Thread: Realloc problem

i am trying to use DMA to create an array . The array is to be filled with the position of free records(records that have been marked free for use using a flag with a value of 1 )

here is the code :

Code:

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


//----struct template---//
struct st_student 
{
    char name[20] ;
    int st_id ;
    int age ;
    char gender ; 
    int flag ; } ; 


int main(void)
{
    //--declare file pointer and open file--//
    FILE *FilePointer ; 
    FilePointer = fopen("students_flag.bin" ,"rb" ) ; 
    
    int i = 0 ; 
    int j = 0 ; 
    int * Dma_Array ; 
    int size_of_flags_array  ; 
    
    struct st_student student_records ; 
    
    
    if(FilePointer == NULL )
    {
        printf("error opening the students flag binary file \n " ) ; 
        exit (1) ; 
    }
    
    Dma_Array = (int * ) malloc( 0 * sizeof(int) ) ; 
    
    while(! feof (FilePointer))
    {
        if(ferror(FilePointer ) ) 
        {
            printf("Ferror on file \n " ) ; 
            exit (1) ; 
        }
        
        //---read the structure---//
        fread(&student_records , sizeof(struct st_student) , 1 , FilePointer ) ;
            
        //_______________________________print flags to screen for reference__________________________________________-//
        printf("flag is %d \n " , student_records.flag ) ; 
        
        //--if flag == 1 record is marked as free for use, copy position of free record to array ----//
        if(student_records.flag == 1)
        {
            Dma_Array = (int *) realloc(Dma_Array , (j+1)*sizeof(int)) ; 
            Dma_Array[j] = i ; 
            j++ ; 
        }//end if 
        i++;
    }//end while
    
    size_of_flags_array = (sizeof(Dma_Array) / sizeof(Dma_Array[0])) ; 
    
    //___________print size of array to screen for reference__________//
    printf("size of flags array is %d \n " , size_of_flags_array )  ; 
    
    //---print position of flags to screen----//
    for(i = 0 ; i < size_of_flags_array ; i ++)
    {
        printf("position of free records is %d \n " , Dma_Array[i] ) ; 
    }//end for 
    
    
    return 0 ; 
}//End of main

output to terminal is

Code:

 
flag is 0 
flag is 0 
flag is 1
flag is 0 
flag is 0 
flag is 1 
flag is 0 
flag is 0 
flag is 1
flag is 0 
flag is 0 
size of flags array is 1 
position of free records is 2

i think there is a problem with my realloc call not sure what though any help is great , thanks

hey im not sure what you mean by that could you expand a little ?

also i just noticed that if i hard code the

Code:

for(i = 0 ; i  < ; size_of_flags_array )

to

Code:

for(i = 0 ; i < 3 ; i ++)

it prints out the correct values . So possibly my problem is in getting the size of the arrray

Code:

size_of_flags_array = (sizeof(Dma_Array) / sizeof(Dma_Array[0])) ;

so possibly it is getting the size of the pointer (4 ) / size of an int (4)
giving 1 which is why its not printing out correctly.
??

Ah okay i understand now . thanks, I will change the malloc and when i realloc.
Thanks : )

yes if i use

Code:

for (i = 0 ; i < j ; i ++ )

it prints out the correct results
and that way im not just hardcoding how many flags there should be in.

in the question i will have to check again for free flags after they have been modified . So i was going to turn that into a function. to keep main less cluttered and it was save me writing out the same piece of code again . my question is if i make it a function when the function ends will the memory taken by malloc and realloc be marked as free by the os ? or would it be better to use malloc in the main and then pass the pointer to the function ?