My own implementation of the Unix sponge command: http://linux.die.net/man/1/sponge

sponge.c

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

typedef struct
{
  char *text;
  size_t len;
} filedata_t;

filedata_t *
read_stdin (void)
{
  filedata_t *result = malloc (sizeof (filedata_t));
  result->len = 0;
  size_t cap = 4096;
  result->text = malloc (cap * sizeof (char));
  for (;;)
    {
      int c = fgetc (stdin);
      if (feof (stdin))
        break;
      result->text[result->len] = (char) c;
      if (++result->len == cap)
        result->text = realloc (result->text, (cap *= 2) * sizeof (char));
    }
  result->text = realloc (result->text, (result->len + 1) * sizeof (char));
  result->text[result->len] = '\0';
  return result;
}

void
write_file (const char *file_name, const filedata_t * fdata)
{
  FILE *fp = fopen (file_name, "wb");
  fwrite (fdata->text, fdata->len * sizeof (char), 1, fp);
  fclose (fp);
}

int
main (int argc, char **argv)
{
  filedata_t *fdata = read_stdin ();
  if (argc == 2)
    {
      write_file (argv[1], fdata);
    }
  else
    {
      printf ("%s", fdata->text);
    }
  free (fdata->text);
  free (fdata);
}