Thread: My C Programming Files I/O are not working

Originally Posted by abmoh

i have looked at both codes. but unfortunately its a little bit too complicated for a beginner like me

It's good you are not using it since it is riddled with errors and bad style.

Originally Posted by abmoh

So that instead of my code showing this:

That's (probably) because you have a bug in your code concerning fscanf. Your compiler might be warning you about it; if not, you could try increasing the warning level. Either way, if you run the code with the debugging statements I added, you should see that there are plenty of unexpected read failures. Do you see that?

Your compiler should also be warning you about these lines:

Code:

inventory[i].varunummer = varunummer;
strcpy(inventory[i].namn, namn);
inventory[i].lagersaldo = lagersaldo;

I hinted that you should remove them because not only are they useless (the assignment to the values of inventory[i] should have been performed by the fscanf), but they are harmful: you are trying to reassign values to inventory[i], but you are using variables that have not even been initialised! Luckily, these lines probably never get executed because the fscanf call always fails.

Why are you not taking any of the advice being given to you? You need to read what I and (especially) laserlight have told you and make the indicated changes.

Originally Posted by abmoh

I did not try to run the code since i did not understand it.

JUST RUN IT! You might understand it after you see it.

Sigh, nevermind. This is wrong:

Code:

if(fscanf(fp, file, "%s %d %d", inventory[i].namn, &inventory[i].lagersaldo, &inventory[i].varunummer) == 3)

It is wrong because you provided an extra argument, i.e., the argument named file. fscanf's first parameter is the FILE pointer, which you provided correctly. The second parameter is the format string, but you provided the format string as the third argument instead.

As for those three lines: if you don't remove them, your code won't work, and I already explained why. If you really don't understand why, that's great: leave them there and see the problematic result, then you'll understand why.

if you take a good look at this you may see what LaserLight is speaking about in the usage of fscanf it has a specif set of rules that the format of the function in the parameters of the function for it to be matched in order for that function to work, just like the functions you wrote will not work if you do not follow that same format you wrote them to work. it all applies across the board.

C library function fscanf()

Take a good hard look at how fscanf is being used without the 'file' included in it.

Code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
struct storagemanipulation
{
    char namn[100];
    int lagersaldo;
    int varunummer;
     
};
// pass opened file into function for reading into struct array
int readFromFile(FILE *fp, int antal_varor, struct storagemanipulation *inventory)
{
    int i = 0;
     for ( ; i < antal_varor; i++)
        fscanf(fp,"%s%d%d",inventory[i].namn, &inventory[i].lagersaldo, &inventory[i].varunummer);
       
  // actually size gotten, it cannot over ride passed in size to get. 
  // so a means to actaully know how many records needed to be allowed
  //for is needed, or malloc to make extra space if it goes over given size
  return i; 
}
   
void saveToFile(FILE *fp, int antal_varor, struct storagemanipulation *inventory)
{    //make sure it is set to 
    //end of file before writing to it, 
    //to prevent over writing of data.
    
    fseek(fp, 0, SEEK_END);
    
    for (int i = 0; i < antal_varor; i++)
        fprintf(fp, "%s %d %d\n", inventory[i].namn, inventory[i].lagersaldo, inventory[i].varunummer);
        
}
 
 
int main (void)
{
    struct storagemanipulation inventory[4];
    char file[30], buffer[1024], name[100];
    int count = 0, size;
    FILE *fp;
     
     //gets user speified file name;
    printf("Enter filename\n");
    scanf(" %s", file);
     
     
     //Open for append; open 
     //(or create if the file does not exist) 
     //for update at the end of the file.
    if ((fp=fopen(file,"a+") ) == NULL )
          
         {
            printf("cannot create file\n");
           exit(1);
        }
          
        // check to see if file is empty
        // for first open if it is a fresh file. 
        // just to play it safe.
         
        fseek (fp, 0, SEEK_END); // go to end of file
        size = ftell(fp); // get size
 
 
        // put back to front of file just in case
        // that part all depends on how your file is, if it is a new file,
        //already has information in it, and you do not want to over write it
        // then you'll need to set it accordingly. 
 
                fseek(fp,0,SEEK_SET);
         
        if ( size == 0 )
        {
            printf("File is empty\n"
            "please add data to file\n");
                         
            while ( count < 4)
            {    
                 
                printf("enter name, item nuber, and number\n");
                 
                 
                fgets(buffer, sizeof buffer, stdin);
                    scanf(" %s%d%d", strcpy(inventory[count].namn,name), &inventory[count].lagersaldo,
                    &inventory[count].varunummer);
                count++;
              
            }        
          
        }
        else
        readFromFile(fp, 4, inventory);
         
        if ( size > 0)
        {
            // size_t instead of int
            // for poratabiltiy, because sizeof/sizeof returns 
            // long int on linux, and I've been told it 
            // can retrun a different data type on others
            // so size_t takes care of that. 
            size_t check = 0;
             
            //              returns actual size of array
            while ( check < sizeof(inventory)/sizeof(inventory[0]) )
            {
                printf("Varunummer: Namn %s\nLagersaldo %d varunummer %d\n",inventory[count].namn , inventory[count].lagersaldo, inventory[count].varunummer);
                check++;
            }
        }
        else
        {
            saveToFile(fp, 4, inventory);
            
            for ( int f = 0; f < 4; f++)
                printf("%s %d %d\n",  inventory[f].namn, inventory[f].lagersaldo,
                    inventory[f].varunummer);
        }
         
      
       fclose(fp);
        return 0;
 }

results

Code:

userx@slackwhere:~/bin
$ ./temp2
Enter filename
filedata
Varunummer: Namn name1
Lagersaldo 10 varunummer 10
Varunummer: Namn name1
Lagersaldo 10 varunummer 10
Varunummer: Namn name1
Lagersaldo 10 varunummer 10
Varunummer: Namn name1
Lagersaldo 10 varunummer 10
userx@slackwhere:~/bin
$ ./temp2
Enter filename
filedata1
File is empty
please add data to file
enter name, item nuber, and number
name30 30 30 
enter name, item nuber, and number
mane40 40 40 
enter name, item nuber, and number
mane50 50 50 
enter name, item nuber, and number
mane60 60 60

name30 30 30
mane40 40 40
mane50 50 50
mane60 60 60
userx@slackwhere:~/bin
$