External declarations of structure templates and prototypes facilitate consistent usage of tags and functions. As a general practice, we will declare structure templates externally, usually at the head of the source file. Sometimes, external structure tag declarations will be placed in a separate header file, which is then made part of the source file by an include directive.

From this example, we can see that using structures with functions is no different than using any scalar data type like int. However, let us consider what really happens when the program runs. When the function read_part() is called, memory is allocated for all of its local variables, including the struct inventory variable, part. As each data item is read, it is placed in the corresponding field of part, accessed with the dot operator. The value of part is then returned to main() where it is assigned to the variable item. As would be the case for a scalar data type, the value of the return expression is copied back to the calling function. Since this is a structure, the entire structure (each of the fields) is copied. For our inventory structure, this isn't too bad - only two floats and an integer. If the structure where much larger, maybe including nested structures and arrays, many values would need to be copied.

Likewise with the call to print_part(). Here, an inventory structure is passed to the function. Recall that in C, all parameters are passed by value - the value of each argument expression is copied from the calling function into the cell allocated for the parameter of the called function. Again, for large structures, this may not be a very efficient way to pass data to functions. In the next section we see a way to remedy this problem.