In a small mixing bowl, whisk together 1 cup sugar and ¼ cup cornstarch. Set aside.
Best to store opened packages of cornstarch in a cool, dry place. Stored properly, it will keep indefinitely. Same for sugar.
Add 4 cups cherries to a saucepan over medium-low heat and cover the pan. Cook cherries until they begin to release their juice and come to a low simmer, about 10 minutes. Stir often and do not let them burn.
Pour sugar mixture into the cherries and juice, mixing as you add it. Bring the mixture back to a simmer and cook until the filling begins to thicken. About 2 minutes or up to 5 minutes for a thicker cherry pie filling.
Fresh cherries will keep for up to 10 days in the fridge and 6 months in the freezer. Removing the stems and pits is ideal before freezing but not required.
Let cool and use as pie filling. This is also great over ice cream. The pie filling will keep in the fridge in an airtight container for about 3 days or in the freezer for about 4 months.