how to read a C function from STDIN and run that in program?

Question

I want to write a program which reads from stdin : scanf("%s %s", foo, args); then run function foo ( which was defined in program before ) with arguments : foo(args); I don't know how to do this in C . I could easily do this in Shell before but seems difficult in C

Answer 1

There's no solution for this problem which is generalized and cross-platform/standard at the same time.

The non-general solution that doesn't rely on non-standard extensions: store name and function pointer pairs in an array, perform search, call function by pointer. For example:

struct namedfunc {
    const char *name;
    void (*fptr)(void);
} fvect[3] = {
    { "foo", foo_impl },
    { "bar", bar_impl },
    { "baz", baz_impl }
};

char buf[0x100];
fgets(buf, sizeof buf, stdin);
char *p = strchr(buf, '\n');
if (p != NULL)
    *p = 0;

for (size_t i = 0; i < sizeof fvect / sizeof fvect[0]; i++) {
    if (strcmp(buf, fvect[i].name) == 0) {
        fvect[i].fptr();
        break;
    }
}

The more general solution that relies on dynamic loading being available: use the dlopen() API, like this:

void *hndl = dlopen(RTLD_SELF, RTLD_LAZY); // try NULL if RTLD_SELF doesn't work
void (*fptr)(void) = dlsym(hndl, buf);
if (fptr != NULL)
    fptr();

dlclose(hndl);

Answer 2

Another hard to debug, highly fragile solution would involve compiling with debug symbols (or a map file), parsing them and calling the function dynamically. That's crazy.

Reasonably though, do you really want every single function in your program callable in this way? Including main? Probably not. Well then, identify a set of functions that should be callable in this way, and build an array of structures that would contain a function name and a function pointer. When presented with function name upon input, scan the array for that function, find the function pointer and call. If they have different prototypes, add another member that would describe the kind of parameter(s) the function would expect. (EDIT: the answer above suggests the same and provides details)