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# hex and unsigned and signed decimal

 P: n/a I am writing a little base conversion utility called base.c. This is what base does. \$ base -127 Signed decimal: -127 Unsigned decimal: 4294967169 Hexidecimal: 0xffffff81 Octal: O37777777601 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 1111 1111 1111 1111 1111 1111 1000 0001 \$ base 127 Signed decimal: 127 Unsigned decimal: 127 Hexidecimal: 0x7f Octal: O177 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0000 0000 0000 0000 0000 0000 0111 1111 However, base has a bug when one inputs sign extended hexadecimal or octal numbers, like this. \$ base 0xffffff81 Signed decimal: 2147483647 Unsigned decimal: 2147483647 Hexidecimal: 0x7fffffff Octal: O17777777777 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0111 1111 1111 1111 1111 1111 1111 1111 Base works like this. I use sscanf %i to convert argv[1] to an int. Then I cast the int to an unsigned int and do the rest of the manipulation. However, as the above example shows, it doesn't work properly when I input a sign extended octal or hex number. The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? Thanks Dec 28 '06 #1
14 Replies

 P: n/a me2 wrote: I am writing a little base conversion utility called base.c. This is what base does. \$ base -127 Signed decimal: -127 Unsigned decimal: 4294967169 Hexidecimal: 0xffffff81 Octal: O37777777601 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 1111 1111 1111 1111 1111 1111 1000 0001 \$ base 127 Signed decimal: 127 Unsigned decimal: 127 Hexidecimal: 0x7f Octal: O177 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0000 0000 0000 0000 0000 0000 0111 1111 However, base has a bug when one inputs sign extended hexadecimal or octal numbers, like this. \$ base 0xffffff81 Signed decimal: 2147483647 Unsigned decimal: 2147483647 Hexidecimal: 0x7fffffff Octal: O17777777777 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0111 1111 1111 1111 1111 1111 1111 1111 Base works like this. I use sscanf %i to convert argv[1] to an int. Then I cast the int to an unsigned int and do the rest of the manipulation. However, as the above example shows, it doesn't work properly when I input a sign extended octal or hex number. The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? Because %i is only for integers. You will need to check the string in argv[1] before scanning it using sscanf. If it is preceded with "0x" then use %x (hexadecimal), if its preceded with just "0" then use %o (octal) for scanning with sprintf, and later you can convert it to other formats. Dec 28 '06 #2

 P: n/a Scorpio wrote: me2 wrote: I am writing a little base conversion utility called base.c. This is what base does. \$ base -127 Signed decimal: -127 Unsigned decimal: 4294967169 Hexidecimal: 0xffffff81 Octal: O37777777601 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 1111 1111 1111 1111 1111 1111 1000 0001 \$ base 127 Signed decimal: 127 Unsigned decimal: 127 Hexidecimal: 0x7f Octal: O177 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0000 0000 0000 0000 0000 0000 0111 1111 However, base has a bug when one inputs sign extended hexadecimal or octal numbers, like this. \$ base 0xffffff81 Signed decimal: 2147483647 Unsigned decimal: 2147483647 Hexidecimal: 0x7fffffff Octal: O17777777777 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0111 1111 1111 1111 1111 1111 1111 1111 Base works like this. I use sscanf %i to convert argv[1] to an int. Then I cast the int to an unsigned int and do the rest of the manipulation. However, as the above example shows, it doesn't work properly when I input a sign extended octal or hex number. The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? Because %i is only for integers. You will need to check the string in argv[1] before scanning it using sscanf. If it is preceded with "0x" then use %x (hexadecimal), if its preceded with just "0" then use %o (octal) for scanning with sprintf, and later you can convert it to other formats. Correction...I meant scanning with sscanf, not sprintf. Sorry. Dec 28 '06 #3

 P: n/a Scorpio wrote: Scorpio wrote: me2 wrote: I am writing a little base conversion utility called base.c. > This is what base does. > \$ base -127 Signed decimal: -127 Unsigned decimal: 4294967169 Hexidecimal: 0xffffff81 Octal: O37777777601 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 1111 1111 1111 1111 1111 1111 1000 0001 \$ base 127 Signed decimal: 127 Unsigned decimal: 127 Hexidecimal: 0x7f Octal: O177 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0000 0000 0000 0000 0000 0000 0111 1111 > However, base has a bug when one inputs sign extended hexadecimal or octal numbers, like this. > \$ base 0xffffff81 Signed decimal: 2147483647 Unsigned decimal: 2147483647 Hexidecimal: 0x7fffffff Octal: O17777777777 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0111 1111 1111 1111 1111 1111 1111 1111 > Base works like this. I use sscanf %i to convert argv[1] to an int. Then I cast the int to an unsigned int and do the rest of the manipulation. However, as the above example shows, it doesn't work properly when I input a sign extended octal or hex number. > The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? Because %i is only for integers. You will need to check the string in argv[1] before scanning it using sscanf. If it is preceded with "0x" then use %x (hexadecimal), if its preceded with just "0" then use %o (octal) for scanning with sprintf, and later you can convert it to other formats. Correction...I meant scanning with sscanf, not sprintf. Sorry. More correction. I just realized that %x and %o are only for printing, and its cannot be used for scanning. So you'll have to find some other method to read hexadecimal and octal values. Dec 28 '06 #4

 P: n/a On Thu, 28 Dec 2006 07:04:32 -0800, Scorpio wrote: Because %i is only for integers. You will need to check the string in argv[1] before scanning it using sscanf. If it is preceded with "0x" then use %x (hexadecimal), if its preceded with just "0" then use %o (octal) for scanning with sprintf, and later you can convert it to other formats. But man sscanf says this: The following conversion specifiers are available: .... i Matches an optionally signed integer; the next pointer must be a pointer to int. The integer is read in base 16 if it begins with 0x or 0X, in base 8 if it begins with 0, and in base 10 otherwise. Only characters that correspond to the base are used. So why isn't it properly reading the hex and octal numbers ? Thanks. Dec 28 '06 #5

 P: n/a me2 wrote: > .... snip ... > However, base has a bug when one inputs sign extended hexadecimal or octal numbers, like this. \$ base 0xffffff81 Signed decimal: 2147483647 Unsigned decimal: 2147483647 Hexidecimal: 0x7fffffff Octal: O17777777777 Binary: 1098 7654 3210 9876 5432 1098 7654 3210 0111 1111 1111 1111 1111 1111 1111 1111 Base works like this. I use sscanf %i to convert argv[1] to an int. Then I cast the int to an unsigned int and do the rest of the manipulation. However, as the above example shows, it doesn't work properly when I input a sign extended octal or hex number. The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? You have experienced integer overflow, after which behaviour is undefined. Therefore the answers you are getting (or anything else) are perfectly valid. -- Merry Christmas, Happy Hanukah, Happy New Year Joyeux Noel, Bonne Annee. Chuck F (cbfalconer at maineline dot net) Dec 28 '06 #6

 P: n/a me2 #include #include int main(int argc, char **argv) { char *end; long long num; if (argc!=2) { fputs("usage: base \n", stderr); return EXIT_FAILURE; } num = strtoll(argv[1], &end, 0); if (*end || errno) { fprintf(stder, "%s: invalid number\n", argv[1]); return EXIT_FAILURE; } printf("signed : %lld\n", num); printf("unsigned: %llu\n", num); printf("hex : %llo\n", num); printf("octal : %llx\n", num); return EXIT_SUCESS; } #v- (code not tested) -- Best regards, _ _ .o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o ..o | Computer Science, Michal "mina86" Nazarewicz (o o) ooo +-------ooO--(_)--Ooo-- Dec 29 '06 #7

 P: n/a Michal Nazarewicz writes: You'd better use strtol() for that which can automatically read decimal, octal or hexadecimal numbers and provides better error checking I guess. You could even use strtoll() for bigger numbers if it's available on your system (I believe it's defined in C99). Or lacking long long, handle the sign specially and read the rest into an unsigned int with strtoul or %u. Then maybe convert to int - taking care to avoid overflow, including for INT_MIN. -- Hallvard Dec 29 '06 #8

 P: n/a On Thu, 28 Dec 2006 11:14:06 -0500, CBFalconer wrote: You have experienced integer overflow, after which behaviour is undefined. Therefore the answers you are getting (or anything else) are perfectly valid. 0xffffff81 is a 32 bit number. How can it be integer overflow ? Are you saying it will interpret it as an unsigned integer and then try to stuff it in a signed int variable and overflow that way ? Dec 29 '06 #9

 P: n/a On Fri, 29 Dec 2006 11:16:00 +0100, Michal Nazarewicz wrote: Changing it to use strtol and a long prevented the overflow. It works properly now. Dec 29 '06 #10

 P: n/a me2

 P: n/a me2 writes: Changing it to use strtol and a long prevented the overflow. It works properly now. Maybe 'long' is wider than 'int' on your host, then. If that's the reason it works, it will break on hosts where both are the same size - which is legal and quite common. (For that matter 'long long' is not required to be wider than 'int' either, but at least I don't know of any counterexamples.) -- Hallvard Dec 29 '06 #12

 P: n/a Hallvard B Furuseth Changing it to use strtol and a long prevented the overflow. It worksproperly now. Maybe 'long' is wider than 'int' on your host, then. If that's the reason it works, it will break on hosts where both are the same size Then strtol() will set errno to appropriate value and thus program will print error message instead of printing wrong value (I hope). -- Best regards, _ _ .o. | Liege of Serenly Enlightened Majesty of o' \,=./ `o ..o | Computer Science, Michal "mina86" Nazarewicz (o o) ooo +-------ooO--(_)--Ooo-- Dec 31 '06 #13

 P: n/a >me2 wrote: >... I use sscanf %i to convert argv[1] to an int. ThenI cast the int to an unsigned int and do the rest of the manipulation.However, as the above example shows, it doesn't work properly when I inputa sign extended octal or hex number.The decimal of 0xffffff81 should be 4294967169, but it isn't. Why ? In article <11**********************@h40g2000cwb.googlegroups .com> Scorpio Because %i is ... Right start, but: >only for integers. wrong details. :-) All of this family of conversions -- %d, %i, %o, %u, and %x -- are "for integers", specifically for int unless modified (e.g., %lu is for long, %hx is for short; note that the hh and ll modifiers are specific to C99). All of them also convert "as if" by strtol() or strtoul() -- or in C99 sometimes strtoll() and strtoull() -- with a base of 16, 10, 8, or 0 depending on the conversion directive; except that in all cases, the behavior is not defined if the number would overflow. Most implementations seem to use the strtol() family of functions internally, so that they all exhibit that family's "clamping" behavior of out-of-range inputs. This is the case above: 0xffffff81 is (presuambly) out of range for strtol(), and %i reads a *signed* integer, so the input is clamped to INT_MAX (or more likely to LONG_MAX, but that is probably the same as INT_MAX on the target platform). -- In-Real-Life: Chris Torek, Wind River Systems Salt Lake City, UT, USA (40°39.22'N, 111°50.29'W) +1 801 277 2603 email: forget about it http://web.torek.net/torek/index.html Reading email is like searching for food in the garbage, thanks to spammers. Jan 2 '07 #14

 P: n/a Chris Torek San Diego Supercomputer Center <* We must do something. This is something. Therefore, we must do this. Jan 2 '07 #15

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