call_math.cpp

斯坦福Energy211/CME211课《c++编程——地球科学科学家和工程师》的课件

// This program demonstrates loading dynamic libraries
// programmatically, and calling their functions

// To compile this program:
//
// g++ call_math.cpp -ldl
//
// where the -ldl option tells the linker to include the
// library /lib/libdl.so, which includes the functions
// dlopen and dlsym

#include <dlfcn.h>
#include <iostream>

using namespace std;

typedef double (*functype)(double);

int main() {

    // /lib/libm.so is the C standard math library
    // Use dlopen to load it into memory.  RTLD_LAZY
    // means only resolve references to functions in
    // the library as they are needed, instead of
    // resolving all of them at once, which is wasteful
    // if most functions in the library won't be used.
    // Note that the path to the library is not included,
    // because dlopen will look in standard directories
    void *plib = dlopen( "libm.so", RTLD_LAZY );
    if ( plib == NULL ) {
        // Library could not be found, abort
        cerr << "Could not open math library\n"; 
        return 1;
    }

    do {
        // Get the name of a function to call
        cout << "Enter the name of the function you would like to call,\n";
        cout << "or enter END to quit: ";
        string s;
        cin >> s;
        if ( s == "END" )
            break;
        
        // Look up the function in the library
        void *psym = dlsym( plib, s.c_str() );
        if ( psym == NULL )
            cerr << "Function not found\n";
        else {
            // We assume the function takes a single argument, a double,
            // and returns a double
            cout << "Enter x-value: ";
            double x;
            cin >> x;
            // functype is a type that represents a pointer to a function
            // that takes a double argument returns a double.  dlsym
            // returns a generic pointer to void, so we have to convert
            // before we can call the function
            functype pfunc = (functype) psym;
            // Now call the function and obtain the return value
            double y = pfunc(x);
            cout << s << "(" << x << ") = " << y << endl;
        }
    } while ( true );

    return 0;
}