The file becomes NULL after printing 254 rows in .txt

Question

I am trying to solve the problem of recursion. The problem is : Given a number n, print out all the possible combinations of set {a, b, ..., a+n-1}. Output the answer in "output-n.txt". 1<=n<=9.

Here's my code:

#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<stdlib.h>

void comb(int n,int curlen,int j,int num[],int flag) {
    int i,k;
    char filename[20] = "output-0.txt";
    filename[7] = n + '0';
    char ans[20] = { 0 };

    FILE* fptr;
        fptr = fopen(filename, "a");


        if (fptr != NULL) {
            if (curlen < n && flag != 0) {
                for (i = 0; num[i] != 0; i++) {
                    ans[i] = num[i] + '`';
                }
                for (k = 0; k <= curlen; k++) {
                    fprintf(fptr, "%c", ans[k]);
                }
                fprintf(fptr, " ");
                
                if (curlen == 0 && num[0] == n) {
                    fclose(fptr);
                }
                else {
                    comb(n, curlen + 1, j + 1, num, 0);
                }

            }
            else {
                for (; j <= n; j++) {
                    num[curlen] = j;
                    comb(n, curlen, j, num, 1);
                }
            }
        }
        

}

int main() {
    int n,flag=0,num[20] = { 0 };
    
    scanf("%d", &n);
    
    comb(n, 0, 1, num, flag);
    

    system("pause");
    return 0;
}

My program is doing fine while 1<=n<=7, but at n=8 and n=9, the FILE pointer becomes NULL in the middle and can only output part of the answers. I looked closely to the output .txt file and realized that both of the files can only output to the 254th row.I wonder did I make any mistake. Thanks for answering.

Answer 1

You have hit the limit for the number of open files.

You keep opening the same file for append and don't always close it. When I rearranged the code so that it opens the file once in main in "w" mode, it works correctly.

#include <stdio.h>
#include <stdlib.h>

void comb(FILE *fptr, int n,int curlen, int j, int num[], int flag) {
    int i,k;
    char ans[20] = { 0 };
    if (curlen < n && flag != 0) {
        for (i = 0; num[i] != 0; i++) {
            ans[i] = num[i] + '`';
        }
        for (k = 0; k <= curlen; k++) {
            fprintf(fptr, "%c", ans[k]);
        }
        fprintf(fptr, " ");
        
        if (curlen == 0 && num[0] == n) {
            //fclose(fptr);
        }
        else {
            comb(fptr, n, curlen + 1, j + 1, num, 0);
        }
    }
    else {
        for (; j <= n; j++) {
            num[curlen] = j;
            comb(fptr, n, curlen, j, num, 1);
        }
    }
}

int main() {
    int n,flag=0,num[20] = { 0 };
    char filename[20] = "output-0.txt";
    if(scanf("%d", &n) != 1 || n < 1 || n > 9)  // sanity check
        return 1;
    filename[7] = n + '0';
    FILE *fptr = fopen(filename, "w");          // open once
    if(fptr != NULL) {
        comb(fptr, n, 0, 1, num, flag);         // pass the file pointer
        fclose(fptr);
    }
    return 0;
}