lasterror

Lasterror is a term used in computer software to refer to the most recently reported error from a system, library, or runtime. It can be represented as an error code, a human-readable message, or both, and is often kept separate from normal program flow to aid debugging and error reporting. In many environments the stored information is thread-local to prevent races between concurrent execution units.

In the Windows API, the last error for the calling thread is retrieved with GetLastError, which returns a numeric code. The code can be translated into a message via FormatMessage. In networking sockets, WSAGetLastError may be used for the last error on the Winsock stack.

In C and POSIX libraries, the variable errno records the last error code for the calling thread; many functions set errno on failure. The corresponding human-readable string is produced with strerror(errno).

Some programming environments expose a dedicated variable or function named lasterror or similar that holds the textual description of the most recent error, including stack traces in certain runtimes. In languages that rely on exceptions, the last error may be captured in an exception object rather than through a separate last-error variable.

Best practices include treating last-error information as potentially untrusted, avoiding leaking internal details in user-facing messages, and logging errors for debugging. Clear separation of error codes, messages, and context helps portability and security.