Q: If I have a char * variable pointing to the name of a function, how can I call that function? Code like
extern int func(int, int); char *funcname = "func"; int r = (*funcname)(1, 2);or
r = (*(int (*)(int, int))funcname)(1, 2);doesn't seem to work.
A: By the time a program is running, information about the names of its functions and variables (the ``symbol table'') is no longer needed, and may therefore not be available. The most straightforward thing to do, therefore, is to maintain that information yourself, with a correspondence table of names and function pointers:
int one_func(), two_func();
int red_func(), blue_func();
struct { char *name; int (*funcptr)(); } symtab[] = {
"one_func", one_func,
"two_func", two_func,
"red_func", red_func,
"blue_func", blue_func,
};
Then,
search the table for the name, and call
via
the
associated function pointer,
with code like this:
#include <stddef.h>
int (*findfunc(char *name))()
{
int i;
for(i = 0; i < sizeof(symtab) / sizeof(symtab[0]); i++) {
if(strcmp(name, symtab[i].name) == 0)
return symtab[i].funcptr;
}
return NULL;
}
...
char *funcname = "one_func";
int (*funcp)() = findfunc(funcname);
if(funcp != NULL)
(*funcp)();
The callable
functions should all have
compatible argument and return types.
(Ideally,
the function pointers would also specify the argument types.)
It is sometimes possible for a program to read its own symbol table if it is still present, but it must first be able to find its own executable (see question 19.31), and it must know how to interpret the symbol table (some Unix C libraries provide an nlist function for this purpose). See also questions 2.15, 18.14, and 19.36.
References:
PCS Sec. 11 p. 168
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