如何访问 C 中 const char *[] 的数组元素

Question

我有一个通过 const char* fix[] 的函数。 我想知道如何访问 const char* fix[] 的元素。

int evaluate(const size_t size, const char* fix[], int  *result)

当我尝试按如下方式访问时，出现分段错误

push(stack,*fix[i]-'0');

而我的推送 function 如下

  void push(Stack *stack,char no)
  {
      stack->stack_array[stack->top++] = no;
  }

我的完整代码如下

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
 #define STACK_EMPTY  -1
 #define STACK_NOT_EMPTY  1
 #define STATUS_INVALID         -1
 #define STATUS_SUCCESS       1 
 #define STATUS_DIVIDE_BY_ZERO  0
 
  
 typedef struct 
 {
     int top;
     unsigned stack_size;
     int *stack_array;
     
     p
 }Stack;
 
 /*Stack Operations : Create , Push and Pop functions*/
  Stack *Create_Stack(unsigned stack_size)
  {
     Stack *stack_mem = (Stack*)malloc(sizeof(stack_size));
      /*Return NULL if the pinter is invalid*/
      if(!stack_mem)
      {
          return NULL;
      }
      stack_mem->top = -1;
      stack_mem->stack_size = stack_size;
      stack_mem->stack_array = (int*)malloc(stack_mem->stack_size * sizeof(int));
      if(!stack_mem->stack_array)
      {
          return NULL;
      }
      return stack_mem;
      
  }
  
  int isStackEmpty(Stack* stack)
  {
      if(stack->top == -1)
      
          return STACK_EMPTY;
          else {
                return STACK_NOT_EMPTY;
          }
      
  }
  void push(Stack *stack,char no)
  {
      stack->stack_array[stack->top++] = no;
  }
  
  char pop(Stack *stack)
  {
      if(!isStackEmpty(stack))
      {
          return stack->stack_array[stack->top--];
      }
      return '#';
  }
  
  char peek(Stack *stack)
  {
      return stack->stack_array[stack->top];
  }

int evaluate(const size_t fix_size, const char* fix[], int  *result)
{
  
    /*Create Stack*/
   
    Stack *stack = Create_Stack(fix_size);
   int i;
   int valu1;
   int valu2;
   int return_result;
   int return_status;
    if (!stack) 
    return -1;
    /*Scan the characters and store the numbers in stack*/
    for(i = 0; i < fix_size;++i )
    {
         printf("%c",*fix[i]);
        if(isdigit(fix[i]))
       // printf(":SIze = %c",*fix[i]);
        push(stack,*fix[i]-'0');
        
        else {
            if(fix_size>2)
            {
                valu1 = pop(stack);
                valu2 = pop(stack);
                switch(*fix[i])
                {
                    
                    case '+':*result = valu1+valu2;
                    return_status = STATUS_SUCCESS;
                    break;
                    case '-':*result = valu1-valu2;
                    return_status = STATUS_SUCCESS;
                    break;
                    case '*':*result = valu1*valu2;
                    return_status = STATUS_SUCCESS;
                    break;
                    case '/':
                    if(valu2 != 0)
                    {
                        *result = valu1/valu2;
                    }
                    else {
                    return_status = STATUS_DIVIDE_BY_ZERO;;
                    }
                    break;
                    
                }
            }
            else {
            return STATUS_INVALID;
            }
    
        }
        
        
    }
    return return_status;
    
}

int main()
{
    
     char exp[] = "231*+9-"; 
     int *result; 
    evaluate(7,exp,&result);

    return 0;
}

谁能告诉我如何访问 const char * 数组 []

提前致谢 ！

Answer 1

编译器标志是你的朋友！ 编写代码时始终启用-Wall -Wextra （和-Werror进行良好测量）。 编译您的代码使其像圣诞树一样亮起，带有警告和错误：

soSeg.c:17:3: error: expected specifier-qualifier-list before ‘p’
   17 |   p
      |   ^

不知道你是如何编译它的，你的结构定义中有一个悬空的“p”

soSeg.c: In function ‘evaluate’:
soSeg.c:84:16: warning: comparison of integer expressions of different signedness: ‘int’ and ‘size_’ {aka ‘const long unsigned int’} [-Wsign-compare]
   84 |   for(i = 0; i < fix_size;++i )
      |                ^

选择一个，使i size_t或fix_size int

soSeg.c: In function ‘main’:
soSeg.c:136:14: warning: passing argument 2 of ‘evaluate’ from incompatible pointer type [-Wincompatible-pointer-types]
  136 |   evaluate(7,exp,&result);
      |              ^~~
      |              |
      |              char *
soSeg.c:70:49: note: expected ‘const char **’ but argument is of type ‘char *’
   70 | int evaluate(const size_t fix_size, const char* fix[], int  *result)
      |                                     ~~~~~~~~~~~~^~~~~
soSeg.c:136:18: warning: passing argument 3 of ‘evaluate’ from incompatible pointer type [-Wincompatible-pointer-types]
  136 |   evaluate(7,exp,&result);
      |                  ^~~~~~~
      |                  |
      |                  int **
soSeg.c:70:62: note: expected ‘int *’ but argument is of type ‘int **’
   70 | int evaluate(const size_t fix_size, const char* fix[], int  *result)
      |                                                        ~~~~~~^~~~~~

但这是您的段错误的根源。 你是：

1. 当需要char **时，仅将char *传递给evaluate()
2. 当result已经是int *时，无论出于何种原因，您也会通过&result
3. 这条线

Stack *stack_mem = (Stack*)malloc(sizeof(stack_size));

不做你认为它做的事。 您正在分配stack_size字节。 相反，你应该做

Stack *stack_mem = malloc(sizeof(Stack)*stack_size);

4. 首先也没有理由将char *[]传递给您的 function。 进行中

int evaluate(const size_t fix_size, const char fix[], int  *result)

（并将所有*fix[i]更改为fix[i] ）也可以。

总而言之，这会导致大量越界访问

==581== Memcheck, a memory error detector
==581== Copyright (C) 2002-2017, and GNU GPL'd, by Julian Seward et al.
==581== Using Valgrind-3.17.0.GIT and LibVEX; rerun with -h for copyright info
==581== Command: ./soSeg
==581== 
==581== 
==581== Invalid read of size 4
==581==    at 0x1091EC: Create_Stack (in /petsc/soSeg)
==581==    by 0x109316: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581==  Address 0x4a78044 is 0 bytes after a block of size 4 alloc'd
==581==    at 0x484D93F: malloc (vg_replace_malloc.c:307)
==581==    by 0x1091C1: Create_Stack (in /petsc/soSeg)
==581==    by 0x109316: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581== 
==581== Invalid write of size 8
==581==    at 0x109204: Create_Stack (in /petsc/soSeg)
==581==    by 0x109316: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581==  Address 0x4a78048 is 4 bytes after a block of size 4 alloc'd
==581==    at 0x484D93F: malloc (vg_replace_malloc.c:307)
==581==    by 0x1091C1: Create_Stack (in /petsc/soSeg)
==581==    by 0x109316: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581== 
==581== 
==581== Invalid read of size 1
==581==    at 0x10934F: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581==  Address 0x2d392b2a313332 is not stack'd, malloc'd or (recently) free'd
==581== 
==581== 
==581== Process terminating with default action of signal 11 (SIGSEGV)
==581==  General Protection Fault
==581==    at 0x10934F: evaluate (in /petsc/soSeg)
==581==    by 0x1094F7: main (in /petsc/soSeg)
==581== 
==581== HEAP SUMMARY:
==581==     in use at exit: 32 bytes in 2 blocks
==581==   total heap usage: 2 allocs, 0 frees, 32 bytes allocated
==581== 
==581== LEAK SUMMARY:
==581==    definitely lost: 28 bytes in 1 blocks
==581==    indirectly lost: 0 bytes in 0 blocks
==581==      possibly lost: 0 bytes in 0 blocks
==581==    still reachable: 4 bytes in 1 blocks
==581==         suppressed: 0 bytes in 0 blocks
==581== Rerun with --leak-check=full to see details of leaked memory
==581== 
==581== For lists of detected and suppressed errors, rerun with: -s
==581== ERROR SUMMARY: 5 errors from 5 contexts (suppressed: 0 from 0)
Segmentation fault

注意我已经修剪了上面的内容，因为您在循环中进行了这些越界访问