explain that undesired output args in C API can be ignored by passing NULL (cf. !470)

doc/texinfo/gmsh.texi

@@ -5286,6 +5286,8 @@ See
 @url{@value{GITLAB-PREFIX}/tutorials/t2.geo,t2.geo}. Also
 available in C++
 (@url{@value{GITLAB-PREFIX}/tutorials/c++/t2.cpp,t2.cpp}),
+C
+(@url{@value{GITLAB-PREFIX}/tutorials/c/t2.c,t2.c}),
 Python
 (@url{@value{GITLAB-PREFIX}/tutorials/python/t2.py,t2.py})
 and Julia
@@ -5365,7 +5367,9 @@ and Julia
 See
 @url{@value{GITLAB-PREFIX}/tutorials/t6.geo,t6.geo}.
 Also available in C++
-(@url{@value{GITLAB-PREFIX}/tutorials/c++/t6.cpp,t6.cpp})
+(@url{@value{GITLAB-PREFIX}/tutorials/c++/t6.cpp,t6.cpp}),
+C
+(@url{@value{GITLAB-PREFIX}/tutorials/c/t6.c,t6.c})
 and Python
 (@url{@value{GITLAB-PREFIX}/tutorials/python/t6.py,t6.py}).
 
@@ -5439,7 +5443,9 @@ See
 Also available in C++
 (@url{@value{GITLAB-PREFIX}/tutorials/c++/t10.cpp,t10.cpp})
 and Python
-(@url{@value{GITLAB-PREFIX}/tutorials/python/t10.py,t10.py}).
+(@url{@value{GITLAB-PREFIX}/tutorials/python/t10.py,t10.py})
+and Julia
+(@url{@value{GITLAB-PREFIX}/tutorials/julia/t10.jl,t10.jl}).
 
 @smallformat
 @verbatiminclude ../../tutorials/t10.geo
@@ -5546,6 +5552,8 @@ See
 @url{@value{GITLAB-PREFIX}/tutorials/t16.geo,t16.geo}.
 Also available in C++
 (@url{@value{GITLAB-PREFIX}/tutorials/c++/t16.cpp,t16.cpp}),
+C
+(@url{@value{GITLAB-PREFIX}/tutorials/c/t16.c,t16.c}),
 Python
 (@url{@value{GITLAB-PREFIX}/tutorials/python/t16.py,t16.py})
 and Julia

tutorials/c/t2.c

@@ -82,8 +82,11 @@ int main(int argc, char **argv)
   // should be freed with `gmshFree()' when not used anymore:
   gmshFree(ov);
 
+  // Note that in the C API, undesired output values can be ignored by passing a
+  // NULL pointer. For example, to ignore the error flag, we can pass NULL as
+  // the last argument:
   const int cl2[] = {5, -8, -7, 3};
-  gmshModelGeoAddCurveLoop(cl2, sizeof(cl2) / sizeof(cl2[0]), 10, 0, &ierr);
+  gmshModelGeoAddCurveLoop(cl2, sizeof(cl2) / sizeof(cl2[0]), 10, 0, NULL);
 
   const int s2[] = {10};
   gmshModelGeoAddPlaneSurface(s2, sizeof(s2) / sizeof(s2[0]), 11, &ierr);
@@ -161,18 +164,15 @@ int main(int argc, char **argv)
   // the surface 11 along the z axis and automatically creates a new volume (as
   // well as all the needed points, curves and surfaces). As expected, the
   // function takes a vector of (dim, tag) pairs as input as well as the
-  // translation vector, and returns a vector of (dim, tag) pairs as output:
-  int *ov2;
-  size_t ov2_n;
+  // translation vector; since we don't plan to use the output vector of (dim,
+  // tag) pairs, we pass NULL instead:
   const int *numElements;
   const size_t numElements_n = 0;
   const double *heights;
   const size_t heights_n = 0;
   const int recombine = 0;
-  gmshModelGeoExtrude(&(ov[2]), 2, 0.0, 0.0, 0.12, &ov2, &ov2_n, numElements,
+  gmshModelGeoExtrude(&(ov[2]), 2, 0.0, 0.0, 0.12, NULL, NULL, numElements,
                       numElements_n, heights, heights_n, recombine, &ierr);
-  gmshFree(ov);
-  gmshFree(ov2);
 
   // Mesh sizes associated to geometrical points can be set by passing a vector
   // of (dim, tag) pairs for the corresponding points: