shared memory in c, question about the output - c

Question

I tried to realize shared memory with C, but I came up with a problem with my output.
I tried to make a thread called TA put the grade into a shared memory space and make "student" thread output the grade.
In the TA thread:
const int SIZE = 4096;
const char *name_ta = "Students_Information_ta";
int studentGrade = (int)((random() % (100 - 80 + 1)) + 80); // TA gives out a grade to a student
char *grade = (char *)&studentGrade;
/* shared memory file descriptor */
int shm_fd_ta;
/* pointer to shared memory object */
void *ptr_ta;
/* create the shared memory segment of ta */
shm_fd_ta = shm_open(name_ta, O_CREAT | O_RDWR, 0666);
/* configure the size of the shared memory segment of ta*/
ftruncate(shm_fd_ta, SIZE);
/* map the shared memory segment of ta in the address space of the process */
ptr_ta = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd_ta, 0);
if (ptr_ta == MAP_FAILED)
{
printf("Map failed\n");
return -1;
}
/* write to the shared memory region */
sprintf(ptr_ta, "%d", grade);
ptr_ta += strlen(grade);
And here is the output sentences in student thread:
/* name of shared memory object */
const char *name_ta = "Students_Information_ta";
/* size of shared memory object in bytes */
const int SIZE = 4096;
int shm_fd_ta;
void *ptr_ta;
/* open the shared memory segment of ta */
shm_fd_ta = shm_open(name_ta, O_RDWR, 0666);
if (shm_fd_ta == -1)
{
printf("shared memory failed\n");
exit(-1);
}
/* map the shared memory segment of ta in the address space of the process */
ptr_ta = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd_ta, 0);
if (ptr_ta == MAP_FAILED)
{
printf("Map failed\n");
exit(-1);
}
printf("The grade assigned by the TA is %d\n", ptr_ta); // student receives a grade
I thought it should gave me the grades which are numbers between 80 and 100, but in fact the output is some very large numbers like 251142144. Perhaps it has output the address. What can I do to fix this mistake?

Answer 1

char *grade = (char *)&studentGrade;
Okay, so grade is of type pointer to character but is pointing to an integer. Not sure why you'd want to do that, but okay.
sprintf(ptr_ta, "%d", grade);
You told sprintf to print an integer but you passed it a pointer to a character that points to an integer. The sprintf function certainly has no idea what to do with such a mismatched pointer, especially when you told it you were going to pass it an integer.
Why not:
sprintf(ptr_ta, "%d", studentGrade);
If you tell it you're going to give it an integer, maybe give it an integer.
--
ptr_ta = mmap(0, SIZE, PROT_READ | PROT_WRITE, MAP_SHARED, shm_fd_ta, 0);
if (ptr_ta == MAP_FAILED)
{
printf("Map failed\n");
exit(-1);
}
Now, ptr_ta is a pointer to the string you printed.
printf("The grade assigned by the TA is %d\n", ptr_ta); // student receives a grade
You told printf you were going to pass it an integer, but instead you passed it a pointer to a string. How is that supposed to work?
I'd change a lot of things, but this is a start:
printf("The grade assigned by the TA is %s\n", (char *) ptr_ta); // student receives a grade