I have encountered this piece of code:
struct test
{
uint32 num_fields;
char array_field [];
};
How can I understand array_field? Is this a gcc extension for the C language?
It's a C99 feature, called flexible array member which is typically used to create a variable length array.
It can only be specified as the last member of a struct without specifying the size (as in array_field [];).
For example, you can do the following and the member arr will have 5 bytes allocated for it:
struct flexi_example
{
int data;
char arr[];
};
struct flexi_example *obj;
obj = malloc(sizeof (struct flexi_example) + 5);
Its pros/cons discussed here:
sizeof applied to the struct doesn't include the size of the array member; you just have to keep track of that yourself.
Such structures are usually allocated on the heap with a calculated size, with code such as the following:
#include <stddef.h>
struct test * test_new(uint32 num_fields)
{
size_t sizeBeforeArray = offsetof(struct test, array_field);
size_t sizeOfArray = num_fields * sizeof(char);
struct test * ret = malloc(sizeBeforeArray + sizeOfArray);
if(NULL != ret)
{
ret->num_fields = num_fields;
}
return ret;
}
