How can I write variables in a shared-memory or read them?

Question

My Program starts Child-Processes with fork to ask the Parent-Process about the UNIX-Time. I've made a Function with a shared memory to write the Ticks (UNIX-Time) and the Client-Number into a struct Array with MAXENTRIES of indexes. I can't figure out why the values passed to the function f_timeLog(int, int) don't show in the Array.

If I print with

printf("From: %d\tTICKS: %d\n", logSM[*counter]->vonWem, logSM[*counter]->ticks);

The Values say always: 0. If I debug the program, it writes the first value logSM[0] right, but further not.

Thanks for help!

typedef struct
{
    int vonWem;
    int ticks;
}timeLog [MAXENTRIES];

void f_timeLog(int who, int ticks)
{
    int *counter;
    timeLog *logSM;
    logSM = (timeLog*) shmat(TimeLog, NULL, (SHM_R|SHM_W));
    counter = (int*) shmat(IDCounter, NULL, (SHM_R|SHM_W));
    P(SemWriteLog);
    logSM[*counter]->vonWem = who;
    logSM[*counter]->ticks = ticks;
    *counter= *counter+1;
    if(*counter >= MAXENTRIES) *counter= *counter - MAXENTRIES;
    V(SemWriteLog);
}

The output is logSM[0] vonWem = X, ticks = xxxxxxxx and logSM[1] and further: vonWem = 0, ticks = 0;

Answer 1

logSM is a pointer to an array, not an array of pointers.

Because logSM is a pointer, we need to dereference it. And because it points to an array of structure object (instance) we can't use the "arrow" operator.

So usage should be like

(*logSM)[*counter].ticks = ticks;

---

A more "natural" solution is to redefine the type-alias timeLog to be just the structure itself.

So something like

typedef struct
{
    int vonWem;
    int ticks;
} timeLog;

Then you can use logSM like any other pointer or array:

logSM[*counter].ticks = ticks;