Using a malloc for an int array in typedef struct

What did i do wrong?

You allocated evaluations but wrote arr_rew[i].evaluations unitialised and without indexing by j.

Change:

reviews* evaluations=malloc(arr_rew[i].number_of_evaluations*sizeof(int));

to

arr_rew[i].evaluations = malloc( arr_rew[i].number_of_evaluations * sizeof(int) ) ;

(or better):

arr_rew[i].evaluations = malloc( arr_rew[i].number_of_evaluations * 
                                 sizeof(*arr_rew[i].evaluations) ) ;

and

fscanf(fp, "%d", arr_rew[i].evaluations) ;

to

fscanf( fp, "%d", arr_rew[i].evaluations[j] ) ;

You also close the file while you are still writing to it, and the use of feof() in this manner is flawed as it is only true after you attempt to read past the end of the file. Instead check the file i/o operations for success and exit the loop if any fail.

    int input_check = 1 ;
    for( int i = 0; input_check != 0; i++ )
    {
        input_check = fscanf(fp, "%d", arr_rew[i].number_of_evaluations);

        if( input_check != 0 )
        {
            arr_rew[i].evaluations = malloc( arr_rew[i].number_of_evaluations * 
                                             sizeof(*arr_rew[i].evaluations) ) ;

            for( int j = 0; input_check != 0; j < arr_rew[i].number_of_evaluations; j++ )
            {
                input_check = fscanf( fp, "%d", arr_rew[i].evaluations[j] ) ;
            }

        }
    }
    fclose( fp ) ;

You need to assign the result of malloc() to arr_rew[i].evaluations. And then you need to index into that array when reading the evaluation values.

And evaluations should be int*, just like the evaluations member of the reviews struct.

void charge_review(review arr_rew[LEN]){
    FILE *fp;
    fp = fopen("recensioni.txt", "r");
    if(fp == NULL) {
        puts("================\nFile didn't opened\n================"); 
    }
    else
    {
        for(i=0;;i++)
        {
            if (fscanf(fp, "%d", arr_rew[i].number_of_evaluations) != 1) {
                break;
            }
            int* evaluations=malloc(arr_rew[i].number_of_evaluations*sizeof(int));
            for(int j=0;j<arr_rew[i].number_of_evaluations;j++)
            {
                fscanf(fp, "%d", &evaluations[j]);
            }
            arr_rew[i].evaluations = evaluations;
        }
        fclose(fp);
    }
}

Other issues:

• Don't use !feof(fp) as the loop condition. See Why is “while ( !feof (file) )” always wrong? for the reason. Check the result of fscanf() instead.
• fclose(fp) needs to be outside the loop.

Thanks to everybody, the code now finally works with the dynamic allocation of memory :D