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C undefined input string length program (programming.dev)
submitted 1 year ago by rastignac@programming.dev to c/c_lang@programming.dev
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      ' char* strInput =(char*) malloc(sizeof(char));
        int ch;
        int letNum = 0;
        while((ch = getchar()) != EOF){
                letNum++;
                strInput = (char*)realloc(strInput,letNum*sizeof(char));
                *(strInput + letNum - 1) = ch;
        }
        printf("\n");
        printf("%s\n",strInput);
        free(strInput);`

This is the contents of main in a program I wrote that takes an undefined number of chars and prints the final string. I don't understand why but it only works if I press ctrl+D twice, and only once if I press enter before.

does anyone get what's going on? And how would you have written the program?

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[-] adriator@lemmy.world 4 points 1 year ago

It's due to the way getchar() and console input work. When you enter "abcdCTRL+D" on the keyboard, here's what happens:

  • abcd characters are added to the stdin buffer and consumed by getchar()
  • CTRL+D (or EOF on Unix, CTRL+Z on Windows) is left in the input buffer (it's not yet consumed by getchar()!))
  • The console is waiting for more input, and the next time you press ENTER or CTRL+D the previous one will be consumed by getchar()

Think about this scenario: What happens if you only enter "abcd" and not press anything else? The program will still be waiting for more input. It needs to receive a signal telling it to stop with the input and proceed with the code execution. But if you press enter, it won't automatically add a new line to the string, because the new line character is still in the input buffer.

[-] RangerHere@programming.dev 2 points 1 year ago

This guy is right. I saw OP's post but I did not have enough time to reply at the time. I came back to reply to OP's post, but you are already right.

[-] rastignac@programming.dev 1 points 1 year ago

Thanks a lot for your answer. This might be because of my shallow understanding of how a buffer works, but I don't understand why EOF isn't consumed by getchar() when the other bytes are consumed. Isn't a char just a number and EOF too (-1 I think)? I probably should try and understand buffers more

this post was submitted on 27 Jun 2023
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