問題與指針和realloc，用程序用C

Question

崩潰的錯誤在main.c

#include <stdio.h> 
#include "poly.h" 

int main (void) 
{ 
    struct poly *p0 = polySetCoefficient (polySetCoefficient (polySetCoefficient (polyCreate() , 0, 4.0), 1, -1.0), 10, 2.0); 

    //polyPrint (p0); 

    return 0; 
} 

在poly.c我剛剛列入，我處理兩種功能，它好像polySetCoefficient的是，竟然放棄我的人問題。我已經評論了printf語句，但是我使用這些來確定程序崩潰的位置。顯然，它在崩潰之前實際上會經歷整個函數，所以我不確定錯誤是從哪裏來的。其次，在我的main.c文件中，我只調用了兩個函數。另一個問題是，每次我經歷polySetCoefficient時，我的以前的條目都被替換爲0。我認爲這可能是因爲我將數組中的元素設置爲0的方式，但我仔細確保將元素的前一個大小設置爲0，直到數組的新大小（不包括最後一個索引）。

struct poly *polyCreate() 
{ 

    struct poly *q; 

    q = malloc(sizeof(struct poly)); 
    q->c = malloc(sizeof(double)); 

     q->c[0] = 0.0; 

    q->size = 0; 

    //printf("polyCreate: %g\n", q->c[0]); 

    return q; 
} 

struct poly *polySetCoefficient(struct poly *p, int i, double value) 
{ 
    //printf("%d\n", i*sizeof(double)); 

    if (p->size < i) 
    { 
     printf("Old: %d, New: %d\n", sizeof(p->c)/sizeof(double), i); 

     p->c = realloc(p->c, i+1*sizeof(double)); 

     printf("New: %d \n", sizeof(p->c)); 

     for(int l = p->size; l <= i; l++) 
     { 

      if(l != i) 
      { 
       p->c[l] = 0; 
       printf("set to 0\n"); 
      } 
      else 
      { 
       p->c[l] = value; 
       printf("F:set to %g\n", p->c[i]); 
      } 
     } 

     printf("Did we come here?\n"); 

     p->size = i; 

    } else { 

     p->c[i] = value; 

    } 

    printf("The %d'th coefficient is %g\n", i, p->c[i]); 

    printf("Cof 0: %g, Cof 1: %g, Cof 10: %g", p->c[0], p->c[1], p->c[10]); 


    return p; 


} 

在poly.h

struct poly 
{ 
    double *c; 
    int size, length; 
}; 


struct poly *polyCreate(); 
struct poly *polyDelete(struct poly *p); 
struct poly *polySetCoefficient (struct poly *p, int i, double value); 
double polyGetCoefficient (struct poly *p, int i); 
int polyDegree (struct poly *p); 
void polyPrint (struct poly *p); 
struct poly *polyCopy (struct poly *p); 
struct poly *polyAdd (struct poly *p0, struct poly *p1); 
struct poly *polyMultiply (struct poly *p0, struct poly *p1); 
struct poly *polyPrime (struct poly *p); 
double polyEval (struct poly *p, double x); 

Answer 1

這個代碼的版本完全編譯並運行plausib （但不是真的正確）沒有崩潰。

一個關鍵的變化是由immibis在他的answer中確定的一個，即更正的尺寸計算。

另一個關鍵變化是當沒有係數10時沒有打印係數10。如果您嘗試儘快打印，則會調用未定義的行爲;什麼都可能發生。

我修正了一些printf()格式;在一臺64位機器上，您需要使用%zu（或%zd）來打印size_t。我刪除了該結構的未使用的length成員。我真的不知道如何整齊地格式化你的單線多線電話。我不介意略長於80個字符，但我通常不會超過120.我絕對更喜歡p1版本，不僅僅是因爲它可能會隨着您的錯誤檢查。目前，我用assert(ptr != 0);應該有一個分配錯誤檢查。

輸出：

The 0'th coefficient is 4 
The 1'th coefficient is -1 
Old: 1, New: 10 
New: 8 
p[1] set to 0 
p[2] set to 0 
p[3] set to 0 
p[4] set to 0 
p[5] set to 0 
p[6] set to 0 
p[7] set to 0 
p[8] set to 0 
p[9] set to 0 
p[10] set to 2 
Did we come here? 
The 10'th coefficient is 2 
Cof 0: 4, Cof 1: 0, Cof 10: 2 

注意，當你擴展到10

代碼，你打一頓第一系數爲零：

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

struct poly 
{ 
    double *c; 
    int size; 
}; 

struct poly *polyCreate(void); 
struct poly *polySetCoefficient(struct poly *p, int i, double value); 

int main(void) 
{ 
    /* I don't know how to format this neatly! */ 
    struct poly *p0 = polySetCoefficient(
     polySetCoefficient(
      polySetCoefficient(polyCreate(), 0, 4.0), 
      1, -1.0), 
     10, 2.0); 

    assert(p0 != 0); 

    /* 
    struct poly *p1 = polyCreate(); 
    p1 = polySetCoefficient(p1, 0, 4.0), 
    p1 = polySetCoefficient(p1, 1, -1.0); 
    p1 = polySetCoefficient(p1, 10, 2.0); 
    */ 

    return 0; 
} 

struct poly *polyCreate(void) 
{ 
    struct poly *q = malloc(sizeof(struct poly)); 
    assert(q != 0); 
    q->c = malloc(sizeof(double)); 
    assert(q->c != 0); 

    q->c[0] = 0.0; 
    q->size = 1; 

    return q; 
} 

struct poly *polySetCoefficient(struct poly *p, int i, double value) 
{ 
    assert(p != 0); 

    if (p->size < i) 
    { 
     printf("Old: %zu, New: %d\n", sizeof(p->c)/sizeof(double), i); 

     p->c = realloc(p->c, (i+1)*sizeof(double)); 
     assert(p->c != 0); 

     printf("New: %zu\n", sizeof(p->c)); 

     for (int l = p->size; l <= i; l++) 
     { 
      if (l != i) 
      { 
       p->c[l] = 0; 
       printf("p[%d] set to 0\n", l); 
      } 
      else 
      { 
       p->c[l] = value; 
       printf("p[%d] set to %g\n", i, p->c[i]); 
      } 
     } 

     printf("Did we come here?\n"); 

     p->size = i; 
    } 
    else 
    { 
     p->c[i] = value; 
    } 

    printf("The %d'th coefficient is %g\n", i, p->c[i]); 

    if (i >= 10) 
     printf("Cof 0: %g, Cof 1: %g, Cof 10: %g", p->c[0], p->c[1], p->c[10]); 

    return p; 
} 

Answer 2

丟失的括號這裏可能是這個問題：

p->c = realloc(p->c, i+1*sizeof(double)); 

將其更改爲：

p->c = realloc(p->c, (i+1)*sizeof(double));