HELP!!!! Super urgent!!!!

slinga

Member
Hey guys,

I have a major assignment due tomorrow. It's for security class and we're implementing AES encryption. I need a function that will convert a hex numbers to ASCII equivalent. IE, I have the number 41, and I need to to turn back into an A. SOMEBODY HELP ME!!!

I thought about manually hardcoding each value in a massive switch statement, but I don't know about some of the weirder values. The project is in C. I really appreciate it.
 

ExCyber

Staff member
Hex and ASCII are just different representations of the underlying numbers; are you trying to read an ASCII representation of a hex number and convert it to the actual number? If so, libc provides the scanf/sscanf function for you, of interest would be the %x conversion:

NAME

scanf, fscanf, sscanf, vscanf, vsscanf, vfscanf - input format conver-

sion

SYNOPSIS

#include <stdio.h>

int scanf(const char *format, ...);

int fscanf(FILE *stream, const char *format, ...);

int sscanf(const char *str, const char *format, ...);

#include <stdarg.h>

int vscanf(const char *format, va_list ap);

int vsscanf(const char *str, const char *format, va_list ap);

int vfscanf(FILE *stream, const char *format, va_list ap);

DESCRIPTION

The scanf family of functions scans input according to a format as

described below. This format may contain conversion specifiers; the

results from such conversions, if any, are stored through the pointer

arguments. The scanf function reads input from the standard input

stream stdin, fscanf reads input from the stream pointer stream, and

sscanf reads its input from the character string pointed to by str.

The vfscanf function is analogous to vfprintf(3) and reads input from

the stream pointer stream using a variable argument list of pointers

(see stdarg(3). The vscanf function scans a variable argument list

from the standard input and the vsscanf function scans it from a

string; these are analogous to the vprintf and vsprintf functions

respectively.

Each successive pointer argument must correspond properly with each

successive conversion specifier (but see `suppression' below). All

conversions are introduced by the % (percent sign) character. The for-

mat string may also contain other characters. White space (such as

blanks, tabs, or newlines) in the format string match any amount of

white space, including none, in the input. Everything else matches

only itself. Scanning stops when an input character does not match

such a format character. Scanning also stops when an input conversion

cannot be made (see below).

CONVERSIONS

Following the % character introducing a conversion there may be a num-

ber of flag characters, as follows:

* Suppresses assignment. The conversion that follows occurs as

usual, but no pointer is used; the result of the conversion is

simply discarded.

a (glibc) Indicates that the conversion will be s, the needed mem-

ory space for the string will be malloc'ed and the pointer to

it will be assigned to the char pointer variable, which does not

have to be initialized before. This flag does not exist in ANSI

C (C89) and has a different meaning in C99.

a (C99) Equivalent to f.

h Indicates that the conversion will be one of dioux or n and the

next pointer is a pointer to a short int (rather than int).

l Indicates either that the conversion will be one of dioux or n

and the next pointer is a pointer to a long int (rather than

int), or that the conversion will be one of efg and the next

pointer is a pointer to double (rather than float). Specifying

two l flags is equivalent to the L flag.



L Indicates that the conversion will be either efg and the next

pointer is a pointer to long double or the conversion will be

dioux and the next pointer is a pointer to long long. (Note

that long long is not an ANSI C type. Any program using this

will not be portable to all architectures).

q equivalent to L. This flag does not exist in ANSI C.

In addition to these flags, there may be an optional maximum field

width, expressed as a decimal integer, between the % and the conver-

sion. If no width is given, a default of `infinity' is used (with one

exception, below); otherwise at most this many characters are scanned

in processing the conversion. Before conversion begins, most conver-

sions skip white space; this white space is not counted against the

field width.

The following conversions are available:



% Matches a literal `%'. That is, `%%' in the format string

matches a single input `%' character. No conversion is done,

and assignment does not occur.

d Matches an optionally signed decimal integer; the next pointer

must be a pointer to int.

D Equivalent to ld; this exists only for backwards compatibility.

(Note: thus only in libc4. In libc5 and glibc the %D is silently

ignored, causing old programs to fail mysteriously.)

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.

o Matches an unsigned octal integer; the next pointer must be a

pointer to unsigned int.

u Matches an unsigned decimal integer; the next pointer must be a

pointer to unsigned int.

x Matches an unsigned hexadecimal integer; the next pointer must

be a pointer to unsigned int.

X Equivalent to x.



f Matches an optionally signed floating-point number; the next

pointer must be a pointer to float.

e Equivalent to f.

g Equivalent to f.

E Equivalent to f.

s Matches a sequence of non-white-space characters; the next

pointer must be a pointer to char, and the array must be large

enough to accept all the sequence and the terminating NUL char-

acter. The input string stops at white space or at the maximum

field width, whichever occurs first.

c Matches a sequence of width count characters (default 1); the

next pointer must be a pointer to char, and there must be enough

room for all the characters (no terminating NUL is added). The

usual skip of leading white space is suppressed. To skip white

space first, use an explicit space in the format.

[ Matches a nonempty sequence of characters from the specified set

of accepted characters; the next pointer must be a pointer to

char, and there must be enough room for all the characters in

the string, plus a terminating NUL character. The usual skip of

leading white space is suppressed. The string is to be made up

of characters in (or not in) a particular set; the set is

defined by the characters between the open bracket [ character

and a close bracket ] character. The set excludes those charac-

ters if the first character after the open bracket is a circum-

flex ^. To include a close bracket in the set, make it the

first character after the open bracket or the circumflex; any

other position will end the set. The hyphen character - is also

special; when placed between two other characters, it adds all

intervening characters to the set. To include a hyphen, make it

the last character before the final close bracket. For

instance, `[^]0-9-]' means the set `everything except close

bracket, zero through nine, and hyphen'. The string ends with

the appearance of a character not in the (or, with a circumflex,

in) set or when the field width runs out.

p Matches a pointer value (as printed by `%p' in printf(3); the

next pointer must be a pointer to void.

n Nothing is expected; instead, the number of characters consumed

thus far from the input is stored through the next pointer,

which must be a pointer to int. This is not a conversion,

although it can be suppressed with the * flag. The C standard

says: `Execution of a %n directive does not increment the

assignment count returned at the completion of execution' but

the Corrigendum seems to contradict this. Probably it is wise

not to make any assumptions on the effect of %n conversions on

the return value.

RETURN VALUE

These functions return the number of input items assigned, which can be

fewer than provided for, or even zero, in the event of a matching fail-

ure. Zero indicates that, while there was input available, no conver-

sions were assigned; typically this is due to an invalid input charac-

ter, such as an alphabetic character for a `%d' conversion. The value

EOF is returned if an input failure occurs before any conversion such

as an end-of-file occurs. If an error or end-of-file occurs after con-

version has begun, the number of conversions which were successfully

completed is returned.

SEE ALSO

strtol(3), strtoul(3), strtod(3), getc(3), printf(3)

CONFORMING TO

The functions fscanf, scanf, and sscanf conform to ANSI X3.159-1989

(``ANSI C'').

The q flag is the BSD 4.4 notation for long long, while ll or the usage

of L in integer conversions is the GNU notation.

The Linux version of these functions is based on the GNU libio library.

Take a look at the info documentation of GNU libc (glibc-1.08) for a

more concise description.

BUGS

All functions are fully ANSI X3.159-1989 conformant, but provide the

additional flags q and a as well as an additional behaviour of the L

and l flags. The latter may be considered to be a bug, as it changes

the behaviour of flags defined in ANSI X3.159-1989.

Some combinations of flags defined by ANSI C are not making sense in

ANSI C (e.g. %Ld). While they may have a well-defined behaviour on

Linux, this need not to be so on other architectures. Therefore it usu-

ally is better to use flags that are not defined by ANSI C at all, i.e.

The Linux version of these functions is based on the GNU libio library.

Take a look at the info documentation of GNU libc (glibc-1.08) for a

more concise description.

BUGS

All functions are fully ANSI X3.159-1989 conformant, but provide the

additional flags q and a as well as an additional behaviour of the L

and l flags. The latter may be considered to be a bug, as it changes

the behaviour of flags defined in ANSI X3.159-1989.

Some combinations of flags defined by ANSI C are not making sense in

ANSI C (e.g. %Ld). While they may have a well-defined behaviour on

Linux, this need not to be so on other architectures. Therefore it usu-

ally is better to use flags that are not defined by ANSI C at all, i.e.

use q instead of L in combination with diouxX conversions or ll.

The usage of q is not the same as on BSD 4.4, as it may be used in

float conversions equivalently to L.

 

slinga

Member
After about 3 hours of random guessing, some nice guess decided to help me. All I had to do was "%c" in a sprintf function call. Argh.
 

ExCyber

Staff member
I'd recommend either avoiding itoa() or embedding it in your source; it's not a standard function (not in ANSI or POSIX), so you can't count on it being available or having consistent behavior in the C library.

It also doesn't do what slinga was looking for, though it is obviously handy.
 
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