Why is fprintf not printing the full data in file even after flushing?

Anubhav Sharma 0
Viewed 44

I am using sieve method to print all the prime numbers to a file. My code is as below:

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

int main(){
    FILE* file = fopen("primeList.txt","w+");
    bool* x;
//  long upLimit=100;
    long upLimit=2000000000;    
    x=(bool*) calloc(1,upLimit);
    long i=upLimit;
    while(i--){
        *(x+i)=true;
    }
    *(x+1)=false;
    *(x+2)=true;
    *(x+3)=true;
    int cnum=2;
    int count=0;
    while(cnum<upLimit){
        if(*(x+cnum)){
            fprintf(file, "%d\n",cnum);
            count++;
            int compNum=cnum*2;
            while(compNum<upLimit){
                *(x+compNum)=false;
                compNum+=cnum;
            }
        }
        cnum+=1;
    }
    fprintf(file, "Count is : %d\n", count);
    fflush(file);
    fclose(file);
    return 0;
}

The last few lines from outfile is as below:

enter image description here

I am expecting count of primes to be the last line. Please help with the issue. Thanks in advance.

printf c
2 Answers

Running the code with debug symbols, I can observe on my machine that it is reliably segfaulting at line 26: *(x+compNum)=false;

Issuing p compNum in gdb gives us the clue: compNum has overflown to negative and now the code is trying to reference memory address that's not within process limits.

In other words, the output is not cut at the "Count" but earlier, in the middle of fprintf in your system (and mine).

mike39960

Possible wrong size allocation.

Code assumes bool is size 1.

bool* x;
long upLimit=2000000000;    

//x=(bool*) calloc(1,upLimit);
// Should be 
x = calloc(sizeof *x, upLimit);

// Add test
if (x) {
  long i=upLimit;
  while(i--){
    *(x+i)=true;
  }
chux - Reinstate Monica0
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