How do I store repeated null characters in a string in C from fgetc?

Question

EDIT: This simply doesn't work reliably with strings. I have changed the entire system to work with int arrays. Eliminated a bunch of other headaches, too. The working version of my MVC is:

#include <stdio.h>
#include <string.h>

int main (){
    int nextChar;
    int augmented[256];
    int index = 0;
    while ((nextChar = fgetc(stdin)) != EOF){
        augmented[index] = nextChar;
        index++;
    }
    for (int i = 0; i <= index;++i){
        printf("%c", augmented[i]);
    }
}

END EDIT

ORIGINAL POST:

I am trying to implement an LZW compressor for an assignment. So far, everything works great on text, but I am putting out garbage if the input file contains a long run of null characters.

Right at the start I store the incoming char as an int to check for EOF and then cast it to a char to concat to my augmented string for dictionary comparison. I have printed out my dictionary after each file and find that with long runs of zeros my dictionary entry is a null string.

I think that whats happening is that it takes a string of zeros and makes it a single zero. Not the desired value. I need to put out ALL those zeros.

I have made a minimal viable code to show the error and have found that it occurs right at the casting stage. How can I build a check for the null character so that I can substitute it for something else that can be stored in a string?

#include <stdio.h>
#include <string.h>

int main (){
    int nextChar;
    char augmented[256] = "\0";
    while ((nextChar = fgetc(stdin) != EOF)){
        char charBuffer[2];
        sprintf(charBuffer, "%c", nextChar);
        strcat(augmented, charBuffer);
    }
    printf("%s",augmented);
}

I've been searching for a couple days and I guess I can't seem to figure out what the correct query should be as I'm not finding any useful results.

If the data contains zeros, then using string functions isn't going to go well. You'd do better to keep an index into `augmented[]` and write to it directly instead of going around the houses. — Weather Vane, May 23 '20 at 23:12
This put me on the right track. I abandoned strings all together and made the whole work on int arrays. — timbitsaregood, May 24 '20 at 00:05
regarding: `while ((nextChar = fgetc(stdin)) != EOF){ augmented[index] = nextChar; index++; }` This code allows an unlimited number of characters to be stored in `augumented[256]` However, that array is only 256 bytes long. Suggest: `while ( index < sizeof( augmented ) && (nextChar = fgetc(stdin)) != EOF){ augmented[index] = nextChar; index++; }` — user3629249, May 24 '20 at 21:52

Bill Morgan · Answer 1 · 2020-05-23T23:38:11.043

1

here are some updates to your program. 0's are converted to '0's. Not exactly sure what you're looking for but hopefully this gets you pointed in the right direction:

#include <stdio.h>
#include <string.h>

int main (){
    int nextChar;
    char augmented[256] = {0}; // zero entire array
    int i = 0;
    while ((nextChar = fgetc(stdin)) != EOF){
        // convert 0 to some other character
        if( nextChar == 0 ) nextChar = '0';
        augmented[i++] = (char)nextChar;
        //check for buffer overflow
        if( i==255) break;
    }
    printf("%s",augmented);
}

edited May 23 '20 at 23:38

answered May 23 '20 at 23:11

Bill Morgan

538
2
14

Thanks for replying. I tried that before (and again now) and it seems to have no effect. – timbitsaregood May 23 '20 at 23:19
This is a good idea, but it converts it to an ascii zero. I need the actual null 0x00. – timbitsaregood May 24 '20 at 00:03
Then remove the if(nextChar==0)nextChar='0'. Also note that the printf will stop at the first 0x00. – Bill Morgan May 24 '20 at 00:39

score 1 · Answer 2 · answered May 23 '20 at 23:37

1

The problem is parenthesis. Change to:

while ((nextChar = fgetc(stdin)) != EOF){

Your code assigned the value of the comparison fgetc(stdin)) != EOF to nextChar.

And you should also initialize charBuffer to zero.

answered May 23 '20 at 23:37

klutt

30,332
17
55
95

Yep, that was definitely an issue. But it was only in my posted code snippet. In the actual code it was as you defined. Good catch! Thank you! Still doesn't solve the null problem, though. – timbitsaregood May 23 '20 at 23:56

How do I store repeated null characters in a string in C from fgetc?

2 Answers2