C缺少数据的结构数组？

Question

I am designing a program in C. Part of the program involves reading a table of data relating to the periodic table and its elements from a file, and putting it in a structure.

So far, it's working rather well. However, for some reason, when I try to display the array, a couple of elements don't show up, but instead blanks. It does show up earlier in the code, though.

main.c

main()
{
    struct periodic *tablePtr;
    tablePtr = createTable();
    printf("%d\t",(tablePtr+90)//Prints "Pa" here as expected
    int i;
    for(i=0;i<num_elements;i++){
        printf("%d\t%s\n",i,(tablePtr+90)->sym);//Prints i, but then just blank.
    }


}

periodic.c (creates the table)

#include "periodic.h"
#include <stdio.h>

struct periodic *createTable(){

    char format[] ="%d\t%s[3]\t \
                         %s[20]\t%f\t \
                         %s[100]\t%f\t \
                         %d\t%f\t%d\t \
                         %d\t%d\t%s[20]\t \
                         %s[7]\t%s[17]\t \
                         %d\t%d\t%f\t \
                         %s[40]\n)";


    struct periodic period_table[num_elements];
    struct periodic *tablePtr = period_table;
    FILE *fp;
    fp = fopen("periodictable.csv","r");

    char buff[200];

    struct periodic *initPtr = tablePtr;
    while(fgets(buff,sizeof(buff),fp)){
        sscanf(buff,format,&(tablePtr->num),&(tablePtr->sym),&(tablePtr->name),&(tablePtr->weight),&(tablePtr->config),&(tablePtr->neg),&(tablePtr->ion_rad),&(tablePtr->vdW_rad),&(tablePtr->IE_1),&(tablePtr->EA),&(tablePtr->oxi_st),&(tablePtr->stn_st),&(tablePtr->melt),&(tablePtr->boil),&(tablePtr->dens),&(tablePtr->type));
        tablePtr++;
    }

    fclose(fp);
    return initPtr;

}

I can give more information as needed.

Answer 1

You have:

struct periodic *tablePtr = period_table;

Here, tablePtr points to an array that is defined locally in the function. And then you return tablePtr from the function. When the function returns, the array is destroyed. Hence, the calling function has a dangling pointer.

Referencing a dangling pointer leads to undefined behavior.

You need to allocate memory from the heap, return a pointer to the dynamically allocated memory, and deallocate the memory in the calling function.

struct periodic *tablePtr = malloc(sizeof(*tablePtr)*num_elements);

and in main , call

free(tablePtr);

before the function ends.

Also, add an explicit return type to main .

int main() 
{
   ...
}