C appending to file after writing

Question

I'm testing out the basic functions to operate files with.

I try to first open/close a file to create it, and then open/close it again to append to it. Lastly, I print out what is in the file.

My code currently looks like the following:

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

int main()
{
  FILE * file;
  char mark;

  /* WRITING: */
  file= fopen("goodbye.c","w");
  if(!file)
    { printf("Couldn't open file.\n");
      exit(EXIT_FAILURE); }
  printf("Enter data to write to .c file:");
  while((mark= getchar())!=EOF)
    {
      putc(mark,file);
    }
  fclose(file);

  /* APPENDING: */
  file= fopen("goodbye.c","a");
  if(!file)
    { printf("Couldn't open file.\n");
      exit(EXIT_FAILURE); }
  char add;
  scanf("%c",add);
  putc(add,file);
  fclose(file);

  /* READING: */
  file= fopen("goodbye.c","r");
  if(!file)
    { printf("Couldn't open file.\n");
      exit(EXIT_FAILURE); }
  while((mark= getc(file))!= EOF)
    {
      printf("%c",mark);
    }
  fclose(file);
}

With this, I'm not able to append to the file. When using getchar(), I type ctrl+d once finished writing in the first place. After this it goes on to printing out what I just wrote, not giving me the chance to append to the file. Does ctrl+d somehow interrupt with scanf? And how to get the result that I was looking for?

Answer 1

Your code only allows you to append a single character to the file, which is a little stingy. It can also (at least in theory) lead to problems on some systems if the last line of the text file does not end with a newline, which it won't if you add something other than a newline. Maybe you need a loop to read multiple characters?

Also, since you don't stop the initial input until EOF, you need to clear the 'error' on stdin with clearerr(stdin) to allow further input to occur. This works correctly on Mac OS X 10.10.1 Yosemite; it should work the same on other Unix systems. I can't answer confidently for Windows-based code unless it is using something like Cygwin to simulate Unix, but I expect it would work in much the same way there, too, even with MSVC.

Incidentally, my compiler complains about a missing & in the call to scanf() at:

char add;
scanf("%c",add);

If your compiler doesn't complain, either turn up the warning level or get a better compiler.

This code works as I'd expect:

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

int main(void)
{
    FILE *file;
    char mark;

    /* WRITING: */
    file = fopen("goodbye.c", "w");
    if (!file)
    {
        printf("Couldn't open file.\n");
        exit(EXIT_FAILURE);
    }
    printf("Enter data to write to .c file:");
    while ((mark = getchar()) != EOF)
    {
        putc(mark, file);
    }
    fclose(file);
    printf("EOF 1\n");

    /* APPENDING: */
    file = fopen("goodbye.c", "a");
    if (!file)
    {
        printf("Couldn't open file.\n");
        exit(EXIT_FAILURE);
    }
    clearerr(stdin);
    char add;
    while (scanf("%c", &add) == 1)
        putc(add, file);
    fclose(file);
    printf("EOF 2\n");

    /* READING: */
    file = fopen("goodbye.c", "r");
    if (!file)
    {
        printf("Couldn't open file.\n");
        exit(EXIT_FAILURE);
    }
    while ((mark = getc(file)) != EOF)
    {
        printf("%c", mark);
    }
    fclose(file);
    return 0;
}

The only substantive changes are adding a loop around the scanf() — though frankly it would be better to use getchar() again, like in the first input loop — fixing the call to scanf(), adding the two printf() statements that report when EOF is detected, and including clearerr(stdin); to allow input to continue.

Sample output

Code without clearerr(stdin):

Enter data to write to .c file:Happiness is a bug-free program.
Happiness is seldom attained.
EOF 1
EOF 2
Happiness is a bug-free program.
Happiness is seldom attained.

Code with clearerr(stdin):

Enter data to write to .c file:Happiness is a bug-free program.
Happiness is seldom attained.
EOF 1
But it helps when you add the clearerr(stdin) to this one.
EOF 2
Happiness is a bug-free program.
Happiness is seldom attained.
But it helps when you add the clearerr(stdin) to this one.