Read the first integer in a binary file

Question

Having a binary file that holds:

# hexdump file.pak 
0000000 09 00 00 00 
0000004

trying to read it with fread results in :

int main(void){
  char *filename = "file";
  FILE *fh = header_open(filename);
  header_init(fh);
  header_read_num_files(fh);
  header_close(fh);
}

FILE *header_open(char *pkg_file){
  FILE *fh;
  // open file with binary/read mode                                                                                                                                               
  if ( (fh = fopen(pkg_file, "ab")) == NULL){
    perror("fopen");
    return NULL; // todo: ...                                                                                                                                                      
  }

  return fh;
}

int header_read_num_files(FILE *fh){
  fseek(fh, 0L, SEEK_SET);
  char c;
  fread(&c, sizeof(char), 1, fh);
  fprintf(stderr,"buff received: %d\n", c);
}

/* write first 0 for number of files */
void header_init(FILE *fh){
  unsigned int x= 9;
  fseek(fh, 0, SEEK_SET);
  fwrite( (const void*)&x, sizeof(int), 1, fh);
}


output: buff received: 112

My system uses the small-endianness conversion. but still the other bytes are set to zero, I cannot really see why I'm getting this output.

An explanation is much appreciated.

You need to check the return values of these functions to make sure they succeeded. — David Schwartz
– David Schwartz, Commented Feb 16, 2012 at 21:58
You are opening the file in "append" mode, not "read" mode. You have to pass "rb" as second argument to fopen! — Gandaro
– Gandaro, Commented Feb 16, 2012 at 22:17
@Gandaro, I've been trying to avoid reopening the file for efficiency, do you think this is relevant? what's the point of the `a' flag then? — Taher
– Taher, Commented Feb 16, 2012 at 22:25
It is used to append stuff to files?! Appending and reading are two completely different things… — Gandaro
– Gandaro, Commented Feb 16, 2012 at 22:30

mikithskegg · Accepted Answer · 2012-02-16 22:29:50Z

1

You open the file with "ab" option. Bur this option does not allow to read from file, only writing to its end is alloowed. Try to open this way

fh = fopen(pkg_file, "w+b")

answered Feb 16, 2012 at 22:29

mikithskegg

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1 Comment

Taher Over a year ago

Thanks a bunch! though I used a+b instead since I don't my file truncated if it exists.

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