May I have some problem with memory allocated - c

Question

I have this code and after the first write in the array(successful data write ) then we get null as message and I don't know why.
I have tried many things, please help me.
my code is:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct course {
int marks;
char *subject;
};
int main(int argc, char *argv[])
{
struct course *ptr;
int i, noOfRecords;
printf("Enter the number of records: ");
scanf("%d", &noOfRecords);
//Memory allocation for noOfRecords structures
ptr = (struct course *)malloc(noOfRecords * sizeof(struct course));
ptr->subject=(char*)malloc(20*sizeof(char));
for (i = 0; i < noOfRecords; ++i) {
printf("Enter the name of the subject and marks respectively:\n");
scanf("%s %d", (ptr + i)->subject, &(ptr + i)->marks);
}
printf("Displaying Information:\n");
for (i = 0; i < noOfRecords; ++i){
printf("%s\t%d\n", (ptr + i)->subject, (ptr + i)->marks);
}
return 0;
}

Answer 1

As stated in question comments, you need to allocate memory for ALL subject members, not just the first:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct course {
int marks;
char *subject;
};
int main(int argc, char *argv[])
{
struct course *ptr;
int i, noOfRecords;
printf("Enter the number of records: ");
scanf("%d", &noOfRecords);
//Memory allocation for noOfRecords structures
ptr = (struct course *)malloc(noOfRecords * sizeof(struct course));
for (i = 0; i < noOfRecords; ++i) {
printf("Enter the name of the subject and marks respectively:\n");
(ptr + i)->subject = (char*) malloc(20*sizeof(char));
scanf("%s %d", (ptr + i)->subject, &((ptr + i)->marks));
}
printf("Displaying Information:\n");
for (i = 0; i < noOfRecords; ++i) {
printf("%s\t%d\n", (ptr + i)->subject, (ptr + i)->marks);
}
return 0;
}
Also, I suggest you to:
Use a #define for subject max length
Always check user input or limit it (truncate or force to retype). You might get unexpected behaviors if user inserts more than 19 chars plus \0 for subject.
Use typedef struct {...} course;. It will be much easier to handle arrays.
Remember to free the memory you claimed.
As a result:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
// From https://stackoverflow.com/a/17387990/2928168
#define MAX_SUBJECT "20"
typedef struct {
int marks;
char *subject;
} Course;
int main(int argc, char *argv[])
{
Course *courses;
int i, noOfRecords;
printf("Enter the number of records: ");
scanf("%d", &noOfRecords);
// Memory allocation for noOfRecords structures
courses = (Course *) malloc(noOfRecords * sizeof(Course));
for (i = 0; i < noOfRecords; ++i) {
printf("Enter the name of the subject and marks respectively:\n");
courses[i].subject = (char*) malloc(atoi(MAX_SUBJECT)*sizeof(char));
scanf("%" MAX_SUBJECT "s %d", courses[i].subject, &(courses[i].marks));
//fgets(courses[i].subject, sizeof(buf), stdin);
}
printf("Displaying Information:\n");
for (i = 0; i < noOfRecords; ++i) {
printf("%s\t%d\n", courses[i].subject, courses[i].marks);
free(courses[i].subject);
}
free(courses);
return 0;
}
Edit
You can even use the trick from https://stackoverflow.com/a/6671729/2928168 to increase performance:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define MAX_SUBJECT 20
#define Q(x) #x
#define QUOTE(x) Q(x)
#define MAX_SUBJECT_STR QUOTE(MAX_SUBJECT)
typedef struct {
int marks;
char *subject;
} Course;
int main(int argc, char *argv[])
{
Course *courses;
int i, noOfRecords;
printf("Enter the number of records: ");
scanf("%d", &noOfRecords);
// Memory allocation for noOfRecords structures
courses = (Course *) malloc(noOfRecords * sizeof(Course));
for (i = 0; i < noOfRecords; ++i) {
printf("Enter the name of the subject and marks respectively:\n");
courses[i].subject = (char*) malloc(MAX_SUBJECT*sizeof(char));
scanf("%" MAX_SUBJECT_STR "s %d", courses[i].subject, &(courses[i].marks));
//fgets(courses[i].subject, sizeof(buf), stdin);
}
printf("Displaying Information:\n");
for (i = 0; i < noOfRecords; ++i) {
printf("%s\t%d\n", courses[i].subject, courses[i].marks);
free(courses[i].subject);
}
free(courses);
return 0;
}