How to print null in the middle of a string?

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Hey guys.
How can I print a null character in the middle of a string and how to tell the program that this is not the null character of string's end?

I want to print characters of a string one by one, but when it reaches to null in the middle, it quits.
I receive a non-printable character and pass it to a function to print the hex value of that non-printable char.

What I have tried:

char	i;
	char	j;

    char chars[] = "0123456789abcdef";
	i = chars[c / 16];
	j = chars[c % 16];
	write (1, "\\", 1);
	write (1, &i, 1);
	write (1, &j, 1);

Posted 11-Feb-23 20:33pm

Navid Rigi Ebrahimi

Updated 12-Feb-23 8:18am

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Richard MacCutchan 12-Feb-23 3:09am

Where is the code thar looks for the null character?

Navid Rigi Ebrahimi 12-Feb-23 3:16am

int main()
{
char str[] = "Coucou\rtu vas bien ?";
str[6] = 0;
printf("%zu\n", sizeof(str));
ft_putstr_non_printable(str);
write (1, "\n", 1);
}

void ft_putstr_non_printable(char *str)
{
int i;
// int len;
char *chars;

i = 0;
chars = "0123456789abcdef";
// len = ft_strlen(str);
printf("%zu\n", sizeof(str));
while (str[i] != '\0')
{
if (str[i] >= 32 && str[i] <= 126)
write (1, &str[i], 1);
else
calcs(str[i], chars);
i++;
}
}

void calcs(char c, char *chars)
{
char i;
char j;

i = chars[c / 16];
j = chars[c % 16];
write (1, "\\", 1);
write (1, &i, 1);
write (1, &j, 1);
}

Richard MacCutchan 12-Feb-23 3:44am

The problem is that the while loop will see the null character first, and terminate the loop. But even if you do check and print it, you do not know if it is in the middle of the string, or the actual string terminator. So if you do capture it and print the hex value, your code could then run on into unassigned memory.

Navid Rigi Ebrahimi 12-Feb-23 4:01am

Yeah, exactly but I have no idea how to fix it. Even sizeof doesn't work because first one refers to array but second one to pointer.

Richard MacCutchan 12-Feb-23 4:25am

You need to pass the size of the string to the function. But you will always have this problem if you treat arrays of characters as strings.

Navid Rigi Ebrahimi 12-Feb-23 5:07am

Yeah, it's the only way. Thanks a lot man.

2 solutions

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OriginalGriff · Answer 1 · 2023-02-11T21:25:00

Solution 1

Pretty much, you can't - in C, a null character is considered a string terminator - encountering one marks the end of the string.

If you want to include nulls in your data, you should be using unsigned char datatypes, and not trying to use "string based shortcuts" like this:

char chars[] = "0123456789abcdef";

as it adds an addition null to the end of the array.

I'd probably recommend using #define to make it obvious:

C

#define byte unsigned char
...
   byte data[noOfBytesNeeded];

Then treat your data as an array of values and print each as a character separately yourself instead of type to con the system into thinking it's a string.

But do be aware that a null character '\0' will print as nothing at all - not even a gap or space between the character before and after as it is defined by the C specification as a Control Character and thus unprintable! The only time you will be able to "see" it is if you print it to a file and use a hex editor to examine the file data.

What are you trying to do that you think you need this?

Posted 11-Feb-23 21:25pm

OriginalGriff

Comments

Navid Rigi Ebrahimi 12-Feb-23 3:49am

I want to print non-printable chars in the middle of string. It's in a question I'm solving right now.

OriginalGriff 12-Feb-23 4:41am

What as? They are control characters, they don't have a "shape" in any font!
And without a shape, you can't "print" them!

Have a look at a hex editor that provides a "text" visualization as well - PSPad in HEx Edit mode does that for example - and it will replace all control codes with a "filler character" like '.', or perhaps '█', or maybe '□' just to space it correctly.

Why do you want to print control characters? They are there to control output: add a new line, or a tab for example.

Navid Rigi Ebrahimi 12-Feb-23 5:06am

All you said is correct and I know it, but as I said, it's in a question. It asks us to print those control characters.

OriginalGriff 12-Feb-23 6:07am

But we can't read your assignment, so we have no idea what the question setter expects to see when you do!

Read the question again - it probably contains some info you missed ...

Navid Rigi Ebrahimi 12-Feb-23 6:57am

Allowed functions : write
• Create a function that displays a string of characters onscreen. If this string contains characters that aren’t printable, they’ll have to be displayed in the shape of
hexadecimals (lowercase), preceeded by a "backslash".
• For example :
Coucou\ntu vas bien ?
• The function should display :
Coucou\0atu vas bien ?
• Here’s how it should be prototyped :
void ft_putstr_non_printable(char *str);

OriginalGriff 12-Feb-23 7:12am

Right - so there is the information you need.
"\n" is not a null - it is Hex 0A - a "newline" character.
Basically think of everything below space as "non printable" and convert it to a hex pair when you print it.

Navid Rigi Ebrahimi 12-Feb-23 8:42am

yeah, i know. i wrote sth to print all the chars below space, but no luck in null, which is a non-printable, like \n or \t or others.

CPallini · Answer 2 · 2023-02-12T08:18:00

You should write a function accepting an array of char (or, better, unsigned char), say a, and its size, say size.
In the function, implement a loop, with n=0,1,..,(size-1):

If a[n] is printable (see isprint[^]) then print it (using, for instance putchar[^]).
On the other hand, if the a[n] then print, as requested, its hex value prefixed by the backslash, i.e.

C
```
printf("\\%02x", a[n]);
```

How to print null in the middle of a string?

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How to print null in the middle of a string?

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