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fread, fwrite issues

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Join Date: Mar 2006

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degamer106 degamer106 is offline

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fread, fwrite issues

Aug 10th, 2006

One of the things I can't get working is reading and writing a structure to and from a file in binary. I'm sure that I'm using fread and fwrite correctly. However, I keep getting crashes which I cannot find ways to debug.

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#include <stdio.h>
 
#define MAX	1
 
typedef struct {
	short int key;
	char name[21];
	char symbol[6];
	float price;
	float high;
	float low;
	short int ratio;
} STOCK;
 
void get_data(STOCK *);
 
int main(void) {
 
	FILE *fp;
//	STOCK data[MAX];
	STOCK data[MAX] = {	{
			{1000}, {"nasdaq"}, {"NAS"}, {100}, {100}, {50}, {2}
		}
	};
 
	if (!(fp = fopen("stockdata.dat", "rb"))) {
//		get_data(data);
		FILE *temp = fopen("stockdata.dat", "wb");
		fwrite(data, sizeof(STOCK), MAX, temp);
		fclose(temp);
	}
 
	STOCK test[MAX];
	fread(test, sizeof(STOCK), MAX, fp);
	printf("%hd", test[0].key);
 
	return 0;
}
#include <stdio.h>

#define MAX	1

typedef struct {
	short int key;
	char name[21];
	char symbol[6];
	float price;
	float high;
	float low;
	short int ratio;
} STOCK;

void get_data(STOCK *);

int main(void) {

	FILE *fp;
//	STOCK data[MAX];
	STOCK data[MAX] = {	{
			{1000}, {"nasdaq"}, {"NAS"}, {100}, {100}, {50}, {2}
		}
	};

	if (!(fp = fopen("stockdata.dat", "rb"))) {
//		get_data(data);
		FILE *temp = fopen("stockdata.dat", "wb");
		fwrite(data, sizeof(STOCK), MAX, temp);
		fclose(temp);
	}

	STOCK test[MAX];
	fread(test, sizeof(STOCK), MAX, fp);
	printf("%hd", test[0].key);

	return 0;
}

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Join Date: Aug 2005

Posts: 148

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Micko

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Re: fread, fwrite issues

Aug 11th, 2006

If you want to write array of structures in file you should use loop to store each member. Coinsider this code:

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#include <string.h>
#define LEN 5
 
typedef struct Test
{
    int x;
    double y;
}Test;
 
 
int main(void)
{
    Test array[LEN];
    Test res[LEN];
    FILE* fp;
    int i;
    /*Open for writing*/
    fp = fopen("test.bin","wb");
    for (i = 0; i <LEN; i++)
    {
        array[i].x = i;
        array[i].y =  i * 2.5;
        fwrite(&array[i], sizeof(Test), 1, fp);
    }
    fclose(fp);
    /* Open for reading*/
    fp = fopen("test.bin", "rb");
    for (i = 0; i <LEN; i++)
    {
        fread(&res[i], sizeof(Test), 1, fp);
    }
    for (i = 0; i <LEN; i++)
    {
        printf ("%d %g\n", res[i].x, res[i].y);
    }
    fclose(fp);
 
    return 0;
}
#include <string.h>
#define LEN 5

typedef struct Test
{
    int x;
    double y;
}Test;


int main(void)
{
    Test array[LEN];
    Test res[LEN];
    FILE* fp;
    int i;
    /*Open for writing*/
    fp = fopen("test.bin","wb");
    for (i = 0; i <LEN; i++)
    {
        array[i].x = i;
        array[i].y =  i * 2.5;
        fwrite(&array[i], sizeof(Test), 1, fp);
    }
    fclose(fp);
    /* Open for reading*/
    fp = fopen("test.bin", "rb");
    for (i = 0; i <LEN; i++)
    {
        fread(&res[i], sizeof(Test), 1, fp);
    }
    for (i = 0; i <LEN; i++)
    {
        printf ("%d %g\n", res[i].x, res[i].y);
    }
    fclose(fp);
    
    return 0;
}

Now, I think you'll know what to do...

Last edited by Micko; Aug 11th, 2006 at 2:01 am.

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Re: fread, fwrite issues

Aug 11th, 2006

I know its possible to write binary files because I've done it hundreds of times, and so has almost everyone else.

This works for me.

 Help with Code Tags 
 C Syntax (Toggle Plain Text) 
#include <stdio.h>
 
#define MAX	1
 
typedef struct {
	short int key;
	char name[21];
	char symbol[6];
	float price;
	float high;
	float low;
	short int ratio;
} STOCK;
 
void get_data(STOCK *);
 
int main(void) {
 
	FILE *fp;
	STOCK data[MAX] = {	{
			{1000}, {"nasdaq"}, {"NAS"}, {100}, {100}, {50}, {2}
		}
	};
	// if the file does not exist, then create it
	if(!(fp = fopen("stockdata.dat", "rb")) )
	{
		if(fp = fopen("stockdata.dat", "wb")) { 
			fwrite(data, sizeof(STOCK), MAX, fp);
			fclose(fp);
			fp = fopen("stockdata.dat", "rb");
		}
		else
		{
			printf("can't write to file\n");
			return 1;
		}
	}
 
	STOCK test[MAX];
	fread(test, sizeof(STOCK), MAX, fp);
	fclose(fp);
	printf("%hd\n", test[0].key);
 
	return 0;
}
#include <stdio.h>

#define MAX	1

typedef struct {
	short int key;
	char name[21];
	char symbol[6];
	float price;
	float high;
	float low;
	short int ratio;
} STOCK;

void get_data(STOCK *);

int main(void) {

	FILE *fp;
	STOCK data[MAX] = {	{
			{1000}, {"nasdaq"}, {"NAS"}, {100}, {100}, {50}, {2}
		}
	};
	// if the file does not exist, then create it
	if(!(fp = fopen("stockdata.dat", "rb")) )
	{
		if(fp = fopen("stockdata.dat", "wb")) { 
			fwrite(data, sizeof(STOCK), MAX, fp);
			fclose(fp);
			fp = fopen("stockdata.dat", "rb");
		}
		else
		{
			printf("can't write to file\n");
			return 1;
		}
	}

	STOCK test[MAX];
	fread(test, sizeof(STOCK), MAX, fp);
	fclose(fp);
	printf("%hd\n", test[0].key);

	return 0;
}

Last edited by Ancient Dragon; Aug 11th, 2006 at 8:13 am.

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degamer106 degamer106 is offline

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Re: fread, fwrite issues

Aug 11th, 2006

so you're allowed to change the state of the file by pointing the pointer elsewhere?

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Re: fread, fwrite issues

Aug 11th, 2006

How would I copy data from one file directly to another? I tried this syntax:

 Help with Code Tags 
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void disp_all(FILE *fp) {
 
	FILE *ftemp = fopen("tempdata.dat", "wb");
 
	fwrite(fp, sizeof(STOCK), 1, ftemp);
	fclose(ftemp);
	ftemp = fopen("tempdata.dat", "rb");
 
	STOCK temp[MAX];
	fread(temp, sizeof(STOCK), 1, ftemp);
	printf("%hd\n", temp[0].key);
 
	rewind(fp);
}
void disp_all(FILE *fp) {

	FILE *ftemp = fopen("tempdata.dat", "wb");

	fwrite(fp, sizeof(STOCK), 1, ftemp);
	fclose(ftemp);
	ftemp = fopen("tempdata.dat", "rb");

	STOCK temp[MAX];
	fread(temp, sizeof(STOCK), 1, ftemp);
	printf("%hd\n", temp[0].key);

	rewind(fp);
}

and it doesn't seem to work

fp is in "rb" mode. I want to write the file that fp is pointing to directly into a temporary file that ftemp is pointing to.

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Re: fread, fwrite issues

Aug 11th, 2006

read the description of the open flags -- one of the allows both read and writes and your program only opens the file once.

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Dave Sinkula Dave Sinkula is offline

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long time no c

Re: fread, fwrite issues

Aug 11th, 2006

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Originally Posted by Ancient Dragon

I know its possible to write binary files because I've done it hundreds of times, and so has almost everyone else.

Writing structures as text files may be preferred for portability and human read/writeability.
http://c-faq.com/struct/io.html

"One of the methods used by statists to destroy capitalism consists in establishing controls that tie a given industry hand and foot, making it unable to solve its problems, then declaring that freedom has failed and stronger controls are necessary." --Ayn Rand