CODEWITHSUNDEEP
c
maria
maria

Reputation: 33

c programming - Extracting string of data from file

I have the following task: I have a file (card) with 5 strings:

U98_25984nhdrwedb \n
U98_5647BGFREdand \n
U98_30984bgtjfYTs \n
U77_76498375nnnnn \n
U98_83645bscdrTRF \n

I need to extract to another file image.txt those strings starting with "U9". The below code without the memory assignment (malloc, calloc) print out the codes correctly to the screen, but it does not print the correct data to the image.txt, where I only get "98_25984nhdrwedb@". I think I am applying the memory allocation incorrectly, but when I use malloc or calloc (before the while loop) it gets worse and print out garbage and I cannot figure out how to set this correctly.

#include <stdio.h>
#include <stdlib.h>
#include <getopt.h>
#include <stdint.h>
    
typedef uint8_t  BYTE;
    
int main()
{
    FILE *input_card = fopen("card","r");    //open the file for reding
    BYTE data[18];
    int i, n = 5;

    FILE* output = fopen("image.txt","w");    //open the output file for writing

    output = malloc(sizeof(data)*18);         //assign memory
    while (!feof(input_card))
    {
        for (i = 1; i <= n; i++)
        {
            fread(data,sizeof(BYTE),18,input_card);
            if(data[i] != 0)
            {
                if (data[0] == 'U' && data[1] == '9')
                {   
                    printf("data: %s",data);
                    fwrite(&data[i],sizeof(BYTE),18,output);
                }
                fclose(output);
            }
        }
    }
    fclose(input_card);
    free(output);
    return 0;
}

Upvotes: 1

Views: 282

Answers (2)

Amin M. Dodin
Amin M. Dodin

Reputation: 136

In the following 2 lines from your code, the second line is incorrect:

FILE* output = fopen("image.txt","w");    //open the output file for writing
output = malloc(sizeof(data)*18);         //assign memory <= This is WRONG

The variable output is a FILE pointer. You should not allocate it using malloc. You should only use it if fopen returns it successfully, which means it has already been allocated by fopen.

This means you don't need this:

free(output); // This is also WRONG

Because this already freed the pointer's allocated data:

fclose(output);

Upvotes: 2

cultab
cultab

Reputation: 51

fopen() returns a FILE pointer so when you try to allocate memory (using malloc, that also returns a pointer) you're replacing the pointer to the FILE with something that points to memory instead of the file. Removing

output = malloc(sizeof(data)*18);         //assign memory

should make it just work.

Upvotes: 2

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