List with struct pointer

Question

I have the following code but the result is null for all components of the structure:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct _TransactionType
{
    char    field1[20];
    char    field2[20]; 
}TransactionType;

int main(int argc, char *argv[]) {
    int i;
    int numreg = 0;
    char  temp[12];
    TransactionType *dbTransaction;

    dbTransaction = (TransactionType*) calloc(10,sizeof(TransactionType));


    for(i=0; i<5;i++)
    {
        memset(temp,0,sizeof(temp));        
        sprintf(temp,"%d",i);
        strcpy(dbTransaction->field1, temp);
        dbTransaction->field1[strlen(dbTransaction->field1)] = '\0';
        strcpy(dbTransaction->field2, temp);
        dbTransaction->field2[strlen(dbTransaction->field2)] = '\0';
        numreg++;
        dbTransaction++;    
    }

    printf("reg = %d\n", numreg);

    for (i=0; i<numreg;i++)
    {
        printf("dbTransaction->field1 = %s\n",(dbTransaction + i)->field1);
        printf("dbTransaction->field2 = %s\n",(dbTransaction + i)->field2);

    }


    return 0;
}

i need to recover the structure values. Please any kind of help will be appreciate Thanks in advance for your help

[Please see this discussion on why not to cast the return value of `malloc()` and family in `C`.](http://stackoverflow.com/q/605845/2173917). — Sourav Ghosh, Jul 04 '16 at 20:22
What's the point of setting the terminators explicitely? They are already there. — Eugene Sh., Jul 04 '16 at 20:24
You must work with a temporary variable, by the time you get to the `printf` the pointer is out of range of memory allocated. — Weather Vane, Jul 04 '16 at 20:26
@WeatherVane not exactly out of range, out of valid occurrences. — Sourav Ghosh, Jul 04 '16 at 20:27

score 1 · Answer 1 · answered Jul 04 '16 at 20:35

You should add error checking and casting of calloc values is discouraged, but the reason your code doesn't work is that you advance dbTransaction pointer in your loop, but never rewind it. The prints you're making are actually of elements 5-9 of the array while you fill elements 0-4.

See the corrected code:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

typedef struct _TransactionType
{
    char    field1[20];
    char    field2[20];
}TransactionType;

int main(int argc, char *argv[]) {
    int i;
    int numreg = 0;
    char  temp[12];
    TransactionType *dbTransaction;
    TransactionType *dbTransactionRoot;

    dbTransaction = (TransactionType*) calloc(10,sizeof(TransactionType));
    dbTransactionRoot = dbTransaction;


    for(i=0; i<5;i++)
    {
        memset(temp,0,sizeof(temp));
        sprintf(temp,"%d",i);
        strcpy(dbTransaction->field1, temp);
        dbTransaction->field1[strlen(dbTransaction->field1)] = '\0';
        strcpy(dbTransaction->field2, temp);
        dbTransaction->field2[strlen(dbTransaction->field2)] = '\0';
        numreg++;
        dbTransaction++;
    }

    printf("reg = %d\n", numreg);

    for (i=0; i<numreg;i++)
    {
        printf("dbTransaction->field1 = %s\n",(dbTransactionRoot + i)->field1);
        printf("dbTransaction->field2 = %s\n",(dbTransactionRoot + i)->field2);

    }


    return 0;
}

You should also consider accessing the pointer as array, with the bracket notation instead of incrementing it, it would allow you to eliminate the helper pointer — Ishay Peled, Jul 04 '16 at 21:16

List with struct pointer

1 Answers1