Thread: Replacing a text file in C

http://www.rt.com/man/fopen.3.html

Using fopen() with mode "w" is correct. If the file isn't updating then there must be something wrong in the rest of your code.

That depends. w is for write. Not update:

w

Truncate file(1,n) to zero length or create text file(1,n) for writing.
The stream is positioned at the beginning of the file.

It depends on the OP's definition of "update".

Quzah.

No. Because if you try to open something that isn't there, fopen has failed, and has opened nothing. However, with the second fopen, you still should be testing to make sure it isn't null (ie: fopen has not failed).

Quzah.

Hmm. I read caduardo21's post differently.

Originally Posted by caduardo21

Another question. This one is tricky for me...
If I try to open a file and it doesn't exist, my FILE pointer will be equal to NULL. Do I have to close the first stream before opening a new one? Here's the situation:

Code:

points = fopen("points.txt", "r");
if(points == NULL)
	create_points_file();
        /* should I fclose(points) here? */

points = fopen("points.txt", "r");
while(fgets(line, sizeof(line), points) != NULL)
	printf("%s", line);
fclose(points);

Essentially, if it opened, close it.

Code:

#include <stdio.h>

int main(void)
{
   static const char filename[] = "myfile.txt";
   FILE *file = fopen(filename, "r");
   if ( !file )
   {
      printf("creating %s\n", filename);
      file = fopen(filename, "w");
      if ( file )
      {
         int i;
         for ( i = 0; i < 5; ++i )
         {
            fprintf(file, "data #%d\n", i + 1);
         }
         fclose(file);
      }
   }
   file = fopen(filename, "r");
   if ( file )
   {
      char line[80];
      while ( fgets(line, sizeof line, file) )
      {
         fputs(line, stdout);
      }
      fclose(file);
   }
   else
   {
      puts("WTF???");
   }

   return 0;
}

/* my output
C:\Test>Test
creating myfile.txt
data #1
data #2
data #3
data #4
data #5

C:\Test>Test
data #1
data #2
data #3
data #4
data #5
*/

[edit]Standing corrected by the subsequent post.

D'oh! If I'd only heed my own advice. Yet another possible way(?).

Code:

#include <stdio.h>

int main(void)
{
   static const char filename[] = "myfile.txt";
   char line[80];
   FILE *file;
   while ( (file = fopen(filename, "r")) == NULL )
   {
      printf("creating %s\n", filename);
      file = fopen(filename, "w");
      if ( file )
      {
         int i;
         for ( i = 0; i < 5; ++i )
         {
            fprintf(file, "data #%d\n", i + 1);
         }
         fclose(file);
      }
   }
   while ( fgets(line, sizeof line, file) )
   {
      fputs(line, stdout);
   }
   fclose(file);
   return 0;
}

But you may want to double-check!