Passing specialized arrays to C functions
Factor handbook > The language > Collections > Specialized arrays

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If a C function is declared as taking a parameter with a pointer or an array type (for example, float* or int[3]), instances of the relevant specialized array can be passed in.

C type specifiers for array types are documented in C type specifiers.

Here is an example; as is common with C functions, the array length is passed in separately, since C does not offer a runtime facility to determine the array length of a base pointer:
USING: alien.syntax specialized-arrays ; SPECIALIZED-ARRAY: int FUNCTION: void process_data ( int* data, int len ) ; int-array{ 10 20 30 } dup length process_data

Literal specialized arrays, as well as specialized arrays created with <T-array> and T >c-array are backed by a byte-array in the Factor heap, and can move as a result of garbage collection. If this is unsuitable, the array can be allocated in unmanaged memory instead.

In the following example, it is presumed that the C library holds on to a pointer to the array's data after the init_with_data() call returns; this is one situation where unmanaged memory has to be used instead. Note the use of destructors to ensure the memory is deallocated after the block ends:
USING: alien.syntax specialized-arrays ; SPECIALIZED-ARRAY: float FUNCTION: void init_with_data ( float* data, int len ) ; FUNCTION: float compute_result ( ) ; [ 100 malloc-float-array &free dup length init_with_data compute_result ] with-destructors

Finally, sometimes a C library returns a pointer to an array in unmanaged memory, together with a length. In this case, a specialized array can be constructed to view this memory using <direct-T-array>:
USING: alien.c-types classes.struct ; STRUCT: device_info { id int } { name char* } ; FUNCTION: void get_device_info ( int* length ) ; 0 int <ref> [ get_device_info ] keep <direct-int-array> .

For a full discussion of Factor heap allocation versus unmanaged memory allocation, see Byte arrays and the garbage collector.

Each specialized array has a underlying slot holding a byte-array with the raw data. Passing a specialized array as a parameter to a C function call will automatically extract the underlying data. To get at the underlying data directly, call the >c-ptr word on a specialized array.