Valgrind: supressed complaints about uninitialized bytes. Valgrind complains if we pass uninitialized memory to a syscall: ==36492== Syscall param sendmsg(msg.msg_iov[0]) points to uninitialised byte(s) ==36492== at 0x6B5E6A: sendmsg (in /usr/lib/system/libsystem_kernel.dylib) ==36492== by 0x10004288E: ngx_signal_worker_processes (ngx_process_cycle.c:527) ==36492== by 0x1000417A7: ngx_master_process_cycle (ngx_process_cycle.c:203) ==36492== by 0x100001F10: main (nginx.c:410) ==36492== Address 0x7fff5fbff71c is on thread 1's stack Even initialization of all members of the structure passed isn't enough, as there is padding which still remains uninitialized and results in Valgrind complaint. Note there is no real problem here as data from uninitialized memory isn't used.