iOS Trouble with NSString

I use NSString in a (typedef struct) data structure.
typedef struct {
...
NSTring *name;
...
} MyStruct;

I use calloc() to allocate the structure. When I set the value of the name just once then it shows correctly for the life of the app.

my_struct->name = @"Hello";

If I modify the string using the NSString function stringWithFormat, then the name is correct until the current function completes, then after that, the name contains garbage.

Here's and example of how I modify it.

my_struct->name = [NSString stringWithFormat"World"];

I can do the following in the beginning of my app, and the corrupted string shows up as soon as the app does any processing outside of the initialization function.

my_struct->name = @"Hello";
my_struct->name = [NSString stringWithFormat"World"];

but if I do the following, then the string never gets corrupted:

my_struct->name = @"Hello";
my_struct->name = @"World";

I need to use stringWithFormat to create name in different ways.

Any clues as to what the issue is?
Thank you,
Michael

Thanks!!!

Thank you so much.
I no longer get trashed text, and thank you for the quick education.