Assigning values to members of structures

问题

I am currently working on a project for a course in C programming and have run into a problem. I am still quite new to the language C but have experience programming Java.

My program runs into a problem when I try to assign a value to a char * variable member of a struct. The code in question follows below:

void ReadFile(ComponentType * Data, int * numEl, int * numNodes)
{
  size_t index;
  FILE * dataFile;
  char * data;
  float * value;

  numEl = malloc(sizeof(int *));
  numNodes = malloc(sizeof(int*));
  Data = malloc(6 * sizeof(ComponentType));

  * numEl = 0;
  * numNodes = 0;
  index  = 0;   

  if((dataFile = fopen("mydata.dat", "r")) == NULL)
      puts("Error - file \"mydata.dat\" could not be opened!");
  else
  { 

      while(!feof(dataFile)) 
      { 

          fscanf(dataFile, "%s", data);
          Data[index].name = (char *)malloc(strlen(data)+1);
          strcpy(Data[index].name, data);

          fscanf(dataFile, "%s", data);
          Data[index].node1 = (char *)malloc(strlen(data)+1);
          strcpy(Data[index].node1, data);

          fscanf(dataFile, "%s", data);
          Data[index].node2 = (char *)malloc(strlen(data)+1);
          strcpy(Data[index].node2, data);

          fscanf(dataFile, "%f", value);
          Data[index].value = * value;


          int i = char2int(CircuitData[index].node1);
          if(i > * numNodes)
              * numNodes = i;

          i = char2int(CircuitData[index].node2);
          if(i > * numNodes)
              * numNodes = i;

          (* numEl)++;
          (* numNodes)++;
          index++;
      }

      fclose(dataFile);
  }

  free(Data);
  free(numEl);
  free(numNodes);       
}

The structure is defined as follows:

typedef struct
{
  char * name;
  char * node1;
  char * node2;
  float value;
} ComponentType;

I have run a debug on the program and what basically happens is that the file is read correctly and the value stored in data is correct however printing out Data[index].name either causes a segmentation fault when formatted as a char * or, in the debug program, prints out 0x0. In short, the values read from the file are not being copied into the structure variable I want them in.

Any suggestions/help would be appreciated. Thank you.

EDIT: I am using Linux 64-bit to code

EDIT 2: I added the entire function in which the error occurs with the changes suggested below

回答1:

First: when you debug your program, check what sizeof(data) is.
You will be surprised to find it is 8 (the size of a pointer), and NOT 6 (the size you allocated)
This is because sizeof measures the size of the type, and data is a char*, (aka. a pointer).

Pointers are 8-bytes. (on a 64-bit system. Other systems are different)
This causes you to allocate 8 bytes for Data[index].name.

Next: You cannot copy data from one array to another using the = operator, as you tried to do with:

Data[index].name = data;

To do it properly:

fscanf(dataFile, "%s", data);
// Measure the length of data, and allocate enough space (including the null-termiantor)
Data[index].name = malloc(strlen(data) + 1);  

// Copy from data into Data[index].name.   For greater safety, look into strcpy_s
strcpy(Data[index].name, data);

来源：https://stackoverflow.com/questions/19255964/assigning-values-to-members-of-structures

标签

structure