C Language, Can function returning pointer return negative integers?

Can function returning pointer return negative integers?

Author	Message
Robert Jon Bredl #1 / 4	Can function returning pointer return negative integers? I'm implementing a hash table and when the client puts a value/item into the table, there are two possibilities. Either the value/item is already in the table or it is not. If it is not, memory needs to be allocated to make another link in one of the chains of the table. This leads to two possible outcomes, either the memory can be allocated or it can not. It's a generic hash table meaning the data is stored into it as void pointers and each item of data has a specific key that indexes it into the hash table. Here's what I'd like to do. If the client tries to put data into the table and data is already attached to the specific key for that data, to return the old data (which will be a pointer) to the client and put the new data in the table. If the key is not already in the table, just store the pointer in the table. But either the memory for the new "bucket" can be allocated or it can't. So here's my problem. I'd like to return any existing data to the client which would be a positive return value because it's a pointer. If the key did not already exist and the malloc succeeded, I'd like to return NULL. And if the malloc fails I'd like to return -1. But the compiler is complaining about me returning -1 because the function is declared to return pointers and pointers can't be negative. For now, I've typecasted the return statement that's returning the -1 and the compiler is not complaining. My concern is something odd happening like returning -1 and the compiler wrapping it to a positive number because pointers can't be negative (Or memory addresses can't be negative at least). Sorry if that's lengthy but I want to give you guys enough info to help me understand yet another aspect of the C programming language. 8) Robert
Thu, 26 Sep 2002 03:00:00 GMT

Lou Zhe #2 / 4	Can function returning pointer return negative integers? Quote: > I'm implementing a hash table and when the client puts a value/item into >the table, there are two possibilities. Either the value/item is already in >the table or it is not. If it is not, memory needs to be allocated to make >another link in one of the chains of the table. This leads to two possible >outcomes, either the memory can be allocated or it can not. It's a generic >hash table meaning the data is stored into it as void pointers and each item >of data has a specific key that indexes it into the hash table. Here's what >I'd like to do. If the client tries to put data into the table and data is >already attached to the specific key for that data, to return the old data >(which will be a pointer) to the client and put the new data in the table. >If the key is not already in the table, just store the pointer in the table. >But either the memory for the new "bucket" can be allocated or it can't. >So here's my problem. I'd like to return any existing data to the client which >would be a positive return value because it's a pointer. If the key did not >already exist and the malloc succeeded, I'd like to return NULL. And if the >malloc fails I'd like to return -1. But the compiler is complaining about me >returning -1 because the function is declared to return pointers and pointers >can't be negative. For now, I've typecasted the return statement that's >returning the -1 and the compiler is not complaining. My concern is something >odd happening like returning -1 and the compiler wrapping it to a positive >number because pointers can't be negative (Or memory addresses can't be >negative at least). Sorry if that's lengthy but I want to give you guys >enough info to help me understand yet another aspect of the C programming >language. 8) While it certainly is improbable that a pointer set to -1 will cause actual run-time trouble (as long as you don't touch what it points to), your compiler was right to complain. Generally, 'cramming' variables or return values is more trouble than it's worth. Lots of standard C functions return more than one value, but only one value comes out of the function outside the parenthesis. The only real pointer overloading done to add NULL to indicate some type of error occurred. As far as your wrapping concern, well, just don't touch *ptr when ptr==-1. BTW: IFAIK, pointers are always unsigned. On Intel/DOS, you get near and far pointers. nears are only 16-bit guys (offset in current segment). fars are 2x16-bits (segment:offset). Setting a near pointer to -1 would yield a near pointer to 0xFFFF. The only big danger comes in when you have value range collisions. It is highly improbable that any pointer you actually use will be 0xFFFF, and as long as you don't point with that crammed value, you can get away with it.... for now. Pointer cramming is a totally ++ungood practice. Even same data type cramming is considered, in most circles, bad code. I've seen lots of code that would return a positive integer result on success, but some negative number, or super-high positive number, to indicate what kind of error occurred. Yuck. That's cramming. Two different types of information, crammed into one data unit. Avoid it. instead of your cramming... ptr = pHashTable(value, item); if ((int)ptr==-1) ; // malloc failed else if (ptr==NULL) ; // malloc did good else ; // here's the existing data can't you do this? iErr = iHashTable(&ptr, value, item); if (!iErr) if (ptr!=NULL) ; // old key replace else ; // new key added else printf("Error #%i occured in iHashTable\n", iErr); -LZ
Thu, 26 Sep 2002 03:00:00 GMT

Cursing a China Blue Stre #3 / 4	Can function returning pointer return negative integers? / odd happening like returning -1 and the compiler wrapping it to a positive / number because pointers can't be negative (Or memory addresses can't be / negative at least). Sorry if that's lengthy but I want to give you guys An address can have the sign or highest order bit set. It is possible that an address can have the same bit pattern as -1. Why not return an address you know can never be duplicated by any other pointer? xyz.h void xyz(...); void xyz_mallocFailed; xyz.c static char xyz_unique; void xyz_mallocFailed = (void)&xyz_unique; void xyz(...) {...} void v = xyz(...); if (v==xyz_mallocFailed) {...} else if (v==NULL) {...} else {...} -- CACS: Collective Against Consensual Sanity v0.123 Now a text site map! http://www.angelfire.com/ca3/cacs/ pretty? http://www.geocities.com/SoHo/Studios/5079/ :)-free zone. Don't speak Latin in front of the books!
Thu, 26 Sep 2002 03:00:00 GMT

Gordon Burdi #4 / 4	Can function returning pointer return negative integers? Quote: >So here's my problem. I'd like to return any existing data to the >client which >would be a positive return value because it's a pointer. What makes you think that a "positive" pointer value that points to valid memory exists? There's no guarantee that it does. (Your implementation might treat pointers as unsigned, but there's no guarantee another one will.) Quote: >If the key did not >already exist and the malloc succeeded, I'd like to return NULL. And if the >malloc fails I'd like to return -1. But the compiler is complaining about me >returning -1 because the function is declared to return pointers and pointers >can't be negative. Are you sure it's not complaining about returning -1 because the function is declared to return pointers and -1 isn't a pointer? Quote: >For now, I've typecasted the return statement that's >returning the -1 and the compiler is not complaining. My concern is something >odd happening like returning -1 and the compiler wrapping it to a positive >number One thing you might have to worry about is malloc() returning -1 when you allocate a hash table entry. One solution is to check if it does return (void *)-1, and if so, leak that chunk of memory and get another one. This should happen at most once. Quote: >because pointers can't be negative What makes you believe that pointers can be non-negative? Quote: >(Or memory addresses can't be >negative at least). What makes you believe this? Where can I get some of it without getting nabbed by undercover cops? Gordon L. Burditt
Fri, 27 Sep 2002 03:00:00 GMT

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