C-初始化结构成员-指针数组。 第二个元素的值不正确

Question

I've got struct with parameters. I want to initialize every single of them and put their addresses into paramList array and number of initialized params in paramListSize field. I want to achieve this without specifying number of params upfront and with as little steps as possible.

I wrote solution that I'm somewhat happy with, but it is not really working :-) Second element is being overwritten by random number and I have no idea why.

Can you help?

I'm using C++ compiler, but writing in C.

Here's code and sandbox https://repl.it/repls/SourRoughDrawing

#include "stdio.h"

struct parameter
{
  int field;
};

struct configuration {
  parameter** paramList;
  int paramListSize;
  parameter param1;
  parameter param2;
  parameter param3;
};

parameter *initParameter(parameter *param, int value, int *paramListSize)
{
  param->field = value;
  (*paramListSize)++;
  return param;
}

int initConfig(configuration *config)
{
  config->paramListSize = 0;
  parameter* test[] =
  {
    initParameter(&config->param1, 100, &config->paramListSize),
    initParameter(&config->param2, 200, &config->paramListSize),
    initParameter(&config->param3, 300, &config->paramListSize)
  };
  config->paramList = test;

  // dirty failed attemp to fix :-)
  for (int parameterIndex = 0; parameterIndex < config->paramListSize; parameterIndex++)
  {
    config->paramList[parameterIndex] = test[parameterIndex];
  }
}

void printConfig(configuration *config)
{
  for (int parameterIndex = 0; parameterIndex < config->paramListSize; parameterIndex++)
  {
    printf("param%d:%d\n", parameterIndex + 1, config->paramList[parameterIndex]->field);
  }
}

int main(void)
{
  configuration config;
  initConfig(&config);
  printConfig(&config);
}

Answer 1

You are not allocating memory for paramlist ; instead, you assign a pointer to a local variable which becomes invalid once its lifetime ends at the end of the function. Accessing it is undefined behaviour then.

The following code shows how to overcome this issue;

int initConfig(configuration *config)
{
  config->paramListSize = 0;
  config->paramList = calloc(3, sizeof(parameter));
  config->paramList[0] = &config->param1;
  config->paramList[1] = &config->param2;
  config->paramList[2] = &config->param3;

  initParameter(&config->param1, 100, &config->paramListSize);
  initParameter(&config->param2, 200, &config->paramListSize);
  initParameter(&config->param3, 300, &config->paramListSize);
}

Answer 2

The problem with your approach is that test is a local variable allocated and the stack and it will be gone once int initConfig(configuration *config) returns.

So this attempt

config->paramList = test;

has to be replaced with allocation of the memory which will survive the function return.

  config->paramList = (parameter**)calloc(config->paramListSize, sizeof(parameter));

Now you you can copy the pointers gathered in temp to paramList

  for (int i = 0; i< config->paramListSize; i++)
  {
    config->paramList[i] = test[i];
  }

We also check if memory allocation was successful and return 0 or - 1 from the int initConfig accordingly.

At the end of main memory is freed.

The program follows your ideas:

#include "stdio.h"
#include "stdlib.h"
#include "string.h"

struct parameter
{
  int field;
};

struct configuration {
  parameter** paramList;
  int paramListSize;
  parameter param1;
  parameter param2;
  parameter param3;
};

parameter *initParameter(parameter *param, int value, int *paramListSize)
{
   param->field = value;

  (*paramListSize)++;

  return param;
}

int initConfig(configuration *config)
{
  config->paramListSize = 0;

  // This array gathers the parameter pointers but it will be lost when `initConfig` returns:
  parameter* test[] =
  {
    initParameter(&config->param1, 100, &config->paramListSize),
    initParameter(&config->param2, 200, &config->paramListSize),
    initParameter(&config->param3, 300, &config->paramListSize)
  };

  // Allocate memory for the list of parameters:
  config->paramList = (parameter**)calloc(config->paramListSize, sizeof(parameter));

  if (config->paramList == NULL) // calloc failed
    return -1;

  // copy the pointers to the paramList
  for (int i = 0; i< config->paramListSize; i++)
  {
    config->paramList[i] = test[i];
  }

  return 0;
}

void printConfig(configuration *config)
{
  for (int i= 0; i < config->paramListSize; i++)
  {
    printf("param %d = %d\n", i + 1, config->paramList[i]->field);
  }
}

int main(void)
{
   configuration config;

   if( initConfig(&config) == 0){  // initConfig returned with success
      printConfig(&config);
      free(config.paramList);       // free the memory.
   }
   else {
      printf("calloc failed - initConfig failed!\n");
   } 
   return 0;
}

Output:

param 1 = 100
param 2 = 200
param 3 = 300