Since a typical Apache installation runs with several processes, and in the case of the worker and event MPMs multiple threads per process, starting Apache as a single process can simplify troubleshooting.
httpd -X
NOTE: When single process mode is used, Apache will not fork new processes or disassociate from the terminal.
This ensures that all communications flow through one process, which allows troubleshooting procedures, such as attaching to the httpd process with gdb, to be used on a single process instead of multiple processes.