C Variable changes after initializing other variables

Question

I am writing a command-line todo list program in C and am getting some weird behavior. Todos are stored in structs which contain an int priority and a char[128] name. They are created using a function that takes in these parameters and returns an allocated and initialized struct with those values.

The returned structs are initially correct, but after initializing 8 more struct variables, the value of the first-initialized structs begins to change. My guess is that previous values are being overwritten by new ones due to not enough memory being allocated. I've fixed this problem temporarily by increasing the size fed to malloc() from sizeof(TaskP) to sizeof(int) + sizeof(char[128]). However, I am sure there is a better way and would much appreciate an explanation of why malloc(sizeof(TaskP) is not allocating enough memory (if that is the case) and the proper convention.

Here is my code:

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

struct Task {
    char description[128];              /* description of the task */
    int priority;                       /* task priority */
};

typedef  struct Task* Task;


Task createTask (int priority, char *desc)
{
  Task newTask = malloc(sizeof(Task));
  if (newTask) {
    newTask->priority = priority;
    strcpy(newTask->description, desc);
  }
  return newTask;
}

int main(int argc, const char * argv[])
{
    Task taskList[10];

    Task task1 = createTask(9, "task 1");
    printf("task1: %d\n", task1->priority);  /* I put printfs */
    Task task2 = createTask(3, "task 2");
    printf("task1: %d\n", task1->priority);  /* after every call */
    Task task3 = createTask(2, "task 3");
    printf("task1: %d\n", task1->priority);  /* to createTask() */
    Task task4 = createTask(4, "task 4");
    printf("task1: %d\n", task1->priority);  /* to watch the value */
    Task task5 = createTask(5, "task 5");
    printf("task1: %d\n", task1->priority);  /* of task1 change */
    Task task6 = createTask(7, "task 6");
    printf("task1: %d\n", task1->priority);
    Task task7 = createTask(8, "task 7");
    printf("task1: %d\n", task1->priority);
    Task task8 = createTask(6, "task 8");
    printf("task1: %d\n", task1->priority);
    Task task9 = createTask(1, "task 9");
    printf("task1: %d\n", task1->priority);
    Task task10 = createTask(0, "task 10");
    printf("task1: %d\n", task1->priority);

    return 0;
}

And here is my output:

task1: 9
task1: 9
task1: 9
task1: 9
task1: 9
task1: 9
task1: 9
task1: 0
task1: 1802723700
task1: 1802723700

UPDATE

I literally figured out the problem seconds after posting to stack overflow. How embarrassing. The problem is that I was only allocating enough memory for a pointer (Task), which has a size of 8 bytes, instead of memory for struct Task, which has a size of 132 bytes. Thank you to everyone who patiently answered and sorry for wasting your time.

Answer 1

This is the problem.

Task newTask = malloc(sizeof(Task));

You are allocating enough memory to hold only a pointer, not a struct Task.

Change it to:

Task newTask = malloc(sizeof(*newTask));

FWIW, having Task as a typedef for struct Task* is not a good idea. It will be better to use:

typedef struct Task Task;
typedef struct Task* TaskPtr;

and then use

Task* newTaskPtr = malloc(sizeof(Task));

or

TaskPtr newTaskPtr = malloc(sizeof(Task));