It would be a grand thing to set out and craft the entire decoding program in one sitting. That’s ambition in action, but it doesn’t demonstrate much programming experience.
\n
\nInstead of facing multiple setbacks, my approach is to tackle the decoding process one step at a time.<\/p>\n
The first step, conquered in last week’s Lesson<\/a>, was to figure out how to translate the hexadecimal character string into a value. That accomplishment can be set aside for the next few steps, which are:<\/p>\n 2. Process standard input one line at a time. Step 2 is to write code that chews standard input one line a time. To accomplish this task, and because this program is a filter, you could use the fgets()<\/em> function to read in a line of text from standard input. Instead, I’ve opted for more control by using the getchar()<\/em> function. That way I can monitor input one character at a time.<\/p>\n For decoding hexcode output, you know that every line is formatted: 24 hex values on a line, each prefixed by a space, evaluates to 72 characters. Add one for the null character, and you need a 73 byte buffer to hold each line of input. Any line longer than 72 characters indicates some type of error, either a file read booboo or data that’s simple not in the hexcode format.<\/p>\n Here’s my solution:<\/p>\n The filter runs from a endless while<\/em> loop (Lines 11 through 33). Line 13 fetches a character from standard input. If the end of file (EOF in Line 14) is encountered, the loop terminates and the program stops. Otherwise, the character Line 18 checks for buffer overflow, which happens when the When the newline is encountered (Line 24), the string is terminated (Line 27) and then displayed (Line 29). This is the point where processing would normally take place, but at this stage the string is output instead. Line 31 resets the I used the following redirection command to create a hexcode formatted file, Running this Lesson’s code on the file No processing takes place; the code simply spits out the formatted input. If you tried the code with a non-formatted file, such as the original This solution confirms that hexcode output can be read and that long lines are rejected. Step 3 is to confirm the hexcode header, which I cover in the next Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":" The first step is to process input one line at a time. Continue reading
\n3. Confirm that the first line is the hexcode header and check the version number.
\n4. Decode each line of text and display the output.
\n5. Check for the end of the formatted hexcode data.<\/p>\n\r\n#include <stdio.h>\r\n\r\n#define LINE_LENGTH 73\r\n\r\nint main()\r\n{\r\n char line_buffer[LINE_LENGTH];\r\n char c;\r\n int buffer_index = 0;\r\n\r\n while(1)\r\n {\r\n c = getchar();\r\n if( c == EOF)\r\n break;\r\n line_buffer[buffer_index] = c;\r\n buffer_index++;\r\n if( buffer_index > LINE_LENGTH)\r\n {\r\n \/* overflow condition *\/<\/span>\r\n puts(\"\\nInvalid hexcode line format\");\r\n return(1);\r\n }\r\n if( c == '\\n')\r\n {\r\n \/* terminate string *\/<\/span>\r\n line_buffer[buffer_index] = '\\0';\r\n \/* display contents *\/<\/span>\r\n printf(\"%s\",line_buffer);\r\n \/* reset index *\/<\/span>\r\n buffer_index = 0;\r\n }\r\n }\r\n\r\n return(0);\r\n}<\/pre>\nc<\/code> is stored in the line_buffer<\/code> char<\/em> array at offset buffer_index<\/code> (Line 16).<\/p>\nbuffer_index<\/code> variable is greater than the line length. This step not only prevents hackers from abusing the code, but it confirms that the input isn’t formatting for hexcode; the program spews out an error message (Line 21) and quits (Line 22).<\/p>\nbuffer_index<\/code> variable, and the endless while<\/em> loop continues.<\/p>\ngettysburg.hexc<\/code>:<\/p>\nhexencode < gettysburg.txt > gettysburg.hexc<\/code><\/pre>\ngettysburg.hexc<\/code> generates this output:<\/p>\na.out < gettysburg.hexc\r\nSTART HEX CODE v1.0\r\n EC C5 DF D8 8A D9 C9 C5 D8 CF 8A CB C4 CE 8A D9 CF DC CF C4 8A D3 CF CB\r\n D8 D9 8A CB CD C5 8A C5 DF D8 8A CC CB DE C2 CF D8 D9 8A C8 D8 C5 DF CD\r\n C2 DE 8A CC C5 D8 DE C2 8A C5 C4 8A DE C2 C3 D9 8A C9 C5 C4 DE C3 C4 CF\r\n C4 DE 8A CB 8A C4 CF DD 8A C4 CB DE C3 C5 C4 86 8A C9 C5 C4 C9 CF C3 DC\r\n CF CE 8A C3 C4 8A C6 C3 C8 CF D8 DE D3 86 8A CB C4 CE 8A CE CF CE C3 C9\r\n CB DE CF CE 8A DE C5 8A DE C2 CF 8A DA D8 C5 DA C5 D9 C3 DE C3 C5 C4 8A\r\n DE C2 CB DE 8A CB C6 C6 8A C7 CF C4 8A CB D8 CF 8A C9 D8 CF CB DE CF CE\r\n 8A CF DB DF CB C6 84 A0 A0\r\nEND HEX CODE<\/code><\/pre>\ngettysburg.txt<\/code> file, you'd see this output:<\/p>\na.out < gettysburg.txt \r\n\r\nInvalid hexcode line format<\/code><\/pre>\n