c++文件读/写无限循环，再次重写txt

Question

I'm trying to learn how to use file handling in C++. I'd like to save and edit the contents of variables, however, there seems to be a problem with my logic because somehow the following program re writes the contents of the file infinitely.

#include <stdio.h>
#include <iostream>

int main(int argc, char const *argv[])
{
    FILE *filePointer = fopen("text.txt", "r+");
    char currentChar;
    int loop = 0;
    currentChar = getc(filePointer);
    while (currentChar != EOF && loop < 100)
    {
        if (currentChar == '=')
        {
            fseek(filePointer, 1, 1);
            if (fputs("LOL", filePointer) == EOF)
            {
                return 2;
            }
            
        }
        
        std::cout << ftell(filePointer) << "Current Char: " << currentChar << std::endl;
        currentChar = getc(filePointer);
        
        loop++;
    }
    fclose(filePointer);
    return 0;
}

The code that the program reads is the following:

" hello = \n yay! "

Answer 1

This program exhibits undefined behavior. From C99 standard (which governs C standard library functions and is incorporated into the C++ standard by reference):

7.19.5.3/6 When a file is opened with update mode ( '+' as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function ( fseek , fsetpos , or rewind ), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file.

You have output ( fputs ) immediately followed by input ( getc ) without an intervening fflush or a file positioning function.

Answer 2

Text is stored contiguously on disk, and just like arrays, if you want to insert it, you must move the original element back, otherwise the effect of the execution is an overlay