I am new c-programmer.I am writing a small student database. I have an array of structs I would like to write it to file. Till now the program works fine. I can print out the data saved in the array called db (abbreviation of database). In order to be able to write data, I opened a new c-file and I wrote a method writeData()
that allows me to write data using FILE-object
and fopen
. Here are the methods of my database located in a header-file:
//header file
#ifndef DB_OPS
#define DB_OPS
#define SIZE 3typedef int bool;
typedef int bool;
#define true 1
#define false 0
struct student{
bool statusFlag;
char lastname[20];
char firstname[20];
int mNr;
char subject[30];
char nationality[20];
};
int createDb(int s);
struct student getData(char * lastname, char * firstname, int matNr, char * courseOfStudy, char * nationality);
void insert_student(struct student * st);
void update_student(int matNr);
bool delete_student(int matNr);
void display_result(bool res, bool operation);
bool search_student(int matNr);
void display_db();
void writeData();//method to write data
void readData();
void print();
#endif
then I defined the method writeData()
. Here is the code:
//a c-file
#include <stdlib.h>
#include <stdio.h>
#include "db_ops.h"
void writeData(){
//when i use fopen, the data saved in db will be damaged
FILE * fpw;
fpw=fopen("database.txt","wb");
if(fpw==NULL){
printf("the file cannot be opened");
exit(1);
}
extern struct student *db;
int i;
for(i=0;i<SIZE;i++){
printf("%s, %s, %d, %s , %s\n",
(db+i)->lastname,(db+i)->firstname,(db+i)->mNr, (db+i)->subject, (db+i)->nationality);
}
fclose(fpw);//When I use fclose(),I get segmentation fault: free(): invalid next size
}
Till now I am not able to find a solution or an explanation for that Problem. When I use fopen, I cannot find my data saved in the array db anymore. I do know if my array db which is a pointer is pointing outside of the array. The second problem consists in using fclose, which generates a segmentation fault.Any suggestions? Here is a piece of code located in a file where I initialized the array db:
// another c-file
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include "db_ops.h"
struct student * db;
struct student * ptrDb;
static int insertCounter=1;
int size=0;
int createDb(int s){
size=s;
db= (struct student *)calloc(3,sizeof(struct student *));
if(db==NULL){
printf("dynamic allocation failed");
return EXIT_FAILURE;
}
ptrDb=db;
printf("database was created\n");
}
struct student getData(char * lastname, char * firstname, int matNr, char * subject, char * nationality){
struct student st;
int i;
if(insertCounter<=size){
//get data
st.statusFlag=1;//flag to show, whether the program gets data or not
//0-byte marks the end of the string
memcpy(st.lastname,lastname,strlen(lastname)+1);
memcpy(st.firstname,firstname,strlen(firstname)+1);
st.mNr=matNr;
memcpy(st.subject, subject,strlen(subject)+1);
memcpy(st.nationality,nationality,strlen(nationality)+1);
//printf("%s,%s,%d,%s,%s\n",st.lastname,st.firstname,st.mNr,st.subject,st.nationality);
return st;
}else if(insertCounter>size){
st.statusFlag=0;
return st;
}
}
//coping input by reference
void insert_student(struct student * st){
printf("statusFlag:%d\n",st->statusFlag);
if(st->statusFlag==1){
*ptrDb=*st;
insertCounter++;
ptrDb++;
}else{
printf("##########################################################\n");
printf("no insert is possible, The maximum size has been reached\n");
printf("##########################################################\n");
}
}
Why
calloc(3,sizeof(struct student *))
is for struct pointer but not struct member? It seems has to becalloc(3,sizeof(struct student))
for 3 structure members.