Reading hex

**jrfall349** · 04-20-2007

Okay, I've got a small problem going on. Here goes.

Code:

struct CD {
  unsigned int ID;
  char *Title;
  char *Artist;
  char *Genre;
  int  Year;
  int  Time;
  int  Number_Tracks;
  struct Songs *Track_Ptr;
};

void dbload()
{
  char dbname[MAXFILELEN];
  FILE *fp1;
  int cdnum;
  int i;
  struct CD CDray, *ptr;
  
  ptr = &CDray;
  
  printf("Enter file to load: ");
  scanf("%s", &dbname);
  
  if ((fp1 = fopen(dbname, "r")) == NULL)
  {  
    printf("Cannot open file\n");
    return;
  }

  fscanf(fp1, "%x\t", ptr->ID);
  printf("%d",&ptr->ID);
                    
  printf("Load successful\n");
}

ID inside the file should be a hex number, for example: 820c100a
I know %x should read it in, but it crashes each time at that point. Also, when I print it, I want to print it as a hex number, which should again be %x unless I'm mistaken.
Any ideas why I'm crashing?

**Dave_Sinkula** · 04-20-2007

Code:

fscanf(fp1, "&#37;x\t", ptr->ID);

warning: format argument is not a pointer (arg 3)

And there are better ways to obtain a string than this:

Code:

  scanf("%s", &dbname);

See the FAQ.

**jrfall349** · 04-20-2007

I'm not concerned about the string grab, as it is working.
I'm not sure what you mean with the pointer...

**MacGyver** · 04-20-2007

Originally Posted by jrfall349

I'm not concerned about the string grab, as it is working.

A "works for me" attitude is not a good one to have. Using scanf() with that format string is dangerous and can lead to your program being taken over by a malicious user.

Originally Posted by jrfall349

I'm not sure what you mean with the pointer...

ptr->ID is an int. You need to be passing the address of an int to scanf() in this case or else it will write to the wrong location in memory.

**jrfall349** · 04-20-2007

I can't believe I forgot that frackin '&' again! Thanks. And I meant I wasn't concerned about the string for now.... But thanks for the heads up on it anyway.

**MacGyver** · 04-20-2007

To read a string safely, try the following:

Declare a new int variable local to your function. Let's call it iEnd.

Replace this line:

Code:

scanf("&#37;s", &dbname);

With this:

Code:

fgets(dbname,sizeof(dbname),stdin);
iEnd = strlen(dbName) - 1;
if(dbname[iEnd] == '\n')
{
	dbname[iEnd] = '\0';
}

Your program should be safe.

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