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Example needed

This is a discussion on Example needed within the C Programming forums, part of the General Programming Boards category; Hi, could anyone provide a small working example on how to write and read binary files. Let say i have ...

  1. #1
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    Example needed

    Hi, could anyone provide a small working example on how to write and read binary files. Let say i have 3 arrays , a char *array , int *array1 and unsigned int *array2. How would i store this on disc in binary format and then read it from the disc (i need all 3 arrays to be in a single binary on disc , is this possible ?)


    thank you very much,

    baxy

  2. #2
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    Quote Originally Posted by baxy View Post
    Hi, could anyone provide a small working example on how to write and read binary files. Let say i have 3 arrays , a char *array , int *array1 and unsigned int *array2. How would i store this on disc in binary format and then read it from the disc (i need all 3 arrays to be in a single binary on disc , is this possible ?)


    thank you very much,

    baxy
    It sounds like you want to use a struct, which takes disparate datatypes that relate to one object, and puts them together into a single object. Think of a student with an ID, a major, an age, a year in school, and several classes and a GPA. They can all go into one struct, and you can make an array of those structs if you like.

    So you'd wind up with just one file of structs - and there are zillions of examples of writing out binary files. Google one of them.

    If you get stuck, come back and post up your code, and tell us what you're stuck on. We're here to help, but want to see some starting effort - which is forum policy.
    baxy likes this.

  3. #3
    qny
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    The declarations: char *array, int *array1, and unsigned int *array2 do not specify arrays. They specify pointers.

    arrays are not pointers and pointers are not arrays. I suggest you read comp.lang.c.FAQ. particularly section 6.
    AndiPersti likes this.

  4. #4
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    Presumably you're talking about dynamically-allocated arrays.
    You can dump anything you want to a binary file.

    Open the file in binary mode.
    Write the data with fwrite.
    Close the file.

    Post your attempt.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  5. #5
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    hi, ok here is my attempt but i don't deg why is it not working. so i took Adak's advice and it is writing sonething but i cannot read it ????

    write.c
    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include "eprintf.h"
    
    
    struct data	{
    		int *count;
    		long *Info;
    
    }data;
    
    int main(){
    		FILE *fileOut;
    		struct data *myData = emalloc(sizeof(*myData));
    		myData->count = (int *) emalloc(sizeof(int) * 100);
    		myData->Info = (long *) emalloc(sizeof(long) * 100);
    		int i = 0;
    		for (i=0;i<100;i++){
    			myData->count[i] = i+i;
    			myData->Info[i] = i*i;
    		}
    
    		fileOut=fopen("test.bin","wb");
    		if (!fileOut)
    		{
    			printf("Unable to open file!");
    			return 1;
    		}
    		fwrite(&myData, sizeof(struct data), 1, fileOut);
    
    		fclose(fileOut);
    		
    		return 0;
    }
    read.c

    Code:
    #include <stdio.h>
    #include <stdlib.h>
    #include "eprintf.h"
    
    	/* Our structure */
    struct data	{
    		int *count;
    		long *Info;
    
    }data;
    
    
    int main(){
    		FILE *fileIn;
    		struct data *myData = emalloc(sizeof(*myData));
            int i;
            myData->count = (int *) emalloc(sizeof(int) * 100);
    		myData->Info = (long *) emalloc(sizeof(long) * 100);
    		fileIn=fopen("test.bin","rb");
    		if (!fileIn)
    		{
    			printf("Unable to open file!");
    			return 1;
    		}
            printf("ggg\n");
    		fread(&myData,sizeof(struct data),1,fileIn);
    		        printf("ggg\n");
    	    for (i=0;i<100;i++){
    	      printf("%d..%ld\n",myData->count[i],myData->Info[i] );
    	    }
    
    		fclose(fileIn);
    		return 0;
    	}

    so it is writing but when it comes to reading i get: Segmentation fault (core dumped)

  6. #6
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    You don't need a struct.

    Your problem is that all you're writing to the file is the two pointers in the struct, and no actual data at all. The values of the pointers are useless when read back in.

    Here's an example of reading and writing an integer array to a binary file.
    Code:
    /* binwrite.c */
    
    #include <stdio.h>
    #include <stdlib.h>
    
    void die(const char *msg) {
        perror(msg);
        exit(1);
    }
    
    int main(void) {
        int a[] = {1, 2, 3, 4, 5};
        size_t i;
        FILE *fp;
    
        fp = fopen("data.bin", "wb");
        if (!fp) die("Can't open data.bin for writing");
    
        for (i = 0; i < sizeof a/sizeof*a; i++)
            fwrite(&a[i], sizeof*a, 1, fp);
    
        fclose(fp);
    
        return 0;
    }

    Code:
    /* binread.c */
    
    #include <stdio.h>
    #include <stdlib.h>
    
    void die(const char *msg) {
        perror(msg);
        exit(1);
    }
    
    int main(void) {
        int a[5];
        size_t i, n = 0;
        FILE *fp;
    
        fp = fopen("data.bin", "rb");
        if (!fp) die("Can't open data.bin for reading");
    
        while (fread(&a[n], sizeof*a, 1, fp) == 1)
            n++;
    
        fclose(fp);
    
        for (i = 0; i < n; i++)
            printf("%d\n", a[i]);
    
        return 0;
    }
    baxy likes this.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  7. #7
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    aha...

    thanks a million

    PS

    but still if i had 3 arrays then every array would need to be in a single file or ... ???
    Last edited by baxy; 10-11-2012 at 10:54 AM.

  8. #8
    - - - - - - - - oogabooga's Avatar
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    Quote Originally Posted by baxy View Post
    but still if i had 3 arrays then every array would need to be in a single file or ... ???
    That's up to you. Write them all to the same file, one after the other (and read them back in the same order), or write them to three separate files. Give it a try.
    baxy likes this.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  9. #9
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    Thnx everything is working, i learned something new today

  10. #10
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    Hi

    well more problems. So now i want to increase the reading and writing buffer. I don't want it to write one byte at the time or read one byte at the time. so i did this when writting:

    Code:
    FILE *fileOut;
    fileOut=fopen("test.bin","wb");
    int tmp[100];
    int i;
    
      if (!fileOut){
    	printf("Unable to open file!");
      }
      
      for(i=0;i<10000;i++){
        
        if (i%100 == 0){
          fwrite(&tmp, sizeof(Int64), 100, fileOut);
        }
        tmp[i%100] = i;
          
      }
    
      fclose(fileOut);
    but when i read it i get all the zerros

    ??

  11. #11
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    When you say "one byte at a time" you actually mean "one integer at a time" since an integer is more than one byte (usually 4 or 8).

    I see three problems in the code you've posted.

    1. You use sizeof(Int64) whereas you should use sizeof(int) (or even better sizeof(tmp[0]), which will be correct even if you change the type of tmp).

    2. i % 100 will be zero when i is 0, so fwrite will be executed once before tmp has been filled.

    3. i % 100 will NOT be zero when i is it's last value (9999) and so the last 100 values won't be output.

    EDIT: One way to fix it:
    Code:
        for (i = 0; i < 10000; i++) {
            tmp[i % 100] = i;
            if ((i + 1) % 100 == 0) {
                fwrite(&tmp, sizeof(tmp[0]), 100, fileOut);
            }
        }
    Last edited by oogabooga; 10-11-2012 at 02:14 PM.
    baxy likes this.
    The cost of software maintenance increases with the square of the programmer's creativity. - Robert D. Bliss

  12. #12
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    THANK YOU ; it is working now

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