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i am trying to open two files, put their content in an array and write array back to the file. However, after i use fwrite function the destination file is empty. Could anybody explain how to achive my goal?

data.txt file content:

1
2
3

i.txt file content:

3
4
5

here is the code:

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
#include<unistd.h>

int main(void)

 FILE *fmain, *fnew, *fp;
 int i = 0,f = 0, length = 150, arr[length], chararr[length], sizearr;
 char line[130];
 int error;
 fmain = fopen("data.txt", "rw+");
 fnew = fopen("i.txt", "rw");

 while(fgets(line, sizeof line, fnew) != NULL)
 arr[f] = atoi(line);
 f++;
 
 fclose(fnew);
 // read data into array from data file

 while(fgets(line, sizeof line, fmain) != NULL)
 arr[f] = atoi(line);
 f++;
 

 fclose(fmain);

 fp = fopen("data2.txt", "w");

 fwrite(arr, sizeof(char), sizeof(arr), fp);


 fclose(fp);

 return 0;

when i manually open data2.txt after i run the program it is empty but i would like to see something like:

1
2
3
3
4
5

1. There is no "rw+" mode for fopen, you probably want the "r+" mode.


2. You need to check if fopen was successful.

Try this:

 ...
 fmain = fopen("data.txt", "r+");
 if (fmain == NULL)
 
 printf("Can'topen file.n"); exit(1);
 
 fnew = fopen("i.txt", "r+");
 if (fnew == NULL)
 
 printf("Can'topen file.n"); exit(1);
 
 ...

You have to first check if the file you opened is not returning NULL and then only you can write to the file. As in here, the size of the data is fixed, write the entire array into a file is using the binary writing mode:

fp = fopen("data2.txt", "wb");
if(fp == NULL)
 
 printf("Error!"); 
 exit(1); 
 
fwrite(arr, sizeof(char), sizeof(arr), fp);
fclose(fp);