I was watching a video on the Linux kernel moving to C99 or C11 possibly and the video was looking at an example for why they are going to do this.
I kept seeing a function being called like this:
list_for_each_entry(pos, &head, member) {
/* this code gets run for each entry? */
}
I've never seen something like this before in C or C++. However, as a Ruby programmer this makes sense to me because I'm used to doing something like this:
arr.each do |item|
# do something with the item
end
arr.each { |item| single_line_of_code_here }
I never knew that C had this ability? I was trying to learn more about this and my guess is this isn't a function but maybe a macro? Can someone explain to me what's happening here?
Edit: Documentation for this function here: https://www.kernel.org/doc/htmldocs/kernel-api/API-list-for-each-entry.html
Apparently, this IS a macro. The soure code for this macro is:
/**
* list_for_each_entry - iterate over list of given type
* @pos: the type * to use as a loop cursor.
* @head: the head for your list.
* @member: the name of the list_head within the struct.
*/
#define list_for_each_entry(pos, head, member) \
for (pos = list_first_entry(head, typeof(*pos), member); \
!list_entry_is_head(pos, head, member); \
pos = list_next_entry(pos, member))
The source code (which contains other iterators as well): https://elixir.bootlin.com/linux/v5.16.1/source/include/linux/list.h#L629
Without using preprocessor macros you can accomplish something akin to this with function pointers.
typedef void (*fp_t)(int);
void int_array_iter(int *arr, size_t n, fp_t f) {
for (size_t i = 0; i < n; i++) {
f(arr[i]);
}
}
void print_int(int i) {
printf("%d\n", i);
}
int main(void) {
int arr[] = { 1, 2, 3, 4, 5, 6 };
int_array_iter(arr, 6, print_int);
return 0;
}