How do I write a string to a 2D array in C? - arrays

Question

So I am working on a multi-threading program with 4 threads. For thread1 I am grabbing input from stdin and then putting it into buffer1, thread2 then reads from buffer1, modifies it, then puts it in buffer2 for thread3 to read, and so on....
After I have read from buffer1 (in my thread2 function), I have successfully altered the data, now I want to put it in buffer2, but I am getting the error of "expression must be a modifiable lvalue". Does anyone know why??
My thread 2 function is below and the error is appearing in buffer2[write2] = updatedStr;
void *line_separator_thread(void *args)
{
// Consuming from buffer 1 and producing to buffer 2
int haveStop = 0;
while (1)
{
// Snag the mutext for a read
pthread_mutex_lock(&mutex1);
// Release the mutex and block on the condition
pthread_cond_wait(&readyBuffer1, &mutex1);
// Process all unread slots from the buffer1 queue
while (buffer1[read1][0] != -1) // Look for no sentinal value
{
haveStop = haveStopInText(buffer1[read1]);
if (haveStop)
break;
// ***If no break, then read from buffer1, and modify \n to be a space?
// str_replace(orig, replace, with)
char *updatedStr = str_replace(buffer1[read1], "\n", " ");
printf("UPDATED STRING: %s\n", updatedStr);
read1 = (read1 + 1) % 5;
// ***Put the updated string in buffer 2?
pthread_mutex_lock(&mutex2);
buffer2[write2] = updatedStr; //ERROR IS HERE!!!
// ***Need to change buffer1[read1][0] So that it doesn't keep looping?
}
// ***Unlock the mutex here? Or outside of the while loop?
pthread_mutex_unlock(&mutex1);
}
return NULL;
Buffer variables and read/write variables are below
char buffer1[5][1000]; //inputThread + lineSeparatorThread
char buffer2[5][1000];
char buffer3[5][1000];
// Set up variables to let the threads know where is it safe to read from and write to
int write1 = 0;
int read1 = 0;
int write2 = 0;
int read2 = 0;
int write3 = 0;
int read3 = 0;
And lastly, below is in my main function where I am creating the 4 threads
pthread_create(&outputThread, &attr, output_thread, NULL);
usleep(100); // Force the program to allow output thread to actually come up and pend on readyBuffer 3 first
pthread_create(&plusSignThread, &attr, plus_sign_thread, NULL);
usleep(100); // Force the program to allow plus_sign_thread thread to actually come up first
pthread_create(&lineSeparatorThread, &attr, line_separator_thread, NULL);
usleep(100); // Force the program to allow line_separator_thread thread to actually come up first
pthread_create(&inputThread, &attr, input_thread, NULL);
pthread_join(inputThread, NULL);
pthread_join(lineSeparatorThread, NULL);
pthread_join(plusSignThread, NULL);
pthread_join(outputThread, NULL);