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Old 10-10-2003, 02:02 PM   #1
Registered: Sep 2003
Location: Chicago
Distribution: Slackware 13 x86_64
Posts: 58

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C, converting hex to text

ok, i appologize for the subject, i'm a newbie coder, and don't really know my way thru alot of this. Anyway, could anyone help me with this little scenario:

i have a form on a webpage, the webpage is driven by a C compiled program that i have been working on, the form passes the arguments on a "query string".. but all non alpha-numeric charactors are converted to %hexvalue.

i was wondering if there was a simpler way to strip off the % sign, take the 2 hex digits that follow the %, and print them back out as the characters they are supposed to represend.. per say,
blah blah blah d'oh
fix me

^--- something along the lines of that.

i've tried a few methods, all with undesireable results or segfaults.

Last edited by Sm0k3; 10-10-2003 at 02:04 PM.
Old 10-10-2003, 08:53 PM   #2
Registered: Oct 2003
Location: Australia
Distribution: Redhat 9
Posts: 95

Rep: Reputation: 15
At a guess,

You could use atoi to convert the %27 to an int.

const char *theValueStr="%27";

char theChar = (char) atoi(theValueStr+1); // the +1 will move the pointer passed the '%';

std::cout << theChar;

Also note that your %00 will give you a null and not a \n (newline)... as you example implies that %00 will give you a \n.. I think a \n is 10. Do a 'man ascii' to get a list of ascii codes.

Old 10-11-2003, 04:29 AM   #3
Registered: Feb 2003
Location: Iran
Distribution: Solaris 10
Posts: 201

Rep: Reputation: 30
you can first find the % sign and then use strtol():

long strtoul ( const char * string, char** endptr, int radix );
just replace a 0x instead of % and then call
myInteger = strtoul (YourStr, NULL, 10);
Now you can:
char Ch = myInteger;
You have now your original character in Ch.

Here is borrowed from anbout strtoul


unsigned long strtoul ( const char * string, char** endptr, int radix );
Convert string to unsigned long integer.
Parses string interpreting its content as an integer value until a character that can not be interpreted is found, and returns an unsigned long value.


String representing an integer number. The number is considered until a non-numeric character is found (only the digits from 0 to radix-1 are considered valid numeric characters, signs are considered valid numeric characters for this parameter). The format used is:
(where whitespaces are any tab or space character and nnnnn is a sequence of valid numbers following the specified radix).
Although the value returned is unsigned, signs in the string will be read and considered.
Address of a pointer. This is filled by the function with the address where scan has ended. Serves to determine where there is the first non-numerical character in the string.
Numeral radix in which the number to be interpreted. Must be 0 or be between 2 and 36. If it is 0 the radix of the string is determined by the initial characters of the string:
initial chars Interpreted by strol as
0x hexadecimal: radix 16
0 octal: radix 8
any number from 1 to 9 decimal: radix 10

Return Value.
The converted unsigned long value from the input string.
If conversion would cause overflow the result is ULONG_MAX.
If an error occurs or no conversion can be made 0 is returned.

Defined in ANSI-C.


/* strtoul example */
#include <stdio.h>
#include <stdlib.h>

int main ()
char szInput [256];
char * pEnd;
unsigned long ul;
printf ("Enter an integer value: ");
gets (szInput);
ul = strtoul (szInput,&pEnd,0);
printf ("Value entered: %lu. Its double: %lu\n",ul,ul*2);
return 0;


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