Question

我正在编写一个程序，以查找所有小于一定数的素数。一旦我的素数列表超过255000个素数，我就会收到错误“细分错误：11”。活动监视器说我的进程仅使用1.2MB。这是我的代码：

#include <stdio.h>
#include <stdlib.h>
#include <math.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
    int N;
    int arraySize = 1;
    int *primes = malloc(100*sizeof(int));
    int isPrime = 1;
    primes[0] = 2;
    int timesRealloc = 0;
    int availableSlots = 100;

    printf("Please enter the largest number you want checked: \n");
    scanf("%d", &N);

    int j = 0;
    int i;
    for (i = 3; i <= N; i++){ 
        j = 0;
        isPrime = 1;
        while (primes[j] <= sqrt(i)) {
            if (i%primes[j] == 0) {
                isPrime = 0;
                break;
            }
            j++;
        }
        if (isPrime == 1){
            primes[arraySize] = i;
            arraySize++;
            availableSlots = availableSlots - 1;
        }
        if (availableSlots == 0){
            timesRealloc++;
            availableSlots = 100;
            primes = realloc(primes, 100*sizeof(int));
        }
    }

    /*
    for (i = 0; i < arraySize; i++){
        printf("%d\n", primes[i]);
    }
    */
    printf("process ID is %d\n", getpid());
    printf("I found %d primes\n", arraySize);
    printf("Memory was reallocated %d times\n", timesRealloc);
    printf("The largest prime I found was %d\n", primes[(arraySize-1)]);


    return 0;
}

Answer 1

如前所述，您离开数组素数

这里的代码经过最少的修改即可正常运行：

#include <math.h>
#include <sys/types.h>
#include <unistd.h>

int main(){
    int N;
    int arraySize = 1;
    int *primes = malloc(100*sizeof(int));
    int isPrime = 1;
    primes[0] = 2;
    int timesRealloc = 0;
    int availableSlots = 100;

    printf("Please enter the largest number you want checked: \n");
    scanf("%d", &N);

    int j = 0;
    int i;
    for (i = 3; i <= N; i++){ 
        j = 0;
        isPrime = 1;
        while (primes[j] <= sqrt(i)) {
            if (i%primes[j] == 0) {
                isPrime = 0;
                break;
            }
            j++;
        }
        if (isPrime == 1){
            primes[arraySize] = i;
            arraySize++;
         /* LINE REMOVED */
        }
        if (arraySize == availableSlots){ /* MODIFIED */
            timesRealloc++;
            availableSlots += 100; /* MODIFIED */
            primes = realloc(primes, availableSlots*sizeof(int)); /* MODIFIED */
        }
    }

    /*
    for (i = 0; i < arraySize; i++){
        printf("%d\n", primes[i]);
    }
    */
    printf("process ID is %d\n", getpid());
    printf("I found %d primes\n", arraySize);
    printf("Memory was reallocated %d times\n", timesRealloc);
    printf("The largest prime I found was %d\n", primes[(arraySize-1)]);


    return 0;
}

更改通过注释指示

对我来说， arraySize 必须重命名为 numberOfPrimes ，而 availableSlots 必须重命名 arraySize

编译和执行：

pi@raspberrypi:/tmp $ gcc -g -pedantic -Wextra p.c -lm
pi@raspberrypi:/tmp $ ./a.out
Please enter the largest number you want checked: 
500000
process ID is 6337
I found 41538 primes
Memory was reallocated 415 times
The largest prime I found was 499979

在 valgrind 下执行：

pi@raspberrypi:/tmp $ valgrind ./a.out
==6354== Memcheck, a memory error detector
==6354== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==6354== Using Valgrind-3.13.0 and LibVEX; rerun with -h for copyright info
==6354== Command: ./a.out
==6354== 
1000
Please enter the largest number you want checked: 
process ID is 6354
I found 168 primes
Memory was reallocated 1 times
The largest prime I found was 997
==6354== 
==6354== HEAP SUMMARY:
==6354==     in use at exit: 800 bytes in 1 blocks
==6354==   total heap usage: 4 allocs, 3 frees, 3,248 bytes allocated
==6354== 
==6354== LEAK SUMMARY:
==6354==    definitely lost: 800 bytes in 1 blocks
==6354==    indirectly lost: 0 bytes in 0 blocks
==6354==      possibly lost: 0 bytes in 0 blocks
==6354==    still reachable: 0 bytes in 0 blocks
==6354==         suppressed: 0 bytes in 0 blocks
==6354== Rerun with --leak-check=full to see details of leaked memory
==6354== 
==6354== For counts of detected and suppressed errors, rerun with: -v
==6354== ERROR SUMMARY: 0 errors from 0 contexts (suppressed: 6 from 3)

素数必须在最后释放，以免发生内存泄漏

仅是一个细节，但输入1时行为不是预期的

pi@raspberrypi:/tmp $ ./a.out
Please enter the largest number you want checked: 
1
process ID is 6388
I found 1 primes
Memory was reallocated 0 times
The largest prime I found was 2

2大于1;-）

这是因为您强制存在2（primes[0] = 2;等）

Answer 2

这里有两个主要问题。

首先，您没有按照自己的想象分配足够的空间。您在此处进行初始分配：

int *primes = malloc(100*sizeof(int));

然后在此处重新分配：

primes = realloc(primes, 100*sizeof(int));

您要重新分配的空间量与原始大小相同。您并没有使数组更大。您可以通过跟踪当前容量并根据需要增加容量来解决此问题。因此初始分配如下所示：

int capacity = 100;
int *primes = malloc(capacity*sizeof(int));

重新分配如下：

timesRealloc++;
availableSlots = 100;
capacity += availableSlots;
primes = realloc(primes, capacity*sizeof(int));

第二个问题是，您将arraySize从1开始写入primes[arraySize]，但是availableSlots的初始值是100。因此，当您达到当前容量时，您实际上是在写入最后一个元素。

将availableSlots初始化为99，而不是10：

int availableSlots = 99;

此外，请不要忘记最后free(primes)，以免造成内存泄漏。

数组超过大约255000 int时出现分段错误

2 个答案: