C: Expanding an array with malloc
Solution 1
Use realloc, but you have to allocate the array with malloc first. You're allocating it on the stack in the above example.
size_t myarray_size = 1000;
mystruct* myarray = malloc(myarray_size * sizeof(mystruct));
myarray_size += 1000;
mystruct* myrealloced_array = realloc(myarray, myarray_size * sizeof(mystruct));
if (myrealloced_array) {
myarray = myrealloced_array;
} else {
// deal with realloc failing because memory could not be allocated.
}
Solution 2
You want to use realloc (as other posters have already pointed out). But unfortunately, the other posters have not shown you how to correctly use it:
POINTER *tmp_ptr = realloc(orig_ptr, new_size);
if (tmp_ptr == NULL)
{
// realloc failed, orig_ptr still valid so you can clean up
}
else
{
// Only overwrite orig_ptr once you know the call was successful
orig_ptr = tmp_ptr;
}
You need to use tmp_ptr
so that if realloc
fails, you don't lose the original pointer.
Solution 3
No, you can't. You can't change the size of an array on the stack once it's defined: that's kind of what fixed-size means. Or a global array, either: it's not clear from your code sample where myarray
is defined.
You could malloc
a 1000-element array, and later resize it with realloc
. This can return you a new array, containing a copy of the data from the old one, but with extra space at the end.
Solution 4
a) you did not use malloc to create it so you cannot expand with malloc. Do:
mystruct *myarray = (mystruct*)malloc(sizeof( mystruct) *SIZE);
b) use realloc (RTM) to make it bigger
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Updated on November 11, 2020Comments
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I'm a bit new to malloc and C in general. I wanted to know how I can, if needed, extend the size of an otherwise fixed-size array with malloc.
Example:
#define SIZE 1000 struct mystruct { int a; int b; char c; }; mystruct myarray[ SIZE ]; int myarrayMaxSize = SIZE; .... if ( i > myarrayMaxSize ) { // malloc another SIZE (1000) elements myarrayMaxSize += SIZE; }
- The above example should make clear what I want to accomplish.
(By the way: I need this for an interpreter I write: Work with a fixed amount of variables and in case more are needed, just allocate them dynamically)