WIFSTOPPED

WIFSTOPPED is a macro used in POSIX-compatible systems (notably Linux) to inspect the wait status of a child process. It is defined in sys/wait.h and is used after calls such as wait, waitpid, or waitid to determine whether a child has been stopped by a signal rather than having exited or been terminated by a signal.

When a wait-related call returns a status value, WIFSTOPPED(status) evaluates to nonzero if the child is currently in a stopped state. If this is the case, the macro WSTOPSIG(status) can be used to obtain the signal number that caused the stop. This is commonly seen when a process is stopped by signals such as SIGSTOP or SIGTSTP, or when a debugger attaches and stops the process.

WIFSTOPPED is distinct from WIFEXITED and WIFSIGNALED, which indicate normal termination or termination by a signal, respectively. A stopped child remains in that state until it is resumed (for example, with SIGCONT) or terminated.

In practice, programs that manage multiple processes, such as shells or debuggers, may use WIFSTOPPED alongside other status macros to drive control logic. To observe events such as continuation after a stop, the WCONTINUED status can be reported when the waiting call is issued with appropriate flags (for example, WCONTINUED or WUNTRACED in waitpid).

Notes: WIFSTOPPED relies on the encoding of the wait status provided by the kernel. Proper handling typically involves checking WIFSTOPPED first, then using WSTOPSIG to identify the stopping signal, and handling other state transitions through WEXITED, WIFSIGNALED, and WIFCONTINUED as appropriate.