Following an [interesting discussion on Reddit](http://programming.reddit.com/info/62v70/comments) about [first class functions](http://en.wikipedia.org/wiki/First-class_function) in C, I was inspired to see what I could do with this new-found knowledge. The result is what I affectionately call “GCCalc”, for reasons that will become clear below.
GCCalc is a simple command line calculator, much like the common [bc](http://en.wikipedia.org/wiki/Bc_programming_language) calculator on many Unix systems. It’s implementation, however, is *very* different than most calculators. While bc is said to have “C-like syntax”, GCCalc’s syntax *is* C. Whatever you enter on the command line automatically gets compiled, loaded, and executed, and the result is returned (as a double) and printed to the screen.
You can either enter expressions like:
round(46.95886*sqrt(1+2/9.99*sin((21%5)*pow(2,8))))
or you can enter whole C statements (as long as they’re on one line, for now) like:
int i; for (i=0;i<10;i++) { printf("hello world!\n"); } printf("goodbye\n");
Unfortunately variables are scoped to the function that wraps them, so they don't persist across multiple entries. However, you can access the last result using the "last" variable (a double).
[Here's the source file](http://tlrobinson.net/projects/gccalc/gccalc.c), and here's a syntax highlighted version:
It's been tested on Mac OS X (Leopard) and Linux (Ubuntu Gutsy), with GCC 4. Compile with "gcc -o gccalc gccalc.c" on OS X, or "gcc -o gccalc gccalc.c -ldl" on Linux.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <dlfcn.h>
#include <unistd.h>
#ifdef __ELF__
#define GCC_FLAGS "-fPIC -shared"
#define EXTENSION "so"
#else
#define GCC_FLAGS "-dynamiclib"
#define EXTENSION "dylib"
#endif
#define HEADERS "#include <stdio.h>\n#include<math.h>"
typedef double(func_return_double)(
double);
unsigned count = 0;
char *cwd;
char tmp_path[1024] = {‘\0’};
void *lib = NULL;
int main(int argc, char **argv)
{
double result = 0.0;
char input_buffer[1024], code_buffer[2048], function_name[32], command_buffer[1024];
// get out current directory, which we’ll use for tmp files (dlopen seems to need absolute paths)
cwd = getcwd(NULL, 0);
while (1)
{
// for unique function and file names (needed for dlopen/dlsym to work correctly)
count++;
// read in the next line
printf(">> ");
fgets(input_buffer, sizeof(input_buffer), stdin);
// format the function name
sprintf(function_name, "f%d", count);
// format the code string: if it doesn’t contain a semicolon, assume it is just an expression
if (strchr(input_buffer, ‘;’))
sprintf(code_buffer, "%s\ndouble %s(double last) { %s\nreturn 0; }", HEADERS, function_name, input_buffer);
else
sprintf(code_buffer, "%s\ndouble %s(double last) { return (%s); }", HEADERS, function_name, input_buffer);
// format the filename string, delete the file if it exists
sprintf(tmp_path, "%s/libtmp%d.%s", cwd, count, EXTENSION);
unlink(tmp_path);
// format the gcc command string
sprintf(command_buffer, "gcc -Wall %s -x c – -o %s", GCC_FLAGS, tmp_path);
// execute gcc command, write out the code
FILE *fp = popen(command_buffer, "w");
fwrite(code_buffer, 1, strlen(code_buffer), fp);
fprintf(fp, "\n");
// pclose waits for gcc to terminate (fclose/close do NOT thus compilation will sometimes not finish prior to the dlopen)
pclose(fp);
void *ptr = NULL;
// open the just-compiled dynamic library
if ((lib = dlopen(tmp_path, RTLD_NOW|RTLD_LOCAL)) == NULL) {
puts(dlerror());
}
// get the function pointer
else if ((ptr = dlsym(lib, function_name)) == NULL) {
puts(dlerror());
}
// execute it
if (ptr != NULL)
{
func_return_double *func = (func_return_double*)ptr;
result = (*func)(result);
// print the result
printf("=> %.*lf\n", (result/((int)result)>1.0)?5:0, result);
}
// clean up: close the library, delete the temp file
dlclose(lib);
unlink(tmp_path);
}
return 0;
}
Thanks to jbert on Reddit for the initial code and inspiration.
If only I had known about this back when The Daily WTF has having their [OMG WTF](http://omg.thedailywtf.com/) crazy calculator programming contest…
