OpenMP multiple writing on dynamic array C - c

Question

By using OpenMP I'm trying to parallelize the creation of a kind of dictionary so defined.
typedef struct Symbol {
int usage;
char character;
} Symbol;
typedef struct SymbolDictionary {
int charsNr;
Symbol *symbols;
} SymbolDictionary;
I did the following code.
#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <stdbool.h>
#include <omp.h>
static const int n = 10;
int main(int argc, char* argv[]) {
int thread_count = strtol(argv[1], NULL, 10);
omp_set_dynamic(0);
omp_set_num_threads(thread_count);
SymbolDictionary **symbolsDict = calloc(omp_get_max_threads(), sizeof(SymbolDictionary*));
SymbolDictionary *dict = NULL;
int count = 0;
#pragma omp parallel for firstprivate(dict, count) shared(symbolsDict)
for (int i = 0; i < n; i++) {
if (count == 0) {
dict = calloc(1, sizeof(SymbolDictionary));
dict->charsNr = 0;
dict->symbols = calloc(n, sizeof(Symbol));
#pragma omp critical
symbolsDict[omp_get_thread_num()] = dict;
}
dict->symbols[count].usage = i;
dict->symbols[count].character = 'a' + i;
++dict->charsNr;
++count;
}
if (omp_get_max_threads() > 1) {
// merge the dictionaries
}
for (int j = 0; j < symbolsDict[0]->charsNr; j++)
printf("symbolsDict[0][%d].character: %c\nsymbolsDict[0][%d].usage: %d\n",
j,
symbolsDict[0]->symbols[j].character,
j,
symbolsDict[0]->symbols[j].usage);
for (int i = 0; i < omp_get_max_threads(); i++)
free(symbolsDict[i]->symbols);
free(symbolsDict);
return 0;
}
The code compiles and runs, but I'm not sure about how the omp block works and if I implemented it correctly. Especially I have to attach the dict with the symbolsDict at the beginning of the loop, because I don't know when a thread will complete its work. However, by doing that probably different threads will write inside symbolsDict at the same time but in different memory. Although the threads will use different access points, dict should be different for every thread, I'm not sure this is a good way to do that.
I tested the code with different threads and creating dictionaries of different sizes.
I didn't have any kind of problem, but maybe it was just chance.
Basically I looked for the theory part around on the documentation. So I would like to know if I implemented the code correctly? If not, what is incorrect and why?

Answer 1

different threads will write inside symbolsDict at the same time but in different memory. Although the threads will use different access points, dict should be different for every thread, I'm not sure this is a good way to do that.
It isn't a good way but it is safe. A cleaner way would be this:
SymbolDictionary **symbolsDict = calloc(
omp_get_max_threads(), sizeof(SymbolDictionary*));
#pragma omp parallel
{
SymbolDictionary *dict = calloc(1, sizeof(SymbolDictionary));
int count = 0;
dict->charsNr = 0;
dict->symbols = calloc(n, sizeof(Symbol));
symbolsDict[omp_get_thread_num()] = dict;
# pragma omp for nowait
for(int i = 0; i < n; i++) {
dict->symbols[count].usage = i;
dict->symbols[count].character = 'a' + i;
++dict->charsNr;
++count;
}
}
Note that the inner pragma is omp for, not omp parallel for so it is using the outer parallel block to distribute its work. The nowait is a performance improvement that avoids a thread barrier at the end of the loop since it is the last part of the parallel section and threads wait for all other threads at the end of the section anyway.