C. A Linked list problem. How to work with linked list?

Question

I'm trying to create a linked list of courses with dynamic allocation. I keep getting errors. can someone tell me what is the problem?

One of the errors is:

Dereferencing NULL pointer 'coursesList'.

Should I allocate memory to each of the fields of the struct?

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <string.h>

struct Course
{
    int a;
    char courseNumber[5];
    char courseName[30];
    struct Course* next;
};

struct Course* updateCoursesList(char courseName[], char courseNumber[]);


struct Course* updateCoursesList(char courseName[], char courseNumber[])
{
    struct Course* coursesList = (struct Course*)malloc(sizeof(struct Course));
    strcpy(coursesList->courseName, courseName);
    strcpy(coursesList->courseNumber, courseNumber);
    coursesList->next = NULL;
    coursesList->a = 2;
    printf("%d", coursesList->a);
    printf("%s", coursesList->courseName);
    printf("%s", coursesList->courseNumber);
    return coursesList;
}

int main()
{
    char newcourse[] = "math";
    char newcoursenumber[] = "54321";
    struct Course* A = updateCoursesList(newcourse, newcoursenumber);

}

You're breaching the size limits of `courseNumber`. Storying `"54321"` requires **six** slots; not five (the terminating nulchar takes the extra slot). Therefore your program invokes *undefined behavior*, and with that come the ghosts and ghouls. — WhozCraig, Dec 23 '20 at 11:44
@WhozCraig, indeed undefined behavior because the memory layout could suggest that printing course number would print `54321math`. I don't see how `coursesList` could become null. — Paul Ogilvie, Dec 23 '20 at 11:55
Thank you, that helped. I changed the size of course number to 6. but I have a differernt error. At this row: strcpy(coursesList->courseName, courseName); The error is : Dereferencing NULL pointer 'coursesList'." — Netanel, Dec 23 '20 at 11:55
You must check if `malloc` succeeded, i.e. `coursesList` is not null. — Paul Ogilvie, Dec 23 '20 at 11:57
Fixing the issue I mentioned, [I cannot reproduce your error](https://coliru.stacked-crooked.com/a/3e11f3c3523f92d1). — WhozCraig, Dec 23 '20 at 12:29
In addition, you should remember to `free` any dynamically allocated memory - `free(A);` in this case. Running your code, I couldn't reproduce the `NULL` pointer error -- but you should always check the return value of `malloc` as @paul points out, e.g. `if (coursesList == NULL) { return NULL; }`. — costaparas, Dec 23 '20 at 14:02
Also aside: use `strncpy` instead -- it is safer, as described in [this](https://stackoverflow.com/questions/1258550/why-should-you-use-strncpy-instead-of-strcpy) post. — costaparas, Dec 23 '20 at 14:04
@costaparas The accepted answer at the [link](https://stackoverflow.com/questions/1258550/why-should-you-use-strncpy-instead-of-strcpy) you suggested has a 20% down-vote. The higher rated [answer](https://stackoverflow.com/a/1258577/2410359) encourages a non-`strncpy()` solution. `strncpy()` is not really safer - it trades one set of problems for another. — chux - Reinstate Monica, Dec 23 '20 at 18:54
@chux that's very true, thanks for pointing this out. Though `strncpy` isn't perfect is still has merit over `strcpy` since it *forces* the programmer to *think* about the size of the buffers; oftentimes that's enough to catch issues early on. But using `strcat` to do a copy is more cryptic than it should be. Perhaps `snprintf` would be better overall. — costaparas, Dec 23 '20 at 23:33
@costaparas I'd consider using `char *courseName;` and later `coursesList->courseName = strdup(courseName);` — chux - Reinstate Monica, Dec 23 '20 at 23:36
@chux that's also suitable, assuming the struct definition is allowed to change here, and the memory is freed afterwards. — costaparas, Dec 23 '20 at 23:46

Nishad C M · Answer 1 · 2020-12-24T04:22:57.360

input to the function is a char array, not a string, strcpy will happen till it find a a "\0".
Please use strncpy with array size to avoid crash, and overwriting of data or corrupting other fields in structure.
courseNumber size if not enough to hold 5 char string plus "\0" ,\0" is required for string opeartions otherwise should print char's in array. keeping value 5 will give output
```
 2
 math
 54321math
```

used "size()-1" intentionally to add "\0" in stncpy , incase if input char array donot have a "\0" will be placed one by memset and stncpy.

 #include <stdio.h>
 #include <stdlib.h>
 #include <ctype.h>
 #include <string.h>

 struct Course
 {
     int a;
     char courseNumber[6];
     char courseName[30];
     struct Course* next;
 };

 struct Course* updateCoursesList(char courseName[], char courseNumber[]);

 struct Course* updateCoursesList(char courseName[], char courseNumber[])
 {
     struct Course* coursesList = (struct Course*)malloc(sizeof(struct Course));
     if (coursesList == NULL) {
         return coursesList;
     }
     memset(coursesList, 0, sizeof(struct Course));

     strncpy(coursesList->courseName, courseName,    sizeof(coursesList->courseName)-1);
     strncpy(coursesList->courseNumber,courseNumber, sizeof(coursesList->courseNumber)-1);
     coursesList->next = NULL;
     coursesList->a = 2;
     printf("%d\n", coursesList->a);
     printf("%s\n", coursesList->courseName);
     printf("%s\n", coursesList->courseNumber);
     return coursesList;
 }


 int main()
 {
     char newcourse[] = "math\0";
     char newcoursenumber[] = "54321\0";
     struct Course* A = updateCoursesList(newcourse, newcoursenumber);
     // do operations here

     // free before exit
     if (A != NULL) {
         free(A);
         // good practice to set vale to null after free
         A = NULL;
     }
     return 0;
 }

`strncpy()` is not enough to insure `printf("%s\n", coursesList->courseName);` works. — chux - Reinstate Monica, Dec 23 '20 at 15:44
size is kept as "sizeof(coursesList->courseNumber)-1" for strncpy, array is set to "0" and this will write one char less than array size, make sure there is a termination char. — Nishad C M, Dec 23 '20 at 15:50
True. (the -1, far to the right) was not visible in my screen w/o scrolling, yet it does maintain the null character termination as you say. See also [Why is strncpy insecure?](https://stackoverflow.com/a/10845959/2410359), — chux - Reinstate Monica, Dec 23 '20 at 16:46
Please remember to free all allocated memory when posting "corrected" code. Also, you [don't](https://stackoverflow.com/questions/605845/do-i-cast-the-result-of-malloc) need to cast the return value of malloc. And also, its more correct to compare the return value with the `NULL` macro rather than `0`. — costaparas, Dec 23 '20 at 23:50

C. A Linked list problem. How to work with linked list?

1 Answers1