2

I've searched all I could and I'm actually not sure how to look for an answer as it's pretty specific.

I have a function that checks if a file exists and if it doesn't, create it and writes some parameters in it. After that another function adds some data in the file. But the first line of data is missing the number of characters that are in the line created with the file.

The code looks like this (I'm only putting the parts I think are relevant, as it's pretty messy) :

the function that checks if the file exists is :

int check_datafile(char *fname, Par *par){
  char filename[64] = "data/";

  strcat(filename, fname);
  FILE* fdesc = NULL;

  fdesc = fopen(filename, "r");
  if (fdesc == NULL){
  fdesc = fopen(filename, "w");
  fprintf(fdesc,"%d %.3f \n", par -> L, par -> t);
  close(fdesc);
  }
  return 1;
}

Then the function that writes is : 

void result_block(Par *par, double M, double M2, double M4, int ntot, char *fname)
{
  char filename[64] = "data/";

  strcat(filename, fname);
  FILE* fichier = NULL;

  fichier = fopen(filename, "a");

  if (fichier != NULL) // File should already exist
  {
    fprintf(fichier, "%.3f %.3f %.3f\n", M/ntot, M2/ntot, M4/ntot);
    fclose(fichier);
  }
  else
  {
    printf("Problem with the file : %s\n", filename);
    exit(0);
  }
}

They are called by

int initialize_mc(Par *par, int *spin) {
  int i, L2 = par->L * par->L;
  double T = par -> t;
  char *f2;
  double ex[1];
  ex[0] = exp(-2/T);

  if (!par->L) {
    printf("Give system size N!\n");
    return 0;
  }

  init_ran(par->seed);

  sprintf(fname, "%3.3d_%5.3f", par->L, par->t);

  check_datafile(fname, par);

  mc(par, spin, ex);

  return 1;
}

And the function result_block is called in the function mc.

Typically, I want the file to look like this :

16 2.210 
205.412 43371.160 2010463301.344
201.876 42319.600 1951381846.720
198.396 40897.632 1836904396.032
197.652 40743.856 1833699088.000
...

And it looks like this :

16 2.210 
371.160 2010463301.344
201.876 42319.600 1951381846.720
198.396 40897.632 1836904396.032
197.652 40743.856 1833699088.000
...

The the first line of data is cut by the same amount of characters that is in the first line of the file.

What can cause this problem?

StuartLC
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muffinman
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    include the whole program with main – amdixon May 31 '15 at 03:45
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    You are not using any kind of warning, `close()` is for file descriptors, it takes an `int` as the parameter, you need `fclose()`, that's just the first observation. Also, pass an open `FILE *` instead of openinig it in the function where you write to it. – Iharob Al Asimi May 31 '15 at 03:51
  • Why are you doing file I/O at all? Just write to stdout and redirect the output at runtime. – William Pursell May 31 '15 at 05:25
  • Thanks, it works now... @WilliamPursell I'm not very good at coding, I actually didn't understand your question. – muffinman May 31 '15 at 15:00
  • I mean, rather than dealing with opening the file inside your program, just let the shell do it. Instead of `./my-program`, write `./my-program > output-file` – William Pursell May 31 '15 at 19:06

1 Answers1

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close(fdesc); needs to be fclose(fdesc);. When you use FILE * you are implicitly using a buffer on your output. You need to call fclose so that the buffer gets flushed.

By calling close you are actually casting your pointer to an int and closing some random file descriptor (which presumably fails most of the time). You're not closing the FILE * at all.

Ewan Mellor
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    @muffinman You're welcome ;-) You should enable warnings in your compiler, as many as you can. That would have flagged the pointer-to-int conversion. I don't ever write C code without almost all the warnings turned on first, for exactly this reason. Also, the warnings are a good guide to what people consider "good style" and it's worth the time to understand why. – Ewan Mellor May 31 '15 at 18:23