
    
    
    	while(newFile == 'Y' || newFile == 'y') {
    		printf("\nEnter the file where the polynomials are found: \n");
    		fscanf(stdin, " %s", fileName);
    Don't you ever get warnings? You should.
    You are defining an array of pointers and passing that to fscanf. Unfortunately, it won't warn here, but other places possibly.

    Read up on pointers and memory:
    http://cpwiki.sourceforge.net/A_pointer_on_pointers
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
    For information on how to enable C++11 on your compiler, look here.
    よく聞くがいい!私は天才だからね! ^_^

  12. #12
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    ok, I've changed the filename array to an array of type char and inserted an &. After compiling, there are 0 errors and 0 warnings, but the program still stops at
    Code:
    				*(a+p) = ch;
    I tried using fgets to write directly to the polynomial array of pointers, and the program compiled but still stopped midway.

  13. #13
    C++まいる!Cをこわせ! Elysia's Avatar
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    Quote Originally Posted by arrgh View Post
    ok, I've changed the filename array to an array of type char and inserted an &. After compiling, there are 0 errors and 0 warnings, but the program still stops at
    No no no. If you read the above article correctly, you should know that when passing an array, the address is passed, so in essence, a pointer is passed. If you add &, then it will pass the address of the pointer instead, which won't do you any good.

    It's better you use fgets instead, since the compiler can warn you if you do wrong then.
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
    For information on how to enable C++11 on your compiler, look here.
    よく聞くがいい!私は天才だからね! ^_^

  14. #14
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    so..
    Code:
    fgets(fileName, NAME, stdin);

  15. #15
    C++まいる!Cをこわせ! Elysia's Avatar
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    No, check the manual for fgets again.
    It should be
    Code:
    fgets(buffer, sizeof(buffer), stdin)
    Quote Originally Posted by Adak View Post
    io.h certainly IS included in some modern compilers. It is no longer part of the standard for C, but it is nevertheless, included in the very latest Pelles C versions.
    Quote Originally Posted by Salem View Post
    You mean it's included as a crutch to help ancient programmers limp along without them having to relearn too much.

    Outside of your DOS world, your header file is meaningless.
    For information on how to enable C++11 on your compiler, look here.
    よく聞くがいい!私は天才だからね! ^_^

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