c - 在 C 中使用指针数组方法为 3D 数组动态分配内存

Question

我遇到了段错误无法纠正......................

如果使用函数(顶部多行注释)我发现分割错误使用目前的代码，我无法猜测段错误。

如果使用固定大小的字符串，如 malloc，那么它工作得很好。

//Array of pointers method
#include<stdio.h>
#include<stdlib.h>
#include<string.h>
int count=0;
char *** input(char ***);
void print(char ***);
/*
void *allocate(char *p,int j)
{
    int i=0;
    do
    {
        p=realloc(p,i+1);
//      printf("p[count][j]= %u\n",p);
        p[i]=getchar();
    }while(p[i++]!=10);
    p[i-1]='\0';
    return p;
 }*/
void save(char ***p)
{
    int j,i;
    FILE *fp;
    fp=fopen("Secret.c","w");
    for(i=0;i<count;i++)
      for(j=0;j<3;j++)
    {
      fprintf(fp,"%s\n",p[i][j]);

  } 
 fclose(fp);

}
void main()
{
    char ***a=0;
    char ch;
//  printf("a = %u\n ",a);
    while(1)
    {
        if(count>0)
        {
            printf("a = %lu\n",a);
            printf("*a = %lu\n",sizeof(*a));
            printf("**a = %lu\n",sizeof(**a));
            printf("***a = %lu\n",***a);
        }
        printf("\n\n\t\t\tMenu\n\n\n");
        printf("i . input\ns . save\np . print\nq . quit");
        printf("\n\nEnter your choice : ");
        scanf(" %c",&ch);
        getchar();
        switch(ch)
        {
            case 'i': a=input(a);break;
            case 's': save(a);
            case 'p': print(a);break;
            case 'q': return;
            default : printf("\n\n\t\t!!! Invalid choice  \n\n");
        }
    }
}
char *** input(char ***p)
{
    int i=0,j;

//  printf("p = %u\n ",p);
    p=realloc(p,sizeof(p)*(count+1));
//  printf("p = %u\n ",p);
//  printf("sizeof(p) = %u\n",sizeof(p));
//  printf("*p = %u\n ",*p);
    for(j=0;j<3;j++)
    {
        p[count]=realloc(p[count],sizeof(p)*(j+1));
//      printf("*p = %u\n ",*p);
//      printf("**p = %u\n ",**p);
        i=0;
        if(j==0)
            printf("Enter the %d name : ",count+1);
        else if (j==1)
            printf("Enter the %d mobile no : ",count+1);
        else
        printf("Enter the %d E-mail : ",count+1);    
//      p[count][j]=malloc(20);
//      gets(p[count][j]);      

//      p[count][j]=allocate(p[count][j],j);
        do
        {
            p[count][j]=realloc(p[count][j],i+1);
            p[count][j][i]=getchar();
        }while(p[count][j][i++]!=10);
        p[count][j][i-1]='\0';
//      printf("***p = %u %s\n ",***p,***p);
    }
    count++;
    return p;   

}
void print(char ***p)
{
    int j,i;
    for(i=0;i<count;i++,printf("\n"))
        for( j=0;j<3;j++)
            printf("p[%d][%d] : %s\t",i,j,p[i][j]);

}

Answer 1

一些一般准则:

您想(重新)分配 p 指向 类型的 N 个实例，而不是 p 类型的 N 个实例，所以 sizeof 的操作数应该是 *p:

p = malloc( sizeof *p * N ); // operand of sizeof is *p, not p
                   ^^

如果p是一个指针数组，使用

p[i] = malloc( sizeof *p[i] * N );

此外，请记住，如果内存无法扩展，realloc 调用将返回 NULL，因此您不希望将结果直接分配回 p，否则你可能会丢失指向你已经分配的内存的指针，从而导致内存泄漏。将结果分配回与 p 相同类型的临时对象:

T *tmp = realloc( p, sizeof *p * M );
if ( tmp )
{
  p = tmp;
}

realloc 是一个潜在的昂贵调用，许多小的 realloc 会导致内部内存碎片。分配更大的 block 比一次扩展一个元素要好。要读取单个输入，您可以这样做:

char *getNextLine( FILE *inputStream )
{
  char staticBuffer[SOME_BUFFER_SIZE] = {0};
  char *dynamicBuffer = calloc( SOME_BUFFER_SIZE, sizeof *dynamicBuffer );
  size_t dynamicBufferSize = SOME_BUFFER_SIZE;
  bool done = false;

  /**
   * If the initial allocation fails, we have real problems.
   */
  assert( dynamicBuffer != NULL );

  while ( !done && fgets( staticBuffer, sizeof staticBuffer, stdin ) )
  {
    /**
     * Check for the newline; if it's there, remove it and set the
     * done flag to exit the loop.
     */
    char *newline = strchr( staticBuffer, '\n' );
    if ( (done = (newline != NULL)) )
    {
       *newline = 0;
    }

    /**
     * Do we need to extend our dynamic buffer?
     */
    if ( strlen( dynamicBuffer ) + strlen( staticBuffer ) > dynamicBufferSize )
    {
      /**
       * Yes.  Double the size of the dynamic buffer.
       */
      char *tmp = realloc( dynamicBuffer, sizeof *dynamicBuffer * ( dynamicBufferSize * 2 ) );
      if ( tmp )
      {
        dynamicBuffer = tmp;
        dynamicBufferSize *= 2;
      }
      else
      {
        // could not extend buffer, print an error and return what we have
        fprintf( stderr, "ERR: Could not extend dynamic input buffer\n" );
        return dynamicBuffer;
      }
    }

    strcat( dynamicBuffer, staticBuffer );
  }

  return dynamicBuffer;
}