Having some trouble with string.h library in C

Question

I have run this code in C,

char *bracket_by_len(char *output, char *word, int length)
{
    if(strlen(word) < 5) {
        output[0] = '<';
        output[1] = '<';

        /*int i = 2;
        while(i != (strlen(word) + 2)) {
            output[i] = word[i - 2];
            i += 1;
        }*/

        strncat(output, word, strlen(word));
        strcat(output, ">>");
return output;
} else {... not using them as 4 letter word for input}

int main()
{
    char word[40], output[40]; // word is not used.
    printf("%s \n.", bracket_by_len(output, "word", 20);
    return 0;
}

The actual code is this:Input code. I then wrote

return output;

I printed this with

printf("Based on size, \t %s.\n", output);

And the output looks like this:

Based on size, <<^S>>.

There are some random characters in the beginning. When I replaced the strncat() function by a while loop with manually copying the letters of the word, the input is alright.

Thanks in advance.

The two characters are random. There are two different characters being added every time I run the program. — Sreenadha Saranga, Oct 21 '21 at 14:29
You'll need to show us significantly more of your code if you want anyone to diagnose the problem and offer a solution. — Adrian Mole, Oct 21 '21 at 14:29
How is `output` declared and initialized? Please post a [mcve]. — interjay, Oct 21 '21 at 14:30
Can you show the whole code? Why start from `output[1]` and not `output[0]`? — kuro, Oct 21 '21 at 14:30
The word is pre-defined, and I'm not using any input. I did start from output[0] in my program. — Sreenadha Saranga, Oct 21 '21 at 14:31
Post the complete code. Not just a little piece. And no image of code. Post it in text. — Support Ukraine, Oct 21 '21 at 14:35
`output[2] = 0;` is missing, required to generate a zero-terminated string. `strcpy(output, "<<");` makes more sense. — Hans Passant, Oct 21 '21 at 14:35
@HansPassant I think '\0' is the way to go, '0' can be interpreted differently depending on the OS — Hollyol, Oct 21 '21 at 14:37
Aside: Why `strncat(output, word, strlen(word));` instead of simple `strcat(output, word);` or `strncat(output, word, 37);`? — chux - Reinstate Monica, Oct 21 '21 at 14:45
I wanted to add some characters after word part, so I didn't use strcat directly. — Sreenadha Saranga, Oct 21 '21 at 14:46
@SreenadhaSaranga Consider `snprintf(output, 40, "<<%s>>", word);` — chux - Reinstate Monica, Oct 21 '21 at 14:48
@shrewmouse, "guy is new" is not an excuse. If he's new he should read [How do I ask a good question?](//stackoverflow.com/help/how-to-ask). Downvotes do reflect the quality or usefulness of the question and are not personal. By the way, it doesn't even cost reputation, since you cannot get lower than 1. — wovano, Oct 21 '21 at 14:54
Does this answer your question? [How do I concatenate two strings in C?](https://stackoverflow.com/questions/8465006/how-do-i-concatenate-two-strings-in-c) — wovano, Oct 21 '21 at 14:56
PS: [Please do not upload images of code/errors when asking a question.](//meta.stackoverflow.com/a/285557/10669875) — wovano, Oct 21 '21 at 14:58

shrewmouse · Accepted Answer · 2021-10-21T14:52:04.043

0

output hasn't been terminated with a NULL. Add it after the left chevrons.

output[0] = '<';
output[1] = '<';
output[2] = 0;
strncat(output, word, strlen(word));
printf("%s\n", output);

If you use sprintf in stead you won't have to worry about adding the null that detail.

sprintf(output, "<<");
strncat(output, word, strlen(word));
printf("%s\n", output);

edited Oct 21 '21 at 14:52

answered Oct 21 '21 at 14:31

shrewmouse

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I did try that. I wrote output[1] by mistake. Sorry. – Sreenadha Saranga Oct 21 '21 at 14:34
Based on your output, I but that the first char of `word` wasn't been initialized – shrewmouse Oct 21 '21 at 14:36
No, I passed my command as function(output, "word", 20); – Sreenadha Saranga Oct 21 '21 at 14:37
Loop through `output` and `word` and print each char as an integer `printf("%d\n",output[i])`. You will be able to find the value of the offending char. ^S is likely a single char. – shrewmouse Oct 21 '21 at 14:40
When I did the loop, I got perfect output as expected. – Sreenadha Saranga Oct 21 '21 at 14:44
@SreenadhaSaranga, set output[2] to null. strings are null terminated. – shrewmouse Oct 21 '21 at 14:44
But I want to add some characters after that. Won't it affect this? – Sreenadha Saranga Oct 21 '21 at 14:48
No. it will only affect the first three characters. '<' ,'<','\0' It's the same as your first three lines where you set the chars of `output` directly. – shrewmouse Oct 21 '21 at 14:50

Having some trouble with string.h library in C

1 Answers1