### 25.7.2 Defining new giac functions

New giac functions can be defined with a C++ program. All data in the program used in formal calculations needs to be gen type. A variable g can be declared to be gen type with

gen g;

In this case, g.type can have different values. The value of g is fetched differently for different types:

- If g.type is _INT_, then g.val is an integer type int.
- If g.type is _DOUBLE_, then g._DOUBLE_val is a real double.
- If g.type is _SYMB, then g._SYMBptr points to a value of type symbolic. The function is contained in the sommet member and its argument(s) in the feuille member.
- If g.type is _VECT, then g._VECTptr points to a value of type vecteur, which behaves like std::vector.
- If g.type is _ZINT, then g._ZINTptr points to a value of integer type zint.
- If g.type is _IDNT, then g._IDNTptr points to an identifier of type identificateur.
- If g.type will be _CPLX, then g._CPLXptr and g._CPLXptr+1 are pointers to the real and imaginary parts of g.

As an example, put the following program in a file called pgcd.cpp.

#include <stdexcept> #include <cmath> #include <cstdlib> #include <giac/config.h> #include <giac/giac.h> using namespace std; #ifndef NO_NAMESPACE_GIAC namespace giac { #endif gen monpgcd(const gen & a0,const gen & b0) { gen q,r,a=a0,b=b0; for (;b!=0;) { r=irem(a,b,q); a=b; b=r; } return a; } gen _monpgcd(const gen & args,GIAC_CONTEXT) { if ((args.type!=_VECT) || (args._VECTptr->size()!=2)) return gensizeerr(contextptr); // return an error vecteur &v=*args._VECTptr; return monpgcd(v[0],v[1]); } const string _monpgcd_s[]="monpgcd"; unary_function_eval __monpgcd(0,&_monpgcd,_monpgcd_s); unary_function_ptr at_monpgcd(&__monpgcd,0,true); #ifndef NO_NAMESPACE_GIAC } #endif

After compiling this with the commands with

g++ -I.. -fPIC -DPIC -g -c pgcd.cpp -o pgcd.lo && ln -sf pgcd.lo pgcd.o && \ gcc -shared pgcd.lo -lc -lgiac -Wl,-soname -Wl,libpgcd.so.0 -o \ libpgcd.so.0.0.0 && ln -sf libpgcd.so.0.0.0 libpgcd.so.0 && \ ln -sf libpgcd.so.0.0.0 libpgcd.so

the new command can be inserted with the insmod command in giac, where insmod takes the full absolute path of the libpgcd.so file as argument.

insmod("/path/to/file/libpgcd.so")

Afterwards, the monpgcd command will be another giac command. For example, input:

monpgcd(30,36)

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