documentclass[12pt]{article}

usepackage{amsmath,amssymb,newcent}

usepackage{pstricks,pst-eucl}

usepackage{auto-pst-pdf}

begin{document}



begin{pspicture}

pstGeonode[PosAngle={90,0,-90}](1,2){M_1}(2,1){M_2}(1,0){M_3}

pstSegmentMark{M_1}{M_2} %%%%pstSegmentMark

pstSegmentMark{M_2}{M_3}

pstSegmentMark{M_3}{M_1}

end{pspicture}



begin{pspicture}

pstGeonode[PosAngle={90,0,-90}](1,2){M_1}(2,1){M_2}(1,0){M_3}

pstMarkSegment{M_1}{M_2}  %%%%pstMarkSegment

pstMarkSegment{M_2}{M_3}

pstMarkSegment{M_3}{M_1}

end{pspicture}

end{document}

The result of compiling :

In pst-eucl documentation (v.1.59), the general syntax is :

But in the latter example, the syntax is :

The default value of is MarkHashh, not pstslashh.

Question:

According to the above result, pstMarkSegment is failed, Should I choose one ?

You should consider the following.

Edit

Responding to your comment below.

documentclass[pstricks,border=12pt,12pt]{standalone}

usepackage{pst-eucl}



begin{document}

multido{i=0+30}{12}{%

begin{pspicture}(5,5)

    pstGeonode(0,0){O}(4,4){A}

    uput[180](A){Angle = $i^circ$}

    pstSegmentMark[SegmentSymbol=MarkHashhh,MarkAngle=i]{O}{A}

end{pspicture}}

end{document}

It is a typo in the documentation. The code beside the examples is always correct. Hence it should be pstSegmentMark. See also http://comedy.dante.de/~herbert/TeXnik/doc/generic/pst-eucl/

For something like SegmentSymbol=... use the name without the preceeding backslash. However, there also exists the macros with the same name, eg MarkHash which can be used for other purposes.