C applications, in particular those using operating system level services, frequently comprise multiple crosscutting con- cerns: network protocols and security are typical examples of such concerns. While these concerns can partially be addressed during design and implementation of an applica- tion, they frequently become an issue at runtime, e.g., to avoid server downtime. A deployed network protocol might not be sufficiently efficient and may thus need to be re- placed. Buffer overflows might be discovered that imply critical breaches in the security model of an application. A prefetching strategy may be required to enhance perfor- mance. While aspect-oriented programming seems attractive in this context, none of the current aspect systems is expres- sive and efficient enough to address such concerns. This paper presents a new aspect system solving these problems. While efficiency considerations have played an important part in the design of the aspect language, the language al- lows aspects to be expressed more concisely than previous approaches. In particular, it allows aspect programmers to quantify over sequences of execution points as well as over alias accesses. We show how the former can be used to modularize the replacement of network protocols and the latter to prevent buffer overflows. We also present an im- plementation of the language as an extension of Arachne, a dynamic weaver for C applications. Finally, we show eval- uations proving that Arachne is fast enough to extend high performance applications, such as the Squid web cache.