Most platforms have have system level C APIs that are exposed to userspace. When using these APIs it's necessary to layout structures as they expect. Therefore

Most platforms have have system level C APIs that are exposed to userspace. When using these APIs it's necessary to layout structures as they expect. Therefore all compilers on the same platform (OS+arch) would usually produce the same layout for structs so that they are compatible. However this isn't universally true. Some platforms might not have a well defined C ABI.