C++ windows threading and mutex issue - c++

Question

I am a bit rusty with threaded programs especially in windows.
I have created a simple mex file in Matlab that is meant to read a number of files with each file being read in its own thread.
The file doesnt do anything really useful but is a precursor to a more complicated version that will use all of the functionality ive put into this file.
Here is the code:
#include <windows.h>
#include "mex.h"
#include <fstream>
typedef unsigned char uchar;
typedef unsigned int uint;
using namespace std;
int N;
int nThreads;
const int BLOCKSIZE = 1024;
char * buffer;
char * out;
HANDLE hIOMutex;
DWORD WINAPI runThread(LPVOID argPos) {
int pos = *(reinterpret_cast<int*>(argPos));
DWORD dwWaitResult = WaitForSingleObject( hIOMutex, INFINITE );
if (dwWaitResult == WAIT_OBJECT_0){
char buf[20];
sprintf(buf, "test%i.dat", pos);
ifstream ifs(buf, ios::binary);
if (!ifs.fail()) {
mexPrintf("Running thread:%i\n", pos);
for (int i=0; i<N/BLOCKSIZE;i++) {
if (ifs.eof()){
mexPrintf("File %s exited at i=%i\n", buf, (i-1)*BLOCKSIZE);
break;
}
ifs.read(&buffer[pos*BLOCKSIZE], BLOCKSIZE);
}
}
else {
mexPrintf("Could not open file %s\n", buf);
}
ifs.close();
ReleaseMutex( hIOMutex);
}
else
mexPrintf("The Mutex failed in thread:%i \n", pos);
return TRUE;
}
// 0 - N is data size
// 1 - nThreads is number of threads
// 2 - this is the output array
void mexFunction( int nlhs, mxArray *plhs[], int nrhs, const mxArray*prhs[] ) {
N = mxGetScalar(prhs[0]);
nThreads = mxGetScalar(prhs[1]);
out = (char*)mxGetData(prhs[2]);
buffer = (char*)malloc(BLOCKSIZE*nThreads);
hIOMutex= CreateMutex(NULL, FALSE, NULL);
HANDLE *hArr = (HANDLE*)malloc(sizeof(HANDLE)*nThreads);
int *tInd = (int*)malloc(sizeof(int)*nThreads);
for (int i=0;i<nThreads;i++){
tInd[i]=i;
hArr[i] = CreateThread( NULL, 0, runThread, &tInd[i], 0, NULL);
if (!hArr[i]) {
mexPrintf("Failed to start thread:%i\n", i);
break;
}
}
WaitForMultipleObjects( nThreads, hArr, TRUE, INFINITE);
for (int i=0;i<nThreads;i++)
CloseHandle(hArr[i]);
CloseHandle(hIOMutex);
mexEvalString("drawnow");
mexPrintf("Finished all threads.\n");
free(hArr);
free(tInd);
free(buffer);
I compile it like this in Matlab:
mex readFile.cpp
And then run it like this:
out = zeros(1024*1024,1,'uint8');
readFile(1024*1024,nFiles,out);
The problem is that when I set nFiles to be less than or equal to 64 everything works as expected and I get the following output:
Running thread:0
.
.
.
Running thread:62
Running thread:63
Finished all threads.
However when I set nFiles to 65 or larger I get:
Running thread:0
Running thread:1
Running thread:2
Running thread:3
The Mutex failed in thread:59
The Mutex failed in thread:60
The Mutex failed in thread:61
.
.
.
(up to nFiles-1)
Finished all threads.
I have also tested it without threading and it works fine.
I cannot see what Im doing wrong or why the cutoff to using the mutex would be so arbitrary so I am assuming there is something I am not taking into account.
Can anyone see where I have a blatant mistake relating to the error Im seeing?

Answer 1

In the documentation for WaitForMultipleObjects, "The maximum number of object handles is MAXIMUM_WAIT_OBJECTS.", which is 64 on most systems.
This is also (almost) a duplicate of this thread. The summary is really just that yes, the limit is 64, and also to use the information in the remarks section of WaitForMultipleObjects to build up a tree of threads to wait on.