filling array of pointers from file in c - c

Question

I want to read data which is written in fprintf(fp,"%s %s %s\n", p->name,p->surname,p->tc); format. I created struct patients **p in with
struct patients **create_array(struct patients **ptr,int length){
int i;
ptr=(struct patients **)malloc(length*sizeof(struct patients));
return ptr;
}
function above create array of pointers and give it to main. Main calls read_file() function to read data from file which is written in known format. But my data is not filled when i try to print them in main it prints meaningless things. I thought problem in reading data that's why i put only reading function. What is my problem? All suggestons are welcome.
#include<stdio.h>
struct patients
{
int importance;
char name[10], surname[10], tc[11];
};
FILE *file_opening(char x[])
{
return (fopen(x,"w+"));
}
writing_file (FILE *fp, struct patients *p)
{
fprintf(fp,"%s %s %s\n", p->name,p->surname,p->tc);
}
struct patients **read_file (FILE *fp,struct patients **p)
{
int i=-1;
do{
i++;
}while(fscanf(fp,"%s %s %s",p[i]->name,p[i]->surname,p[i]->tc) !=EOF);
return p;
}
void show_all_patients(struct patients **p, int start_index, int length){
int i;
for(i=start_index;i<length;i++)
printf("%s %s %s",p[i]->name,p[i]->surname,p[i]->tc);
}
struct patients **create_array(int length){
return (struct patients **)malloc(length*sizeof(struct patients));
}
int menu(void){
int choice;
printf("1)take patient\n2)show all patients\n3)exit");
scanf("%d",&choice);
return choice;
}
main(){
int i=0,j,choice,cured_patient=0,length=1;
FILE *fp;
struct patients **ptr;
char file_name[40]="patient_list.txt";
ptr=create_array(length);
fp=file_opening(file_name);
ptr=read_file(fp,ptr);
do{
choice=menu();
if(choice==1){
printf("%s %s %s\n",ptr[i]->name,ptr[i]->surname,ptr[i]->tc);
i++;
}
else if(choice==2){
show_all_patients(ptr,i,length);
}
}while(choice!=3);
for(j=i;j<length;j++)
writing_file(fp,ptr[j]);
fclose(fp);
}

Answer 1

Problems with the code:
There is no return value of writing_file. Add void as the return type.
void writing_file (FILE *fp, struct patients *p)
{
fprintf(fp,"%s %s %s\n", p->name,p->surname,p->tc);
}
You need #include <stdlib.h>.
Without that the return value of malloc is assumed to be an int, which can result in strange problems.
Syntax errors:
Given your definition of ptr, the lines
ptr=create_array(length);
and
ptr=read_file(fp,ptr);
are syntactically invalid. I get the following errors with gcc:
soc.c: In function ‘main’:
soc.c:58:7: error: assignment to expression with array type
ptr=create_array(length);
^
soc.c:60:7: error: assignment to expression with array type
ptr=read_file(fp,ptr);
If your compiler does not report errors on those two lines, it's time to use a different compiler.
Reading and writing to the same file
You are using the same FILE* to read from and write to. It's not clear whether you meant to do that or it was an error on your part.
When you open the file using:
return (fopen(x,"w+"));
the contents of the file are truncated. See http://en.cppreference.com/w/c/io/fopen for more info. Pay special attention to the table under Parameters. It says:
"w+" | write extended | Create a file for read/write | destroy contents | create new
If you want to just read the data from the file, use
return (fopen(x,"r"));
If you want to read the data from the file and write back the data to it, read it first using the above mode, close the file, then reopen it using:
return (fopen(x,"w"));
Handling the array of patients
I think you should use:
FILE* fp = NULL;
int length=10;
struct patients *ptr = NULL;
ptr=create_array(length);
fp=file_opening(file_name);
read_file(fp, ptr, length);
Adjust the rest of your code appropriately.
Add code to deallocate memory
Every call to malloc should have a corresponding call to free. I would add
void delete_array(struct patients *ptr)
{
free(ptr);
}
and call it from main before the end of the function.

Answer 2

Things that I would change.
Return value and input arguments of create_array.
The function can also be simplified to:
struct patients* create_array(int length)
{
return malloc(length*sizeof(struct patients));
}
Return value and arguments to read_file
struct patients* read_file (FILE *fp ,struct patients *p)
{
int i=-1;
do
{
i++;
} while(fscanf(fp, "%s %s %s",p[i].name, p[i].surname, p[i].tc) != EOF);
return p;
}
Make sure that you provide the maximum number of characters to be read. Otherwise, you might end up reading more than the array are capable of holding.
Change
} while(fscanf(fp, "%s %s %s",p[i].name, p[i].surname, p[i].tc) != EOF);
to (given the size of the arrays in your struct)
} while(fscanf(fp, "%9s %9s %10s",p[i].name, p[i].surname, p[i].tc) != EOF);