advanced patterns with `fopen` and File Mode in C - Seeking Clarity on Buffering
I'm building a feature where I'm testing a new approach and I'm experiencing unexpected behavior when using `fopen` with the mode `"r+"` in C for reading and writing to a file. I have a file named `data.txt`, and I expected to read from it and write updates without losing any existing content. However, after writing to the file, I find that the original content is being overwritten instead of appended. Here's the relevant code snippet: ```c #include <stdio.h> #include <stdlib.h> int main() { FILE *file = fopen("data.txt", "r+"); if (file == NULL) { perror("behavior opening file"); return EXIT_FAILURE; } char buffer[100]; fgets(buffer, sizeof(buffer), file); printf("Read from file: %s\n", buffer); // Attempting to write after reading fseek(file, 0, SEEK_END); // Move to end of file fprintf(file, "\nNew entry!"); fclose(file); return EXIT_SUCCESS; } ``` I use `fseek` to move to the end of the file before writing, but it seems like the file's content is not preserved. When I check `data.txt` after running the program, the original content is gone, and only "New entry!" is present. I've confirmed that `data.txt` contains data before running my program, and I run it multiple times to verify the behavior. I suspect this might be related to how file buffering works with `fopen` or the mode flags, but I'm not sure how to ensure the original content remains intact while still being able to append new data. Am I missing something in my approach to file handling? Any insights on this would be greatly appreciated! My development environment is macOS. Has anyone else encountered this? This is for a desktop app running on CentOS. I've been using C for about a year now. Any examples would be super helpful.