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How to write/read data to/from structure array using write/fread?

You have the right idea, but this should fix your problem.

for (int var = 0; var < i; ++var) {
    fwrite(&S[i], sizeof(struct Student), 1, sourcefile);

The argument 1 in fwrite after the sizeof statement indicates you only want to add one Student at a time.

Please see the doc for fwrite: http://www.cplusplus.com/reference/cstdio/fwrite/

And also:

          while (fgetc(sourcefile) != EOF) {
          fread(&A[k], sizeof(struct Student), 1, sourcefile);

Please see the doc for fread: http://www.cplusplus.com/reference/cstdio/fread/

I know it's the cplusplus website, but it's the only place I could find good docs on the functions.

Also, the below array initializes to 0 elements:

struct Student A[k];

You may want to try managing a linked list for this instead.

Might end up looking something like:

struct StudentList {
    struct Student* student;
    struct StudentList* next;

struct StudentList* students = NULL;

while(!feof(sourcefile)) {
    struct StudentList* newlink = malloc(sizeof(struct StudentList));
    newlink->student = malloc(sizeof(struct Student));
    newlink->next = NULL;

    fread(newlink->student, sizeof(struct Student), 1, sourcefile); 

    if(NULL != students) {
        struct StudentList* iter;
            NULL != iter->next;
            iter = iter->next); // iterate to last link

        iter->next = newlink; // add new link to the end
    } else students = newlink; // add new link to the beginning

Then to free your list, just use something like:

struct StudentList* iter, *head;
    NULL != iter;)
    head = iter;
    iter = head;

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