Question

c++ newbie here.

I’ve been improving my memory management skills over the last few days, and my program no longer leaks memory according to valgrind. In fact, I get no warnings from valgrind at all.

However, when I add openmp loops into my code, I start to get the following errors in valgrind (memcheck): (but no definitely lost blocks)

==6417== 304 bytes in 1 blocks are possibly lost in loss record 3 of 4
==6417==    at 0x4C279FC: calloc (vg_replace_malloc.c:467)
==6417==    by 0x4011868: _dl_allocate_tls (dl-tls.c:300)
==6417==    by 0x6649871: pthread_create@@GLIBC_2.2.5 (allocatestack.c:570)
==6417==    by 0x62263DF: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==6417==    by 0x42A2BB: Blade::updatePanels() (blade.cpp:187)
==6417==    by 0x418677: VLMsolver::initialiseBlade() (vlmsolver.cpp:590)
==6417==    by 0x415A1B: VLMsolver::start(std::string) (vlmsolver.cpp:80)
==6417==    by 0x40B28C: main (charybdis.cpp:176)

and:

==6417== 1,568 bytes in 1 blocks are still reachable in loss record 4 of 4
==6417==    at 0x4C28FAC: malloc (vg_replace_malloc.c:236)
==6417==    by 0x6221578: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==6417==    by 0x6226044: ??? (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==6417==    by 0x622509B: GOMP_parallel_start (in /usr/lib/x86_64-linux-gnu/libgomp.so.1.0.0)
==6417==    by 0x41AF58: VLMsolver::segmentCirculations() (vlmsolver.cpp:943)
==6417==    by 0x415E4B: VLMsolver::solveManager() (vlmsolver.cpp:177)
==6417==    by 0x415A4B: VLMsolver::start(std::string) (vlmsolver.cpp:91)
==6417==    by 0x40B28C: main (charybdis.cpp:176)

Is this a case of valgrind not understanding the openmp? Or is it something that might become sinister?

Note that when I run valgrind with helgrind, I get thousands of “possible data race during read” (and write) messages. However my program (a fluid dynamics solver) gives the same results for both the openmp and serial codes. I can provide the helgrind errors and relevant sections if you are interested in that for this problem.

Otherwise for now, here is the offending code for the second message: and line 943 is the pragma line.

for (int b = 0;b < sNumberOfBlades;++b) {
*VLMSOLVER.CPP LINE 943 is next*:
#pragma omp parallel for collapse(2) num_threads(2) firstprivate(b) 
    for (int i = 0;i<numX;++i) {
        for (int j = 0;j<numY;++j) {
            if (j == 0) {
                blades[b].line[i*numNodesY+j].circulation = blades[b].panel[i*numY+j].circulation;
            } else {
                blades[b].line[i*numNodesY+j].circulation =  blades[b].panel[i*numY+j].circulation - blades[b].panel[i*numY+j-1].circulation;
            }
            if (j==numY-1) {
                blades[b].line[i*numNodesY+j+1].circulation = -1 * blades[b].panel[i*numY+j].circulation;
            }

        }
    }
    if (sBladeSymmetry) {
        break;
    }
}

int k = numX*numNodesY;
for (int b = 0;b < sNumberOfBlades;++b) {
    for (int i = 0;i<numX;++i) {
        for (int j = 0;j<numY;++j) {
            if (i == 0) {
                blades[b].line[k+i*numY+j].circulation = - 1 * blades[b].panel[i*numY+j].circulation;
            } else {
                blades[b].line[k+i*numY+j].circulation = -1 * blades[b].panel[i*numY+j].circulation + blades[b].panel[(i-1)*numY+j].circulation;
            }
            if (i==numX-1) {
                blades[b].line[k+(i+1)*numY+j].circulation =  blades[b].panel[i*numY+j].circulation;
            }
        }
    }
    if (sBladeSymmetry) {
        break;
    }
}

Answer 1

Still reachable is not a memory leak.

Still reachable means that a block of memory has not been freed but there are still valid pointers to the start of that block in registers or memory that has not been freed.

You need to have a look at the Valgrind FAQ. The actual reason for libgomp causing the warning is explained here.