# Plot segvaults after several mesh adaptations

**URL:** <https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212>\
**Category:** General Discussion\
**Created:** [January 10, 2023, 8:20am UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212 "2023-01-10T08:20:15Z")\
**Posts on this page:** 7\
**Page:** 1

<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [January 10, 2023, 8:20am UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/1 "2023-01-10T08:20:15Z")

</div>

I can’t upload the example since new users can’t upload apparently.  
so source included at bottom.

Attached file appears to die in plot after several adaptations  
with the following stack trace,  
FreeFem++ FreeFem++ - version 4.12 (Thu Jan 5 20:06:10 EST 2023 - git no git) 64bits

Warning: too few vertices to split all internal edges  
We lost 6 edges. Sorry!  
– adaptmesh statistics: Nb triangles 17246 , h min 0.0496506 , h max 3.2041  
area = 398 , M area = 28179.4 , M area/( |Khat| nt) 3.77348  
infiny-regularity: min 0.251023 max 24.7795  
anisomax 13.2528, beta max = 1.71978 min 0.061045  
– mesh: Nb of Triangles = 17246, Nb of Vertices 9000  
– Solve :  
min 4.13289e-32 max 1  
number of required edges : 0  
Warning: too few vertices to split all internal edges  
We lost 8 edges. Sorry!

Program received signal SIGSEGV, Segmentation fault.  
bamg::Vertex::Smoothing (this=0x1c3b580, Th=…, BTh=…,  
tstart=@0x1a4d2e0: 0x19182f8, omega=omega@entry=1)  
at …/bamglib/Mesh2.cpp:1417  
1417 tria-\>SetUnMarkUnSwap(1);  
(gdb) bt  
#0 bamg::Vertex::Smoothing (this=0x1c3b580, Th=…, BTh=…,  
tstart=@0x1a4d2e0: 0x19182f8, omega=omega@entry=1)  
at …/bamglib/Mesh2.cpp:1417  
#1 0x0000000000bad8f9 in bamg::Triangles::SmoothingVertex (  
this=this@entry=0x1579db0, nbiter=nbiter@entry=3, omega=omega@entry=1)  
at …/bamglib/Mesh2.cpp:4460  
#2 0x0000000000aba595 in Adaptation::operator() (this=,  
stack=0x1539290) at lgmesh.cpp:974  
#3 0x0000000000d3861e in E\_F\_F0F0\<Fem2D::Mesh const\*\*, Fem2D::Mesh const\*\*, Fem2D::Mesh const\*\>::operator() (this=0x1551860, s=0x1539290)  
at AFunction.hpp:1021  
#4 0x000000000095b284 in ListOfInst::operator() (this=0x1548ca0, s=0x1539290)  
at AFunction2.cpp:794  
#5 0x00000000008e2ea6 in FFor (s=0x1539290, i0=, i1=0x1548a60,  
i2=0x1548aa0, ins=0x1548ca0) at AFunction.cpp:284  
#6 0x00000000008d8bc2 in E\_F0\_CFunc4::operator() (this=,  
s=) at ./…/fflib/AFunction.hpp:908  
#7 0x000000000095b284 in ListOfInst::operator() (this=0x15494f0, s=0x1539290)  
at AFunction2.cpp:794  
#8 0x000000000095b284 in ListOfInst::operator() (this=0x153a7d0, s=0x1539290)  
at AFunction2.cpp:794  
#9 0x00000000008d2f81 in CListOfInst::eval (this=0x7fffffffbeb8, s=0x1539290)  
at ./…/fflib/AFunction.hpp:1486  
#10 lgparse () at lg.ypp:360  
#11 0x00000000008d7bda in Compile () at lg.ypp:853  
#12 0x00000000008d8604 in mainff (argc=2, argv=0x7fffffffd388) at lg.ypp:1027  
#13 0x00007ffff5568840 in \_\_libc\_start\_main (  
main=0x8c1480 \<main(int, char\*\*)\>, argc=2, argv=0x7fffffffd388,  
init=, fini=, rtld\_fini=,  
stack\_end=0x7fffffffd378) at …/csu/libc-start.c:291  
#14 0x00000000008c5209 in \_start ()  
(gdb)

load “medit”  
load “msh3”  
// mesh Th=Th2; // myqs(-1,0,1,1,-2,-1,5,4,1,0,1,1,0);  
real x0=-1; real y0=0; real w=1; real h=1;  
real ax0=-10; real ay0=-12; real aw=20; real ah=20 ;  
real bx0=1; real by0=0; real bw=1; real bh=1;

int m=10;  
int p=10;  
int n=30;  
// outside  
border F01(t=0, 1) {x=ax0+(aw_t); y=ay0; }  
border F11(t=0, 1) {x=ax0; y=ay0+ah-ah_t; }  
border F10(t=0, 1) {x=ax0+aw; y=ay0+ah_t; }  
border F00(t=0, 1) {x=ax0+aw-aw_t; y=ay0+ah; }  
// first  
border D01(t=0, 1) {x=x0+(w_t); y=y0; }  
border D11(t=0, 1) {x=x0; y=y0+h-h_t; }  
border D10(t=0, 1) {x=x0+w; y=y0+h_t; }  
border D00(t=0, 1) {x=x0+w-w_t; y=y0+h; }

border E01(t=0, 1) {x=bx0+(bw_t); y=by0; }  
border E11(t=0, 1) {x=bx0; y=by0+bh-bh_t; }  
border E10(t=0, 1) {x=bx0+bw; y=by0+bh_t; }  
border E00(t=0, 1) {x=bx0+bw-bw_t; y=by0+bh; }

mesh Th2=buildmesh(  
F01(n)+F10(n)+F00(n)+F11(n)+  
E01(-p)+E10(-p)+E00(-p)+E11(-p)+  
D01(-m)+D10(-m)+D00(-m)+D11(-m));

mesh Th=Th2; // myqs(-1,0,1,1,-2,-1,5,4,1,0,1,1,0);  
real tol=.5;  
real cut=.01;

for(int i=0; i\<10; ++i)  
{  
//fespace Xh(Th,RT03d);  
//fespace Xh(Th,RT0);  
fespace Xh(Th,P1);  
Xh psi, wx;  
macro grad(u) [dx(u),dy(u)]// def of grad operator  
// Solve  
solve potential(psi, wx)  
= int2d(Th)(  
grad(psi)'\*grad(wx)) // scalar product  
//+ on(E00,psi=0) + on(E10,psi=0) + on(E01,psi=0) + on(E11,psi=0)

- on(D00,psi=0) + on(D10,psi=0) + on(D01,psi=0) + on(D11,psi=1)
- on(F00,psi=1) + on(F10,psi=1) + on(F01,psi=1) + on(F11,psi=1)  
//)  
//+ on(C, psi = [uinfty1,uinfty2]'\*[y,-x])  
//+ on(S, psi=-lift) // to get a correct value  
;  
//plot(psi, wait=1);  
plot(Th, wait=0);

// this segfaults after a few iterations.  
// changing hmin to 1/100 instead of 1/1000 seems to work better  
Th = adaptmesh(Th, [dx(psi), dy(psi)], splitpbedge=1, abserror=0, cutoff=cut, err=tol, inquire=0, ratio=1.5, hmin=1./10);  
tol=tol/2;  
cut=cut/2;  
} // i

cout\<\<" done "\<\<endl;

//plot(Th2,wait=1);

---

<div class="post-metadata">

**Author:** ![cmd](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/cmd/32/67_2.png) [@cmd](https://community.freefem.org/u/cmd)\
**Post date:** [January 10, 2023, 9:53am UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/2 "2023-01-10T09:53:19Z")

</div>

Try changing the `nbvx` parameter. For example:

```auto
Th = adaptmesh(Th,u1,u2,...,nbvx = 1e5);

```

---

<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [January 10, 2023, 10:28am UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/3 "2023-01-10T10:28:12Z")

</div>

Thanks. I can play with it but generally a segfault is a bug .  
If it is just a parameter value, it looks like the mesh is getting too fine  
for the “smoother” whatever that does. For now I just disabled plotting  
and dumped to a txt file and display in R.

---

<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [January 10, 2023, 10:32am UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/4 "2023-01-10T10:32:24Z")

</div>

The suggested change appears to work but the mesh is pretty dense  
everywhere not just around the edges. Before it was getting coarse  
in the space between squares and border. Since I’m running this  
at home I’d like as few extra elements as possible.

---

<div class="post-metadata">

**Author:** ![cmd](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/cmd/32/67_2.png) [@cmd](https://community.freefem.org/u/cmd)\
**Post date:** [January 10, 2023, 3:03pm UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/5 "2023-01-10T15:03:00Z")

</div>

Have you tried setting `abserror=1`?

---

<div class="post-metadata">

**Author:** ![marchywka](https://avatars.discourse-cdn.com/v4/letter/m/ee59a6/32.png) [@marchywka](https://community.freefem.org/u/marchywka)\
**Post date:** [January 10, 2023, 3:44pm UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/6 "2023-01-10T15:44:40Z")

</div>

Thanks, that seems to work with 1e4 and the grid looks more organizaed at 1e4 or 1e5.  
Is the segfault just a pathological case or is it worth looking at? I was not going to  
look at the code unless there is some point.

Th = adaptmesh(Th, [dx(psi), dy(psi)], splitpbedge=1, nbvx=1e5, abserror=1, cutoff=cut, err=tol, inquire=0, ratio=1.5, hmin=1./10);

---

<div class="post-metadata">

**Author:** ![cmd](https://yyz2.discourse-cdn.com/flex030/user_avatar/community.freefem.org/cmd/32/67_2.png) [@cmd](https://community.freefem.org/u/cmd)\
**Post date:** [January 10, 2023, 4:23pm UTC](https://community.freefem.org/t/plot-segvaults-after-several-mesh-adaptations/2212/7 "2023-01-10T16:23:44Z")

</div>

Glad that helped. The segfault is probably just a pathological case.