# Gerris Flow Solver Programming Course for Dummies

(Difference between revisions)
 Revision as of 17:40, 29 October 2007Zaleski (Talk | contribs) (→References - added reference to Gerris Tutorial)← Previous diff Revision as of 23:46, 29 October 2007Popinet (Talk | contribs) (Use tag for formatting C code)Next diff → Line 80: Line 80: -
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struct Point {                                                                                                                                                                                                                                                                                                                                                                                                       struct Point {
char  name;                                                                                                                                                                                                                                                                                                                                                                                                          char  name;
double  x, y;                                                                                                                                                                                                                                                                                                                                                                                                        double  x, y;
};                                                                                                                                                                                                                                                                                                                                                                                                                   };
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+ An example of usage An example of usage -
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﻿main()                                                                                                                                                                                                                                                                                                                                                                                                              ﻿main()
{                                                                                                                                                                                                                                                                                                                                                                                                                    {
+                                                                                                                     struct Point my_point;    /* declaration */

-                                                                                                                                  struct Point my_point;    /* declaration */                                                                                                                                                                                                                                                    +                                                                                                                     my_point.x = 0. ;
-                                                                                                                                                                                                                                                                                                                                                                                                                                 +                                                                                                                     my_point.y = 1.;
-                                                                                                                                  my_point.x = 0. ;                                                                                                                                                                                                                                                                              +                                                                                                                     my_point.name = ‘A’;
-                                                                                                                                  my_point.y = 1.;                                                                                                                                                                                                                                                                               +
-                                                                                                                                  my_point.name = ‘A’;                                                                                                                                                                                                                                                                           +
}                                                                                                                                                                                                                                                                                                                                                                                                                    }
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+ name, x and y are members of the structure of type “Point” called my_point. We also give it a name that can be exported (printed, passed to other functions) as a character. This example shows why it is useful to use structures to store several relevant informations or data together. name, x and y are members of the structure of type “Point” called my_point. We also give it a name that can be exported (printed, passed to other functions) as a character. This example shows why it is useful to use structures to store several relevant informations or data together. Line 105: Line 104: ==== An example: the structure GfsNorm in Gerris ==== ==== An example: the structure GfsNorm in Gerris ==== -
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struct _GfsNorm {                                                                                                                                                                                                                                                                                                                                                                                                    struct _GfsNorm {
Line 111:                                                                                                                                                                                                                                                                                                                                                                                                                         Line 110:
};                                                                                                                                                                                                                                                                                                                                                                                                                   };
typedef struct _GfsNorm GfsNorm                                                                                                                                                                                                                                                                                                                                                                                      typedef struct _GfsNorm GfsNorm
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+ From ''domain.h''. From ''domain.h''. -
+
﻿GfsNorm gfs_domain_norm_residual (GfsDomain * domain,                                                                                                                                                                                                                                                                                                                                                               ﻿GfsNorm gfs_domain_norm_residual (GfsDomain * domain,
FttTraverseFlags flags,                                                                                                                                                                                                                                                                                                                                                                                              FttTraverseFlags flags,
Line 145:                                                                                                                                                                                                                                                                                                                                                                                                                         Line 144:
return n;                                                                                                                                                                                                                                                                                                                                                                                                            return n;
}                                                                                                                                                                                                                                                                                                                                                                                                                    }
-
+ From ''domain.c'' From ''domain.c''

## Revision as of 23:46, 29 October 2007

This wiki is an unfinished project !

## Preface

This course is about Gerris, a general-purpose fluid mechanics code developped by Stephane Popinet at NIWA, Wellington, New Zealand .

The intended audience is typical first year graduate students with very little experience of C or some Fortran knowledge, but willing to work hard and learn.

Gerris is a free, open source code available at [[1]] .

## Course 1

### Introduction

This course is about how Gerris ([[2]] ) is programmed. It is intended to help in understanding the Gerris source code and learning how to modify it usefully.

This course is not about the numerical methods in Gerris (however it would be good for students of this course to learn about them, for instance in the J. Comput. Phys. article) .

Gerris is closely linked to Gts, the GNU triangulated surface library, also written by Stephane Popinet

Gerris and Gts are programmed in a style analogous to that of Glib, Gnome and GTK . It is a style of C programming that offers several advantages:

• Most aspects of Object-Oriented Programming (OOP) , such as the existence of classes with their own methods and inheritance.
• The ability to interface to other programming languages. (As far as I know, this feature is not used in Gerris/Gts)

To implement Object-Oriented Programming, Gerris/Gts uses its own “Object system”. This system is analogous to the Glib object system (Gobject), but not identical to it. Learning more about Gobject can be very useful.

### References

• Introduction and in-depth discussion of the Gobject system (in order of decreasing ease of reading):
• Introduction to the Gerris/Gts object system :

You can find one in the [Gerris Tutorial Section on Objects].

### Useful tips

#### Navigate using the TAGS file in emacs or vim

- Create the TAGS file :
```% cd src
% make tags
```
1. In emacs:
1. open any *.c or *.h file with emacs.
2. Position the cursor on a function or variable.
3. do ESC . to find its definition. (or M-. using the emacs Meta key convention )
4. do ESC * to return to the previous location (s) .
2. In vim:
1. to be written ...

#### No order in which to read the code

There is no good order in which to read the code (I have not found it yet)

#### Keep a C precedence and associativity table nearby

A lot of macros and functions such g_assert come from the Glib. Keep a bookmark to the Glib documentation: [[3]]

### C basics

#### Introduction to Structures

`struct Point {  char  name;  double  x, y;};`

An example of usage

`﻿main(){  struct Point my_point;    /* declaration */   my_point.x = 0. ;   my_point.y = 1.;  my_point.name = ‘A’;}`

name, x and y are members of the structure of type “Point” called my_point. We also give it a name that can be exported (printed, passed to other functions) as a character. This example shows why it is useful to use structures to store several relevant informations or data together.

#### An example: the structure GfsNorm in Gerris

`struct _GfsNorm {  gdouble bias, first, second, infty, w;};typedef struct _GfsNorm GfsNorm`

From domain.h.

`﻿GfsNorm gfs_domain_norm_residual (GfsDomain * domain,				  FttTraverseFlags flags,				  gint max_depth,				  gdouble dt,				  GfsVariable * res){  GfsNorm n;  gpointer data[2];   g_return_val_if_fail (domain != NULL, n);  g_return_val_if_fail (res != NULL, n);   gfs_norm_init (&n);  data[0] = res;  data[1] = &n;  gfs_domain_cell_traverse (domain, FTT_PRE_ORDER, flags, max_depth, 			   (FttCellTraverseFunc) add_norm_residual, data);#ifdef HAVE_MPI  domain_norm_reduce (domain, &n);#endif /* HAVE_MPI */  gfs_norm_update (&n);   dt *= dt;  n.bias *= dt;  n.first *= dt;  n.second *= dt;  n.infty *= dt;  return n;}`

From domain.c

Notice the use of

• glib basic types gdouble, gpointer
• glib functions g_return_val_if_fail