Udf全局變量-外部變量External Variables，可在不同文件間共享

2022-03-25 by:CAE仿真在線來源:互聯網

Udf全局變量-外部變量External Variables,可在不同文件間共享,如果要累計不同時間步的數據,就可以考慮這個方式。

A.5.2. External Variables

If you have a global variable that is declared in one source code file, but a function in another source file needs to use it, then it must be defined in the other source fi1e as an external variable. To do this, simp1y precede the variable declaration by the word extern as in

extern real volume;
If there are several files referring to that variable then it is convenient to include the extern definition in a header (.h) file, and include the header file in all of the .c files that want to use the external variable. Only one .c file should have the declaration of the variable without the extern keyword. Below is an example that demonstrates the use of a header file.

Important: extern can be used only in compiled UDFs.

A.5.2.1. Example
Suppose that there is a global variable named volume that is declared in a C source file named file1.c

#include "udf.h"
real volume; /* real variable named volume is declared globally */

DEFINE_ADJUST(compute_volume, domain)
{
/* code that computes volume of some zone */
volume = ....
}
If multiple sourc files want to use volume in the1r computations, then volume can be declared as an external variab1e in a header file (for example, extfile.h)

/* extfile.h
Header file that contains the external variable declaration for
volume */

extern real volume; /*聲明udf全局變量,這樣在h文件聲明就可以了*/
Now another file named file2.c can declare volume as an external variable by simply including extfile.h.

/* file2.c

#include "udf.h"
#include "extfile.h" /* header file containing extern declaration
is included */

DEFINE_SOURCE(heat_source,c,t,ds,eqn)
{
/* code that computes the per unit volume source using the total
volume computed in the compute_volume function from file1.c */
real tota1_source = ...;
real source;
source = total_source/volume;
return source;

}

但如果用static 靜態變量呢?這個不行

因為靜態變量只在定義他的文件范圍內有效,即時把static定義成external都不行。這點要注意。

但如果累計的變量一直在同一個文件內部,則就有用了。

如果static變量在一個function內定義,則在文件內一直保留,包括他的值也保留。如果在頭部定義,則對整個文件一直保留其值。

The static operator has different effects depending on whether it is applied to local or global variables. When a local variable is declared as static the variable is prevented from being destroyed when a function returns from a call. In other words, the value of the variable is preserved. When a global variable is declared as static the variable is "file global." It can be used by any function within the source file in which it is declared, but is prevented from being used outside the file, even if is declared as external. Functions can also be declared as static. A static function is visible only to the source file in which it is defined.

Important: static variables and functions can be declared only in compiled UDF source files.

A.5.3.1. Example - Static Global Variable
/* mysource.c /*

#include "udf.h"

static real abs_coeff = 1.0; /* static global variable */
/* used by both functions in this source file but is
not visible to the outside */

DEFINE_SOURCE(energy_source, c, t, dS, eqn)
{
real source; /* local variable
int P1 = ....; /* local variable
value is not preserved when function returns */
dS[eqn] = -16.* abs_coeff * SIGMA_SBC * pow(C_T(c,t),3.);
source =-abs_coeff *(4.* SIGMA_SBC * pow(C_T(c,t),4.) - C_UDSI(c,t,P1));
return source;
}

DEFINE_SOURCE(p1_source, c, t, dS, eqn)
{
real source;
int P1 = ...;
dS[eqn] = -abs_coeff;
source = abs_coeff *(4.* SIGMA_SBC * pow(C_T(c,t),4.) - C_UDSI(c,t,P1));
return source;
}