Libraries offer reusable functionality through application programming interfaces (APIs) with usage constraints such as call conditions and orders. Constraint violations, i.e., API misuses, commonly lead to bugs and even security issues. In this paper, we introduce IMChecker, a constraint-directed static analysis toolkit to vet API usages in C programs powered by a domain-specific language (DSL) to specify the API usages. First, we propose a DSL, which covers most API usage constraint types and enables straightforward but precise specification by studying real-world API-misuse bug patches. Then, we design and implement a static analysis engine to automatically parse specifications into checking targets, identify potential API misuses and prune the false positives with rich semantics. We have instantiated IMChecker for C programs with user-friendly graphic interfaces and evaluated the widely used benchmarks and real-world projects. The results show that IMChecker outperforms 4.78-36.25% in precision and 40.25-55.21% w.r.t. state-of-the-arts toolkits. We also found 75 previously unknown bugs in Linux kernel, OpenSSL and applications of Ubuntu, 61 of which have been confirmed by the corresponding development communities.