SIG_ERR

From cppreference.com
Utilities library
General utilities
Relational operators (deprecated in C++20)
Defined in header <csignal>
#define SIG_ERR /* implementation defined */

A value of type void ( * ) ( int ) . When returned by std::signal , indicates that an error has occurred.

See also

sets a signal handler for particular signal
(function)
C documentation for SIG_ERR