fread, fwrite issues
 

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.

#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;

}

Re: fread, fwrite issues
 

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

#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...

Re: fread, fwrite issues
 

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.

#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;

}

Re: fread, fwrite issues
 

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

Re: fread, fwrite issues
 

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

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 :sad:

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.

Re: fread, fwrite issues
 

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

Re: fread, fwrite issues
 

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

Re: fread, fwrite issues
 

ok, i read the website that ancient dragon posted up and it was pretty helpful. I managed to get the file copying function of my program to work :)