C Can't read the last property in struct properly - c

Question

I'm having trouble getting the last property in a struct.
This is my struct
#define MAX_FILE_NUM 64
#define MAX_FILE_NAME_LENGTH 15
typedef struct
{
char file_name[MAX_FILE_NAME_LENGTH]; //file name
int size; //size of the file
int used; //0 represents the file is not used and 1 represents the file is used
int block; //the index of data block that stores the file
int block_count; //the number of blocks the file used
int file_descriptor_count; //the number of file descriptors are referring to the file
}file_detail;
Assuming the code update the directory info at i = 0
file_detail * directory_block_ptr = (file_detail *)malloc(4096);
int create(char *name){
if(name == NULL || find_file(name) != -1 || strlen(name) > MAX_FILE_NAME_LENGTH || super_block_ptr->files >= MAX_FILE_NUM){
printf("%s\n", "ERROR: file create condition not satified");
return -1;
}
//create the file
for(int i = 0; i < MAX_FILE_NUM; i++){
//update directory info
if((directory_block_ptr+i)->used == 0){
strcpy((directory_block_ptr+i)->file_name, name);
(directory_block_ptr+i)->size = 0;
(directory_block_ptr+i)->file_descriptor_count = 0;
(directory_block_ptr+i)->used = 1;
(directory_block_ptr+i)->block = -1;
(directory_block_ptr+i)->block_count = 0;
printf("%d\n", (directory_block_ptr+i)->file_descriptor_count);//correct output 0
break;
}
}
printf("%d\n", (directory_block_ptr+0)->file_descriptor_count);//incorrect output
//open the file
int result = fs_open(name);
if(result != -1)
{
printf("%s: %s %s\n", "File", name, "created successfully");
return 0;
}
return -1;
}
But when I'm trying to access the file_decsriptor_count outside of the for loop, I get random numbers like 1868958256.
create("test");

Answer 1

I'm going to go out on a limb here and propose that the memory from malloc is uninitialized, and that's what's going on here.
In the loop (and rewriting into proper array format as suggested by #Gerhardh):
for (int i = 0; i < MAX_FILE_NUM; i++) {
//update directory info
if( directory_block_ptr[i].used == 0) {
// assign stuff
printf("%d\n", directory_block_ptr[i].file_descriptor_count);//correct output 0
break;
}
}
The OP says they get the correct value, but we don't know that it's the [0] index, right? If there's junk in position directory_block_ptr[0].used, then that position will not be touched and the loop will move onto the next one. We just don't know which one is being found.
Recommend to OP: change the printf in the loop to be:
printf("%d set in position [%d]\n", directory_block_ptr[i].file_descriptor_count, i);
and see if it's telling you position [0] or [1] or whatever. My guess is not-zero.
Recommend to OP, change your initialization at the top to be:
#define DIRECTORY_BLOCK_SIZE 4096
file_detail *directory_block_ptr = malloc(DIRECTORY_BLOCK_SIZE);
memset(directory_block_ptr, 0, DIRECTORY_BLOCK_SIZE);
This allocates the same amount of memory, but insures that it's all set to zero, then run it again.
Also, this is more a matter of style than of substance, I'd do the loop this way by defining a much simpler variable name for access throughout the loop, it's easier to follow.
for(int i = 0; i < MAX_FILE_NUM; i++){
file_detail *fd = &(directory_block_ptr[i]); // shorthand
//update directory info
if(fd->used == 0){
strcpy(fd->file_name, name);
fd->size = 0;
fd->file_descriptor_count = 0;
fd->used = 1;
fd->block = -1;
fd->block_count = 0;
printf("%d set in position [%d]\n", fd->file_descriptor_count, i);
break;
}
}