May I have some problem with memory allocated

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;
}

`ptr->subject=(char*)malloc(20*sizeof(char));` only allocates memory for the member of the *first* `struct` element in the array. You need to do that inside the loop, for every element. — Weather Vane, Nov 07 '20 at 13:36
OT: regarding: `int main(int argc, char *argv[])` Since those parameters are not used, the compiler outputs two warning messages about unused parameters. This can be fixed by either using the alternate valid signature for `main()` `int main( void )` or the first couple of statements in the body of `main` be: `(void)argc;` and `(void)argv;` — user3629249, Nov 07 '20 at 19:31
OT: regarding statements like: `ptr = (struct course *)malloc(noOfRecords * sizeof(struct course));` and `ptr->subject=(char*)malloc(20*sizeof(char));` 1) the returned type is `void*` which can be assigned to any pointer. Casting just clutters the code (and is error prone) 2) always check (!=NULL) the returned value to assure the operation was successful. If not successful (==NULL) then call `perror( "malloc for ... failed" ) to output to `stderr` both your error message and the text reason the system thinks the error occurred — user3629249, Nov 07 '20 at 19:35
OT: regarding: `scanf("%d", &noOfRecords);` when calling any of the `scanf()` family of functions, always check the returned value (not the parameter values) to assure the operation was successful. Note: those functions return the number of successful 'input format conversion specifers' (or EOF). Suggest: `if( scanf("%d", &noOfRecords) != 1 ) { fprintf( stderr, "scanf for number of records failed\n" );` This is not recoverable with the current code logic, so the next statement should be: `exit( EXIT_FAILURE );` — user3629249, Nov 07 '20 at 19:37

emi · Answer 1 · 2020-11-07T21:20:58.703

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;
}

May I have some problem with memory allocated

1 Answers1

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