adohertyd adohertyd - 3 months ago 7
C Question

Copying data from one text file to another in C

I'm writing a basic program that copies a string from an existing text file and copies the text into a new text file. I'm almost there but I'm having a few small issues. First, I output the line of text to the screen after copying and it's giving me 3 random characters after the string. I want to know why this is happening. Also, the program is creating the new text file but not putting the string into the file.

Here's my code:

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

int main(void)
{
char content[80];
char newcontent[80];

//Step 1: Open text files and check that they open//
FILE *fp1, *fp2;
fp1 = fopen("details.txt","r");
fp2 = fopen("copydetails.txt","w");

if(fp1 == NULL || fp2 == NULL)
{
printf("Error reading file\n");
exit(0);
}
printf("Files open correctly\n");
//Step 2: Get text from original file//
while(fgets(content, strlen(content), fp1) !=NULL)
{
fputs (content, stdout);
strcpy (content, newcontent);
}
printf("%s", newcontent);
printf("Text retrieved from original file\n");

//Step 3: Copy text to new file//
while(fgets(content, strlen(content), fp1) !=NULL)
{
fprintf(fp2, newcontent);
}
printf("file created and text copied to it");
//Step 4: Close both files and end program//
fclose(fp1);
fclose(fp2);
return 0;
}

Answer

You need to change:

while(fgets(content, strlen(content), fp1) !=NULL)

you need the sizeof the array content, not the length.

while(fgets(content, sizeof(content), fp1) !=NULL)

Even if you had initialised content before using it strlen() would return 0 and you would have read nothing from the file.

Also, if you want to re-read the input file when writing the new file you need to either fclose() the input file and fopen() it or rewind() it.

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