使用fopen（）的最佳方法是什么

Question

I am in my internship and Im a working on a projet of SLAM simulation, and I'm having a little problem while writing my cloud points into my file.

Here is my code, I have two functions :

        void ecriture_fichier(double x, double y){
            FILE *fichier = NULL;
            fichier = fopen("/home/stagiairepierrick/points/points.txt", "a");
            if(fichier != NULL ){ 
                fprintf(fichier, "%lf %lf \n",x,y);
            } else {
                std::cerr << "probleme ecriture" << std::endl;
            }
            fclose(fichier);
        }

        void ajout_nombre_lignes(long nombre_lignes){
            FILE *fichier = NULL;
            fichier = fopen("/home/stagiairepierrick/points/points.txt", "r+");
            if(fichier != NULL ){ 
                fseek(fichier, 0, SEEK_SET);
                fprintf(fichier, "%ld \n", nombre_lignes);
            } else {
                std::cerr << "probleme ecriture nombre de lignes" << std::endl;
            }
            fclose(fichier);
        }

The function ajout_nombre_ligne is called just before ecriture_fichier . I want the first lign to be the number of ligne (variable nombre_lignes ), however when I execute this code, I get the following result in my file:

8070 
00000 5.200000 
5.200000 5.200000 
5.200000 5.200000 
5.200000 5.200000 
5.200000 5.200000 
5.200000 5.200000 

For each char added on my first line, I can see that one my char of the second line is being removed, I understand why but I can't find any solution.

Could you guys help me?

Answer 1

There's nothing wrong with the behaviour.

If you write a character mid file or somewhere not at the end, you overwrite whatever is at that position.

If you want to write somewhere else than to the end (append), you will have to read and write the parts of the file you did not modify.

Also, you might want to use std::ifstream / std::ofstream / std::fstream instead.

Answer 2

fseek is primarily for searching in binary files - there are no line-based files in C/C++.

I understand that you want to append a line, and then update the first line based on the total number of lines - and the only possibility I can see is to use space/zero to pad the number of lines to ensure that it is always the same length, ie something like:

 fprintf(fichier, "%10ld \n", nombre_lignes);

I am not sure that the desired behavior is standardized.