使用指針

Question

我已經收到以下錯誤增加時，當我在終端編譯我的程序獲取一個glibc的錯誤：

*** glibc detected *** ./a.out: double free or corruption (fasttop): 0x089660a0 *** 
======= Backtrace: ========= 
/lib/i386-linux-gnu/libc.so.6(+0x6ebc2)[0xb7621bc2] 
/lib/i386-linux-gnu/libc.so.6(+0x6f862)[0xb7622862] 
/lib/i386-linux-gnu/libc.so.6(cfree+0x6d)[0xb762594d] 
./a.out[0x8048668] ./a.out[0x8048fa3] 
/lib/i386-linux-gnu/libc.so.6(__libc_start_main+0xf3)[0xb75cc113] 
./a.out[0x80484c1] 
======= Memory map: ======== 
08048000-0804a000 r-xp 00000000 08:06 1835029 /home/fasih/poly/a.out 
0804a000-0804b000 r--p 00001000 08:06 1835029 /home/fasih/poly/a.out 
0804b000-0804c000 rw-p 00002000 08:06 1835029 /home/fasih/poly/a.out 
08966000-08987000 rw-p 00000000 00:00 0   [heap] 
b7400000-b7421000 rw-p 00000000 00:00 0 
b7421000-b7500000 ---p 00000000 00:00 0 
b757f000-b759b000 r-xp 00000000 08:06 4195258 /lib/i386-linux-gnu/libgcc_s.so.1 
b759b000-b759c000 r--p 0001b000 08:06 4195258 /lib/i386-linux-gnu/libgcc_s.so.1 
b759c000-b759d000 rw-p 0001c000 08:06 4195258 /lib/i386-linux-gnu/libgcc_s.so.1 
b75b1000-b75b3000 rw-p 00000000 00:00 0 
b75b3000-b7729000 r-xp 00000000 08:06 4195237 /lib/i386-linux-gnu/libc-2.13.so 
b7729000-b772b000 r--p 00176000 08:06 4195237 /lib/i386-linux-gnu/libc-2.13.so 
b772b000-b772c000 rw-p 00178000 08:06 4195237 /lib/i386-linux-gnu/libc-2.13.so 
b772c000-b772f000 rw-p 00000000 00:00 0 
b772f000-b7757000 r-xp 00000000 08:06 4195267 /lib/i386-linux-gnu/libm-2.13.so 
b7757000-b7758000 r--p 00028000 08:06 4195267 /lib/i386-linux-gnu/libm-2.13.so 
b7758000-b7759000 rw-p 00029000 08:06 4195267 /lib/i386-linux-gnu/libm-2.13.so 
b776c000-b776f000 rw-p 00000000 00:00 0 
b776f000-b7770000 r-xp 00000000 00:00 0   [vdso] 
b7770000-b778e000 r-xp 00000000 08:06 4195224 /lib/i386-linux-gnu/ld-2.13.so 
b778e000-b778f000 r--p 0001d000 08:06 4195224 /lib/i386-linux-gnu/ld-2.13.so 
b778f000-b7790000 rw-p 0001e000 08:06 4195224 /lib/i386-linux-gnu/ld-2.13.so 
bfed6000-bfef7000 rw-p 00000000 00:00 0   [stack] Aborted 

這是我的代碼，它出現在polyAdd功能要發生的事情，但我也得到一個斷言在乘法失敗，所以它可能是兩個？

#include <stdio.h> 
    #include <stdlib.h> 
    #include <math.h> 
    #include <stdbool.h> 
    #include "poly.h" 

    int polyDegree(struct poly *p) 
    { 
     return p->length-1; 
    } 
    struct poly *polyCreate() 
    { 
     struct poly *p = (struct poly *)malloc(sizeof(struct poly)); 
     p->coeff=(double *)malloc(sizeof(double)); 
     p->size=1; 
     p->length=0; 
     return p; 

    } 
    struct poly *polySetCoefficient (struct poly *p, int i, double value) 
    { 
     if(p == NULL) 
      return; 

     if(i>=p->size) 
     { 
      do 
      { 
       p->size = p->size*2; 
      }while(i>=p->size); 

      p->coeff = (double *)realloc(p->coeff, p->size*sizeof(double)); 
     } 

     while(i >= p->length) 
     { 
      p->coeff[p->length] = 0; 
      p->length++; 
     } 

     p->coeff[i] = value; 

     return p; 
    } 
    struct poly *polyDelete(struct poly *p) 
    { 
     if (p){ 
     free(p);} 

     return 0; 
    } 
    struct poly *polyCopy(struct poly *p) 
    { 
     struct poly *nP = polyCreate(); 
     nP->size =p->size; 
     nP->length = p->length; 
     int i = 0; 
     for (i = 0; i<(nP->size);i++) 
     { 
      nP->coeff[i] = p->coeff[i]; 
     } 
     return nP; 
    } 
    struct poly *polyAdd(struct poly *p0, struct poly *p1) 
{ 
    int i; 
    struct poly *pF = polyCreate(); 
    if (p0->length > p1->length) 
    { 
     pF = polyCopy(p0); 

     for (i=0;i<p1->length;i++) 
      pF->coeff[i] += p1->coeff[i]; 
    } 
    else if (p1->length >= p0->length) 
    { 
     pF = polyCopy(p1); 

     for (i=0;i<p0->length;i++) 
      pF->coeff[i] += p0->coeff[i]; 
    } 
    return pF; 

} 
    struct poly *polyPrime (struct poly *p) 
    { 
     struct poly *pF = polyCreate(); 
     pF->size = p->size; 
     int i,j,k; 
     int n = p->size; 
     double a[n-1]; 
     for (i = 1; i <=n;i++) 
     { 
      a[i-1] = i * p->coeff[i]; 
     } 
     for (i = 0; i < n; i++) 
     { 
      pF->coeff[i] = a[i]; 
     } 
     return pF; 
    } 

    struct poly *polyMultiply (struct poly *p0, struct poly *p1) 
    { 
     struct poly *product = polyCreate(); 
     product->length = p0->length + p1->length - 1; 
     product->size = p0->size + p1->size; 
     product->coeff = (double *)malloc(product->size*sizeof(double)); 

     int i,j; 
     for(i=0;i<product->length;i++) 
      product->coeff[i] = 0; 

     for(i=0;i<p0->length;i++) 
      for(j=0;j<p1->length;j++) 
       product->coeff[i+j] += p0->coeff[i] * p1->coeff[j]; 

     return product; 
    } 


    double polyGetCoefficient(struct poly *p, int i) 
    { 
     double val =p->coeff[i]; 
     return val; 
    } 
    int checkZero (double a[], int n) 
    { 
    int x = 0; 
    for (x = 0; x < n; x++) 
    { 
    if (a[x] != 0) 
    return 1; 
    } 
    return 0; 
    } 

    double polyEval(struct poly *p, double x) 
    { 
     int i,n; 
     double eval=0; 
     if (!p) 
      return 0; 
     if (p) 
     n = p->length; 
     if (n == 0) 
      return 0; 
     for (i = 0; i<=n;i++) 
     { 
      if (p->coeff[i] == 0)continue; 
      if (i == 0)eval += p->coeff[0]; 
      else 
       eval += p->coeff[i]* pow (x,i); 
     } 
     return eval; 
    } 

    void polyPrint (struct poly *p) 
    { 
     int x=0,y,z; 
     int n; 
     n = p->size; 
     double a[p->size]; 
     for (x = p->size; x >= 0; x--) 
     { 
      a[x] = p->coeff[x]; 
     } 

     bool check,neg,zero = true; 
     if (!checkZero (a,n)) 
     { 
      printf("0\n"); 
     } 
     else{ 
     for(x=(n-1);x>=0;x--) 
     { 
      check = false; 
      neg = false; 
      if (x < (n-1) && a[x+1] == 0 && a[x] != 0 && a[x] > 0 && !zero) 
       printf(" + "); 
      else if (x < (n-1) && a[x+1] == 0 && a[x] != 0 && a[x] < 0 && !zero) 
       { 
        printf (" - "); 
        a[x] = a[x] * -1.00; 
       } 
      if (a[x] == 0) 
       continue; 
      if (a[x] != 0.0&& x > 1) 
      { 
       if (a[x] == -1)printf("-x^%d",x); 
       else if (a[x] == 1)printf ("x^%d",x); 
       else 
       printf("%gx^%d",a[x],x); 
       check = true;zero = false; 
      } 
      else if (x == 1) 
      { 
       if (a[x] == 1)printf("x"); 
       else 
       printf("%gx",a[x]); 
       check = true; zero = false; 
      } 
      else if (x == 0) 
      { 
       printf("%g",a[x]); 
       check = false; zero = false; 
      } 
      if (a[x-1] < 0 && x > 0){ 
       printf (" - ");a[x-1] = a[x-1] * -1.00;} 
      else if (x > 0&& a[x-1] != 0)printf(" + "); 
     } 
     printf("\n");} 
    } 

Answer 1

這是一個錯誤：

for (i = 0; i<=n;i++) 
{ 
    if (p->coeff[i] == 0)continue; 
    if (i == 0)eval += p->coeff[0]; 
    else 
     eval += p->coeff[i]* pow (x,i); 
} 

在C運行從0到n-1數組索引; i<=n將評估p->coeff[n]，這是垃圾數據。 （這也是垃圾的格式 - 我建議通過indent(1)運行整個程序; Linux內核使用indent(1)參數：-npro -kr -i8 -ts8 -sob -l80 -ss -ncs -cp1 -il0;我喜歡這些。）

這是同樣的錯誤，以稍微不同的形式：

n = p->size; 
double a[p->size]; 
for (x = p->size; x >= 0; x--) 
{ 
    a[x] = p->coeff[x]; 
} 

我預計p->size是大小的數組，不是最大的數組下標。

這裏有一個不同的錯誤：

if (a[x] != 0.0&& x > 1) 
    { 

如何很少0.0在浮點運算等於0.0你可能會感到震驚。千萬不要這樣直接比較 - 相反，檢查差值是否接近零，以便將其稱爲零。 （沒有什麼是關於0.0在這裏 - 浮動全線點比較需要謹慎處理。）

Answer 2

在這樣它可以是值得通過Valgrind的運行程序的情況：

valgrind ./your_program 

這可能會是能夠精確地查明任何錯誤寫入發生的位置。修復你發現的問題，再次運行valgrind，並檢查你是否擁有全部。請注意，儘管valgrind不會，至少在我上次檢查時，警告你無法訪問堆棧分配的數組。

Valgrind可作爲來源here，但我建議使用您的操作系統選擇的包管理方法來抓住它，如果可能的話。