c++ - "Writefile" for RS232 communication using MFC hangs forever, but -

i maintaining mfc program can send data computer computer b through rs232. transmits data smoothly, hangs forever. there 2 threads sending same data com port in sequence. first thread sends data successfully, second thread hangs @ code "writefile". when second thread on computer hangs @ "writefile", send meaningless data "1" computer b computer a. hanging @ "writefile" on computer stop hanging, , computer b see data sent second thread on computer a.

here rs232 log computer b .

the picture shows 2 threads on computer initiate there own test , send messages computer b. each thread complete there own test , send test_done computer b @ same time. computer b see test_done sent first thread on computer a(the second thread hanging @ writefile @ point.) until manually send "1" computer computer b.

here code sending message computer computer b. length of cmd 255.

bool serialport::autohandlerres(unsigned char* cmd){ while(wait_transfer.islocked())     sleep(1000); wait_transfer.lock();  cstring out; bool retb; uchar endchar[2]={0x0d,0x0a}; out=cstring(cmd); dword num = out.getlength()+2; cmd[num-2]=endchar[0]; cmd[num-1]=endchar[1]; retb=writefile(this->m_hcom, cmd, num, &num, null); sleep(1000); wait_transfer.unlock();  return retb;} 

my question possible cause causing thread b hanging @ "writefile"? why hanging not happen on thread a? thank you!

from the msdn page serial communications :

if 1 thread blocked waiting i/o operation complete, other threads subsequently call communications api blocked until original operation completes. instance, if 1 thread waiting readfile function return, other thread issued writefile function blocked.

you issue blocking readfile prevent writefile completing. in case race condition between threads ; writes complete before read called, not always.

the best way prevent not call readfile carelessly, instead wrap in same lock writes, , wait on receive comm event before reading. raw win32 calls setcommmask , waitcommevent. can use clearcommerror detect how many bytes should read (because receive event doesn't tell how data received).

you can use overlapped io allow simultaneous io. find cleaner, it's not simpler.