fopen is not opening a file stream... how do I resolve this?

I am trying to write a file and append data to it.

Here is a snippet of my code. thresh is an unsigned short.

    FILE *fp_th;
    fp_th = fopen("threshold.txt", "a");
    printf("opening file failed: %s\n", strerror(errno));
fprintf(fp_th,"%d ", thresh);
    fclose(fp_th);

Before it has worked fine, but I've changed some of my code and all of a sudden it doesn't print out anymore.

I've confirmed that fopen is开发者_运维技巧n't opening a file stream with

printf(" check fp_th = %p \n", fp_th);

It prints out check fp_th = 00000000.

EDIT: Added printf("opening file failed: %s\n", strerror(errno)); after fopen. Output says no error. Does it lie? Odd..

The odd thing is that I am writing and appending a similar file, and yet that file works fine. How do you resolve a fopen that returns a NULL? Why does it happen? Thanks!

---

Firstly

fprintf(fp_th,"%d ", thresh);

Will normally buffer output until you close the file, or the internal FILE* buffer is full. You might not see any output in the file immediatly

You could fflush() the FILE* to have it written out to the file when you decide.

fprintf(fp_th,"%d ", thresh);
fflush(fp_th);

Secondly

printf(" check fp_th = %d \n", fp_th);

This does not check that fp_th is invalid. It just prints the pointer value of fp_th as a signed integer, and -7323824 might be as good as a value as any, this will be even less meaningful, and probably undefined if the size of your pointers are not the same as the size of an int.

To print a pointer you should use %p

printf(" check fp_th = %p \n", fp_th);

fopen returns NULL if it fails, you should check for that to learn if opening the file failed or not.

FILE *fp_th;
fp_th = fopen("threshold.txt", "a");
if(fp_th == NULL) {
   printf("opening file failed: %s\n", strerror(errno));
   return;
}

---

First, always check the return value of fopen for NULL, which means failure.

To find out what went wrong during the file open, you can use perror.

  if (fp_th == NULL) {
    perror ("Error opening threshold file");
    exit(EXIT_FAILURE);
  }

If you still cannot figure out while you get permission denied or No such file or directory, you can use strace (assuming Linux) and to look for the corresponding system call.

---

Before it has worked fine, but I've changed some of my code and all of a sudden it doesn't print out anymore.

If your code changed the current working directory, this can cause the file to not be found, as you're not providing a full path. In this case, fp_th should be a null pointer, which is something for which you could check. Your print suggests that it's not returning a null pointer, but rather did open the file. For details, see fopen.

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I had a similar problem today. My code was openning N-1 files but mysteriously crashed in the Nth file, although that file definitely existed ...

perror revealed that I actually never closed all these files -to my embarassment, I had forgotten fclose(fd)...

I didn't use perror so far, but I now realize how valuable it is. :)