Allocation and freeing of memory for pointer to structures

[TommyB]

Hi,

I have a little programm that uses an array of pointers to a structure. Everything works fine until I free up the memory.

Here is the sample code:
#include <stdio.h>
#include <math.h>
#include <string.h>
#include <stdlib.h>

#define MAX_SMS_LINES 15



struct SMSInfoType {
char SMSNumber[5];
char TelNumber[20];
char Time[20];
char lines[MAX_SMS_LINES][255];
};


void main()
{
int i, nrows;
struct SMSInfoType **arraySMS;


nrows=5;
arraySMS=malloc(nrows*sizeof(struct SMSInfoType *));
if(arraySMS==NULL){
printf("Memory allocation error\n");
exit(-1);
}

for(i=0;i<nrows;i++){
arraySMS[i]=malloc(sizeof(struct SMSInfoType));
if(arraySMS[i]==NULL){
printf("Memory allocation error\n");
exit(-1);
}

strcpy((*arraySMS)[i].SMSNumber,"3");
strcpy((*arraySMS)[i].TelNumber,"1234567890123456789");
strcpy((*arraySMS)[i].Time,"14:30:00");

}

for(i=0;i<nrows;i++){
printf("SMSNumber[%d]: %s TelNumber[%d]: %s Time[%d]: %s\n",i,(*arraySMS)[i].SMSNumber,i,(*arraySMS)[i].TelNumber,i,(*arraySMS)[i].Time);
}

for(i=0;i<nrows;i++){
printf("Free arraySMS[%d]\n",i);
/***************** and here it crashes !!! ***********/
free(arraySMS[i]);
}

printf("Free arraySMS\n");
free(arraySMS);
printf("Everything freed\n");
printf("end\n");

}


Has anyone an idea, what is wrong in this code and why I get an error when trying to free up the memory?

Thanks a lot

[hdanw]

Hi,

I have a little programm that uses an array of pointers to a structure. Everything works fine until I free up the memory.


strcpy((*arraySMS)[i].SMSNumber,"3");
strcpy((*arraySMS)[i].TelNumber,"1234567890123456789");
strcpy((*arraySMS)[i].Time,"14:30:00");


Has anyone an idea, what is wrong in this code and why I get an error when trying to free up the memory?

Thanks a lot

or more importantly
this

(*arraySMS)[i]
 

is different from this

(*(arraySMS[i]))
 

do you see the difference?

*arraySMS is the value that arraysms is pointing to this is your table
and *(arraySMS[0]) this is your data.

Again : arraysms points to a list of pointers
and arraysms[i] is one of the pointers on that list.

This error will haunt you again. Keep in mind it is always becuase you wrote to space that was not part of your allocation. Or you are trying to free space that was not allocated.


Have you ever heard of new? and delete?

Also, there is a better way to do what you are doing.

Each time you allocate space a chunk of memory the size is dependant on the OS and memory settings, but the intire chunk is used.

if your allocation block is 4 kilobytes then you are using 4kb for each index.

Try it like this,

allocate the index like you did above,

struct SMSInfoType **arraySMS;
 
 
nrows=5;
arraySMS=malloc(nrows*sizeof(struct SMSInfoType *));
smsdata = (... *) new structuretag[nrows];
 
 

But only allocate once for the data and use another temp var to hold that address.

SMSInfoType * arraySMSblock = malloc(nrows*sizeof(struct SMSInfoType));
 

And then assign offsets into the second data block into the first allocated array

for( int i = 0; i < nrows; i++)
{
     arraySMS[i] = &arraySMSblock[i];
}
 

now you are only allocating one 4kb block instead of 5, and save yourself about 16kb of overhead.

and cleanup is easy

free(arraySMS);
free(arraySMSblock);
 

Actually, I think you may have to calculate the offset of each using malloc. malloc is outdated, if you use new, the offsets are calculated for you. I said MAY. I haven't used malloc since 1992;

Happy coding.

Dan -

sorry for the double post.

[gpraghuram]

The problem happens becos of the way you are doing strcpy

//Your code commented
                 //strcpy((*arraySMS)[i].SMSNumber,"3");
    //strcpy((*arraySMS)[i].TelNumber,"1234567890123456789");
    //strcpy((*arraySMS)[i].Time,"14:30:00");
 
//Option 1
    strcpy((*(arraySMS[i])).SMSNumber,"3");
    strcpy((*(arraySMS[i])).TelNumber,"1234567890123456789");
    strcpy((*(arraySMS[i])).Time,"14:30:00");
//option 2
    //strcpy(arraySMS[i]->SMSNumber,"3");
    //strcpy(arraySMS[i]->TelNumber,"1234567890123456789");
    //strcpy(arraySMS[i]->Time,"14:30:00");
 
 

Use either option 1 or option 2

Thanks
Raghuram

[weaknessforcats]

for(i=0;i<nrows;i++){
arraySMS[i]=malloc(sizeof(struct SMSInfoType));
if(arraySMS[i]==NULL){
printf("Memory allocation error\n");
exit(-1);
}

strcpy((*arraySMS)[i].SMSNumber,"3");
strcpy((*arraySMS)[i].TelNumber,"1234567890123456789");
strcpy((*arraySMS)[i].Time,"14:30:00");

It looks like your strcpy() uses an i == nrows. That's outside the array.

[TommyB]

Thanks a lot for your replies:

Option 1 of gpraghuram works perfectly for me.

@weaknessforcats:
I checked by printf("%d\n",i) and the maximum i that is used in strcpy is 4. So i<nrows is always fullfilled.

@hdanw years ago a took a C++ course and I hope there was something about new and delete there. However I have to admit that due to never writting real C++ programs I completle forgot this possibility.
As I am writing in plain C I am afraid there new and delete does not exist.
Thanks a lot for enlighten me about the difference between

   (*arraySMS)[i]

and

(*(arraySMS[i]))

.
As I am writing C very rarely these counter stuff makes always knots into my brain.

Thanks a lot to all of you
Thomas

[weaknessforcats]

@weaknessforcats:
I checked by printf("%d\n",i) and the maximum i that is used in strcpy is 4. So i<nrows is always fullfilled.

I don't think so. Here is your code:

for(i=0;i<nrows;i++){
arraySMS[i]=malloc(sizeof(struct SMSInfoType));
if(arraySMS[i]==NULL){
printf("Memory allocation error\n");
exit(-1);
}

strcpy((*arraySMS)[i].SMSNumber,"3");
strcpy((*arraySMS)[i].TelNumber,"1234567890123456789");
strcpy((*arraySMS)[i].Time,"14:30:00");

1) There is no printf() before those strcpy() calls.
2) when you leave the loop, i == nrows.

That makes the strcpy() to one element past the end of your array.