How can I use a function pointer instead of a switch statement?
A slightly different approach from the link posted by ars: You can use the value from the switch statement as an array index in an array of function pointers. So instead of writing
switch (i) {
case 0: foo(); break;
case 1: bar(); break;
case 2: baz(); break;
}
you can do this
typedef void (*func)();
func fpointers[] = {foo, bar, baz};
fpointers[i]();
Alternatively you can use the function pointers instead of numbers as described in ars's answer.
Here's a page that does a pretty good job of explaining this in C++:
http://oopweb.com/CPP/Documents/FunctionPointers/Volume/CCPP/FPT/em_fpt.html
EDIT:
Above link is broken as of June 2020. Cached here:
https://web.archive.org/web/20170606042951/http://oopweb.com/CPP/Documents/FunctionPointers/Volume/CCPP/FPT/em_fpt.html
Related
I have a dynamically linked ELF executable on Linux, and I want to swap a function in a library it is linked against. With LD_PRELOAD I can, of course, supply a small library with a replacement for the function that I compile myself. However, what if in the replacement I want to call the original library function? For example, the function may be srand(), and I want to hijack it with my own seed choice but otherwise let srand() do whatever it normally does.
If I were linking to make said executable, I would use the wrap option of the linker but here I only have the compiled binary.
One trivial solution I see is to cut and paste the source code for the original library function into the replacement - but I want to handle the more general case when the source is unavailable. Or, I could hex edit the needed extra code into the binary but that is specific to the binary and also time consuming. Is something more elegant possible than either of these? Such as some magic with the loader?
(Apologies if I were not using the terminology precisely...)
Here's an example of wrapping malloc:
// LD_PRELOAD will cause the process to call this instead of malloc(3)
// report malloc(size) calls
void *malloc(size_t size)
{
// on first call, get a function pointer for malloc(3)
static void *(*real_malloc)(size_t) = NULL;
static int malloc_signal = 0;
if(!real_malloc)
{
// real_malloc = (void *(*)(size_t))dlsym(RTLD_NEXT, "malloc");
*(void **) (&real_malloc) = dlsym(RTLD_NEXT, "malloc");
}
assert(real_malloc);
if (malloc_signal == 0)
{
char *string = getenv("MW_MALLOC_SIGNAL");
if (string != NULL)
{
malloc_signal = 1;
}
}
// call malloc(3)
void *retval = real_malloc(size);
fprintf(stderr, "MW! %f malloc size %zu, address %p\n", get_seconds(), size, retval);
if (malloc_signal == 1)
{
send_signal(SIGUSR1);
}
return retval;
}
The canonical answer is to use dlsym(RTLD_NEXT, ...).
From the man page:
RTLD_NEXT
Find the next occurrence of the desired symbol in the search
order after the current object. This allows one to provide a
wrapper around a function in another shared object, so that,
for example, the definition of a function in a preloaded
shared object (see LD_PRELOAD in ld.so(8)) can find and invoke
the "real" function provided in another shared object (or for
that matter, the "next" definition of the function in cases
where there are multiple layers of preloading).
See also this article.
Just for completeness, regarding editing the function name in the binary - I checked and it works but not without potential hiccups. E.g., in the example I mentioned, one can find the offset of "srand" (e.g., via strings -t x exefile | grep srand) and hex edit the string to "sran0". But names of symbols may be overlapping (to save space), so if the code also calls rand(), then there is only one "srand" string in the binary for both. After the change the unresolved references will then be to sran0 and ran0. Not a showstopper, of course, but something to keep in mind. The dlsym() solution is certainly more flexible.
I'm trying to parallelize switch ... case (c++) using OpenMP directive, but despite my best efforts, the code goes slower than normal sequential execution.
I have used #pragma parallel, #pragma sections,
I have tried to rewrite the switch case with an if ... else statement
but with no good result ...
switch (number) {
case 1:
f1();
break;
case 2:
f2();
break;
case 3:
f3();
break;
case 4:
fn();
break;
}
Then there is a second problem, OpenMP won't break or return.
The switch cases cannot be implemented in Openmp, just by adding pragma's like parallel, section. The threads running along the parallel section divide work among themselves via the loop index or else they do the same work in a conditional loop. Openmp section needs to know either how many elements it needs to work on or a master condition which determines start and end. You want to make the input section as parallel instead of the functions (f1, f2, .. fn), so I am guessing you are processing a lot of "number". One way is to collect these numbers in a array/vector. Then, you can make a parallel for along this vector/array, calling the corresponding function.
while(some_condition_on_numbers)
{
// Collect Numbers in a vector / some array
}
#pragma omp parallel for
for(int counter = 0; counter < elements_to_process; counter++)
{
F(array_of_number[counter]);
}
F(int choice)
{
if(choice = 1) {f1(); }
if(choice = 2) {f2(); }
..
}
Someone asked me today how to convert the following if statement to switch:
int i=5;
if(i>10)
{
Do Something;
}
else
{
Do Something else;
}
And I proposed that assuming i is an integer with only positive values:
int i=5;
switch(i)
{
case 1:
case 2:
case 3:
case 4:
case 5:
case 6:
case 7:
case 8:
case 9:
case 10: Do Something else;
break;
case default: Do Something;
break;
}
Is there any other, more elegant way of doing it?
There is no direct way of doing that in C#, you can try the following.
int range = (number - 1) / 10;
switch(range)
{
case 0:
Console.WriteLine("1 to 10");
break;
case 1:
Console.WriteLine("11 to 20");
break;
//.,......
default:
break;
}
Not really sure how much clear it would be, IMO, the above approach would reduce the code readability, its better if you use if-else for the range checking. Also the above would work only if the range is constant.
First, note your two code examples (as were originally written) were not equivalent:
You're not Do[ing] Something Else when i==10 in your switch example.
But the original if code was checking on i>10 (as opposed to i>=10).
So case 10: should be explicitly included in your list of cases that fall through to Do Something Else.
To answer your question, I believe in some languages (eg pascal, objective C, I think?) a range similar to this is allowed:
switch(i)
{
case 1-10: // or is it "case 1 .. 10" ?
Do Something else;
break;
case default:
Do Something;
break;
}
But this is not supported in C#. Refer the very final point in the code discussion regarding "stacking" of case labels here MSDN switch(C#)
I started learning Dart today, and I've come across something that my google skills are having trouble finding.
How do I have a fall-through in a non-empty case?
My use case is this: I'm writing a sprintf implementation (since dart doesn't have this too), which would work except for this fall-through thing. When parsing the variable type you can, for example, have "%x" versus "%X" where the upper case type tells the formatter that the output is supposed to be uppercase.
The semi-pseudocode looks like:
bool is_upper = false;
switch (getType()) {
case 'X':
is_upper = true;
case 'x':
return formatHex(is_upper);
}
The other ways I can think of doing this, would one of the following
1:
switch (getType()) {
case 'X': case 'x':
return formatHex('X' == getType());
}
2:
var type = getType();
if (type in ['x', 'X']) {
return formatHex('X' == getType());
}
Now, the second choice almost looks good, but then you have to remember that there are eleven cases, which would mean having eleven if (type in []), which is more typing that I'd like.
So, does dart have some // //$FALL-THROUGH$ that I don't know about?
Thanks.
The Dart specification gives a way for a switch case to continue to another switch case using "continue":
switch (x) {
case 42: print("hello");
continue world;
case 37: print("goodbye");
break;
world: // This is a label on the switch case.
case 87: print("world");
}
It works in the VM, but sadly the dart2js switch implementation doesn't yet support that feature.
From the dart language tour, your example of (2) should be correct.
var command = 'CLOSED';
switch (command) {
case 'CLOSED': // Empty case falls through.
case 'NOW_CLOSED':
// Runs for both CLOSED and NOW_CLOSED.
executeClose();
break;
}
It would be an error if you tried to do something as follows
var command = 'OPEN';
switch (command) {
case 'OPEN':
executeOpen();
// ERROR: Missing break causes an exception to be thrown!!
case 'CLOSED':
executeClose();
break;
}
EDIT: This seems to only have worked due to a bug in the implementation of Dart. See the Dart 1.x language spec, section "17.15 Continue".
The bug is now fixed, as reported in a comment, so this no longer works. Keeping for historical reasons.
Also, a simplification of Lasse's answer is to use a simple continue; instead of continue <label>; as in:
bool is_upper = false;
switch (getType()) {
case 'X':
is_upper = true;
continue;
case 'x':
return formatHex(is_upper);
}
You can't have a non-empty case body in Dart that falls through, this will raise an error.
What I tend to do with anything other than very simple switch statements is to refactor all the common code out into functions, so that you don't have this multi-level control flow in the switch itself.
In other words, something like:
switch (getType()) {
case 'X':
return formatHex(true);
case 'x':
return formatHex(false);
}
There's no reason why you need to have fallthrough. It comes in handy when the actions in a case section can be carried out in toto at the end of another case section, but this method can do that without fallthrough and without making your switch statement complex.
It can also handle more complex cases where there are common actions that aren't included in toto at the end. For example, you may want to do something at the start or in the middle of the case section. Calling common functions handles that more than well enough:
switch (getType()) {
case 'X':
doSomethingOnlyForUpperCase();
doSomethingCommon();
doSomethingElseOnlyForUpperCase();
return formatHex(true);
case 'x':
doSomethingCommon();
return formatHex(false);
}
I actually also do this for languages (such as C) that support this sort of non-empty fall-through since I believe it aids in readability and maintainability.
Dart in 202207, just empty, instead of continue label
PermissionStatus recordStatus =
await Permission.requestSinglePermission(
PermissionName.Microphone);
switch (recordStatus) {
case PermissionStatus.allow:
case PermissionStatus.always:
case PermissionStatus.whenInUse:
return;
default:
break;
}
So I've recently been trying out D, as many programmers I know are recommending I learn it over C++.
I'm using the DMD Compiler v2.057, and this is my code:
while(cliLoopC)
{
write("?> ");
string iPt_ = std.string.tolower(readln());
switch (iPt_)
{
default: writeln(E_URECCOM); break;
case "test":
writeln("Hello!");
break;
}
}
The program is that, whenever I type in test, so it should go to the case instead of the default, it just prints the contents of E_URECCOM (which is a constant char[] that contains UNRECOGNISED COMMAND\n).
I don't see what's happening to make it do this. Any ideas?
Yes, there's a \n at the end of readln. Try using std.string.strip(readln());
Well... I haven't tried out D, but maybe readln() is including the CR, LF or CR/LF at the end of the string and it should be included in the compared string?
case "test\n":
?
Edit:
Adam D. Ruppe's answer is the correct answer in saying:
Yes, there's a \n at the end of readln. Try using std.string.strip(readln());
I just wanted to throw that in there since my answer still has the check
My answer:
The default case is the catch all case, so it looks best (and is conventional) at the end
like this
while(cliLoopC)
{
write("?> ");
string iPt_ = std.string.tolower(readln());
switch (iPt_)
{
case "test":
writeln("Hello!");
break;
default: writeln(E_URECCOM); break;
}
}