printing error using fprintf

**ahming** · 05-26-2005

i don't understand the difference between using

fprintf and printf for error statements.

e.g.

Code:

void mergeSort(float array[], int size)
{
  int* tmpArrayPtr = (int*)malloc(size*sizeof(int)); 

  if (tmpArrayPtr != NULL) 
  {
    mergeSortRec(array, size, tmpArrayPtr);
  }
  else
  {
    fprintf(stderr, “Not enough memory to sort list.\n”);
    exit(1);
  }

  free(tmpArrayPtr);
}

as to ->

Code:

void mergeSort(float array[], int size)
{
  int* tmpArrayPtr = (int*)malloc(size*sizeof(int)); 

  if (tmpArrayPtr != NULL) 
  {
    mergeSortRec(array, size, tmpArrayPtr);
  }
  else
  {
    printf(“Not enough memory to sort list.\n”);
    exit(1);
  }

  free(tmpArrayPtr);
}

**FillYourBrain** · 05-26-2005

fprintf allows you to tell it where to print it. In this case, it prints it to stderr rather than stdout. Normally, you see stderr and stdout going to the same place (the screen) but they can be redirected to different places such as a file.

**itsme86** · 05-26-2005

printf() prints to stdout, and fprintf() you used prints to stderr. In the good old days, programs would write output to stdout that a majority of the time people wanted to store in a file. Take UNIX's sort command for instance. It writes the sorted data to stdout so a lot of people would do something like sort > somefile.txt. That worked fine unless there was an error. Redirecting output using > only redirects stdout. So by printing to stderr, the error messages would go to the terminal instead of getting stuck in the file.

**ahming** · 05-26-2005

oohhh i see. so fprintf gives you more control over where it is being directed to either stderr or stdout. even though printf uses stdout, i don't see how it'd get stuck in a file, as it always prints to the screen?

**itsme86** · 05-26-2005

But stdout doesn't print on the screen if you redirect it. Look:

Code:

itsme@itsme:~/C$ cat spoon.c
#include <stdio.h>

int main(void)
{
  printf("I like spoons.\n");
  fprintf(stderr, "You like spoons.\n");

  return 0;
}
itsme@itsme:~/C$ ./spoon
I like spoons.
You like spoons.
itsme@itsme:~/C$ ./spoon > spoon.txt
You like spoons.
itsme@itsme:~/C$ cat spoon.txt
I like spoons.
itsme@itsme:~/C$

If I didn't fprintf() to stderr on the second line then it would have gotten stuck in spoon.txt along with the first line.

**FillYourBrain** · 05-26-2005

itsme86 showed how to do it in his post (in bold)

edit: never mind

**itsme86** · 05-26-2005

Look at this:

Code:

itsme@itsme:~/C$ cat list.txt
carrot
apple
donkey
banana
itsme@itsme:~/C$ sort list.txt
apple
banana
carrot
donkey
itsme@itsme:~/C$ sort list.txt > sortedlist.txt
itsme@itsme:~/C$ sort list.duh > sortedlist.txt
sort: open failed: list.duh: No such file or directory
itsme@itsme:~/C$

If sort printed the error to stdout it would have just been thrown in sortedlist.txt in that last attempt.

**dwks** · 05-26-2005

So in other words, you should use printf() for regular ouput, and fprintf(stderr, "error") for error messages.

Note that

Code:

printf("Hello, World!\n");

Is the same as

Code:

fprintf(stdout, "Hello, World!\n");

They both print to stdout.

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