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I am trying to finally learn C but I am getting stuck with some stupid things. In this case, what I am trying to achieve, is to create a program that will manage phone contacts. The "tool" allows to insert a contact or delete and search for contacts. It uses a binary file in C and I configured the code so it will work with records and fields of variable length. In this case, my records are in the following format (there is a header that points to the last deleted record):

record_length|name|phone|

My current requirement is that when I choose to delete a record, I will overwrite the record contents with:

record_length * next_record*

This token * identifies that the record status is deleted. Until this point, I am ok with the program. The problem is about a last function of the program, which is compress file. I want to add a function that will read the whole file and copy the VALID records to a new file, removing any unused space (for example, if I delete a record with length of 20 and later or if I overwrite the record with a new one with length 10, the process will keep the last 10 junk chars in the file). The function is actually pretty simple, the basic idea is:

  • Go to the beginning of my file and read the header to store it in a temporary variable
  • fread from file with sizeof(int) to get the record length until fread returns NULL (end of file)

  • fread one single character

    • if this char == *, I want to skip the record and move to the next
    • else, go back 1 position in the file and copy the valid chars of the record to the new file.

    And now comes the stupid question: how do I verify, wether the character is an asterisk? I am so frustrated because I was able to implement the whole code, the only thing that I am missing is this this implementation. Here is what I am trying to do:

    #define MAX_REGI_SZ 70
int main (void) {
  char registro[MAX_REGI_SZ];
  ...
  ...
  ...
  fread(registro,sizeof(char),1,in);                   
  if (strcmp(registro,"\*")) {                         
     /* move to the next record */
  }                                                    
  else {
    /* valid record, copy it over to the new file */
  }                        
...
}

    I have already tried to compare with "*", tried to declare a char token and then make token = "*" and then use token in the strcmp, tried to compare registro == "*" directly, this is simply impossible to achieve. I have been trying to find a solution to this for days and haven't found anything anywhere, so I am giving up and asking here to the masters of C.

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Rodrigo
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  • Yes, and it gives me "Operation between types "char" and "char*" is not allowed." – Rodrigo Oct 12 '15 at 20:07
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    Also make sure you use single quotes around `*` for it to considered a `char` instead of `char*` – jayant Oct 12 '15 at 20:11
  • How can you `strcmp` the input when you only input 1 character? The var `registro` is uninitialised, has no string terminator, so it's **undefined behaviour**. – Weather Vane Oct 12 '15 at 20:20
  • hmmm, i didn't know of that. well, for my surprise, i just understood strcmp. at first i thought it would return 1 in case of both strings being the same so now i tried this: if (!strcmp(registro,"\*")) { instead of this: if (strcmp(registro,"\*")) { i haven't noticed the slight detail of using double quote or single quote, i will try to use '*' instead of "\*" later. i just don't get the difference of both cases – Rodrigo Oct 12 '15 at 20:27

1 Answers1

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Have you tried using fgetc(). 

#define MAX_REGI_SZ 70

int main (void) 
{
   char registro[MAX_REGI_SZ];
   ...
   ...
   ...
   registro[index]=fgetc(/*Your file pointer*/)                 
   if (registro[index] == '*') // or if (registro[index] == 42)
   {                         
       /* move to the next record */
   }                                                    
   else 
   {
     /* valid record, copy it over to the new file */
   }                        
   ...
  }

Note instead of if(registro[index] == '*' you could also use if(registro[index] == 42) that is compare it with the ASCII value of the character.ASCII Values Link

Mutex202
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