test.d

#!/usr/sbin/dtrace -s

pid$target:libdovecot.so.0.0.0:safe_sendfile:entry {
        self->out_fd = arg0;
        self->in_fd  = arg1;
        self->offset = arg2;
        self->count  = arg3;
}

pid$target:libdovecot.so.0.0.0:safe_sendfile:return {
        printf("%d %d %x %d -> %d", self->out_fd, self->in_fd, self->offset, self->count, arg0);
}