使用更多内存时C中的程序崩溃

Question

I have to do as a school assigment small program in C that will read standart input and prints some standart output. To be more specific, it's about reading numbers and sorting them.

(you can skip this, it's just for understanding the code) First line of the input should determine how many lines of numbers there will be. Second line is ammount of numbers in next line. Third line are to concrete numbers. Fourth line is ammount of numbers in next line and so on until it reaches K number of lines. Restrictions are 0 < K <= 10 (max 10 sequences), each sequence can contain max 10.000.000 numbers and each number's value is max 10.000.000

Example Input:

1. 2 //which means that there will be 2 sequences (lines) of numbers and their corresponding ammount
2. 3 //in the first sequence there will be 3 numbers
3. 5 99912 45 //first sequence
4. 6 //in the second sequence there will be 6 numbers
5. 9489498 22131 0 521313 7988956 5 //second sequence

Ouptup:

0 5 5 45 22131 99912 521313 7988956 9489498

So I have done a working program but it seems to be unstable with higher values. However I can't determine when and where exactly the program fails. On my computer, I have tested all possible max values and it returned correct output in reasonable time, but on a school server where tests are done it just can't handle high values and fails.

There is one thing, that the program should only use C, not C++, but I am not very sure of differences between them and as I was using C++ compiler, it's possible that my code isn't just raw C.

I am a C beginner and this is something like "Hello world" for me, so please, can you just quick look through the code and say what can cause the unstability? Thanks

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

int main(void) {    
    int k, n, i, y, x, index = 0;
    int *numbers = (int*) malloc(100000000 * sizeof(int));
    if(numbers == NULL){
        exit(1);
    }
    scanf("%d", &k);
    for (x = 0; x < k; x++) {
        y = 0;
        scanf("%d", &n);
        while(scanf("%d", &i) > 0){
            numbers[index++] = i;
            if(++y == n){
                break;
            }
        }
    }
    for(y = 0;y < index;y++){   //find and print all 0's, because later I will use 0 as a
                                //already used (printed) element in array and ignore it      
        if(numbers[y] == 0){
            if(y == index-1){
                printf("0");
            }else{
                printf("0 ");
            }
        }
    }
    int smallest, smallestIndex;
    for(x = 0;x < index;x++){   //print all other numbers in ascending order
        smallest = 0;
        for(y = 0;y < index;y++){  //find current smallest number
            if((numbers[y] < smallest || smallest == 0) && numbers[y] != 0){
                smallest = numbers[y];
                smallestIndex = y;
            }
        }
        numbers[smallestIndex] = 0;
        if(smallest > 0){
            if(x == index-1){
                printf("%d", smallest);
            }else{
                printf("%d ", smallest);
            }
        }
    }
    free(numbers);
    numbers = NULL;
    return 0;
}

Answer 1

Based on the information you give, I think this is simply a resource limitation on the server. The server simply runs out of memory and your malloc() fails. I suggest you debug or do this:

if(numbers == NULL){
    printf("malloc() failed\n");
    exit(1);
}

Answer 2

The code for printing the initial zeros is suspicious:

for(y = 0;y < index;y++){   //find and print all 0's, because later I will use 0 as a
                            //already used (printed) element in array and ignore it      
    if(numbers[y] == 0){
        if(y == index-1){
            printf("0");
        }else{
            printf("0 ");
        }
    }

Suppose you have a sequence with 0 as the last element (eg 1 2 3 4 5 0); i guess this code will print just 0 with no space after it, and the subsequent code will print 1 2 3 4 5 , so you will get something like 01 2 3 4 5 .

I understand that you want the output to be as beautiful as possible, that is, without a space at the end. Please also note that a newline ( \\n ) at the end of output might be good.

Answer 3

I rewrote beginning parts of your program to get you on the right path. This should help you but I can't be sure since I don't really know what is causing your program to crash.

This implements the realloc function which should make your program drastically more efficient than it is now. If you don't know what realloc is you can read about it here , and here .

#include <stdio.h>
#include <stdlib.h>
#define BUFFER 256                                                  //for memory management         

int main(void) 
{    
    int k, n, i, y , x, index = 0, bff;                             //declare integer 'bff' and set it to BUFFER
    int *numbers = NULL, *tmp;                                      //declare a pointer (numbers) for allocated memory, and a pointer (tmp) for the realloc function

    if(!(numbers = malloc(BUFFER * sizeof(int))))                   //allocate space for 'bff' integers
    {
        exit(1);                                                    //allocation failed
    }
    scanf("%d", &k);
    for (x = 0; x < k; x++) 
    {
        scanf("%d", &n);
        while(scanf("%d", &i) > 0)
        {
            if(bff <= index)                                        //if the size of index grows larger than the amount of space we allocated
            {
                bff += BUFFER;                                      //increase the size of bff by BUFFER
                if(!(tmp = realloc(numbers, bff * sizeof(int))))    //resize our allocated memory block using the tmp pointer 
                {
                    free(numbers);                                      //allocation failed so free already allocated memory
                    exit(1);                                        //and terminate the program
                }
                numbers = tmp;                                      //make numbers point to the same location as tmp
                numbers[index++] = i;                               
                if(++y == n) break;
            }
        }
    }
    .
    .
    .
    free(numbers);
    return 0;
}

Keep in mind there are more efficient ways to use realloc. I just posted this here to get you on the right track. Good luck!

Answer 4

You are allocating the wrong amount of memory. The specification states that each sequence can contain 10 million values whereas you allocate a fixed amount. There may be up to k*10 million values of input, and you cannot know that the amount you allocate is enough.

As pointed out by m0skit0, the problem may also be due to over-allocation.

To fix the problem you should allocate the needed amount of memory, no more, no less. Use the sequence length provided for each sequence to do that. Also, you need to check the return value of malloc and realloc . If the return value is NULL then the allocation failed and you should print an error message and exit .