Segmentation Fault caused by a read into the client - c

Question

I wrote this code for a simple client/server application:
This is a portion of the code of the server:
while(TRUE){
struct sockaddr_in clientName = {0};
int simpleChildSocket = 0;
unsigned int clientNameLength = sizeof(clientName);
simpleChildSocket = accept(simpleSocket,(struct sockaddr *)&clientName, &clientNameLength);
if (simpleChildSocket == -1) {
fprintf(stderr, "Cannot accept connections!\n");
close(simpleSocket);
exit(EXIT_FAILURE);
}
/* write out our message to the client */
snprintf(buffer, sizeof(buffer), "OK %s", MESSAGE);
write(simpleChildSocket, buffer, strlen(buffer));
/* Start server operations */
array = NULL, j = 0, k = 0, numberSample = 0;
tmpValue = NULL;
do{
do{
memset(buffer, '\0', sizeof(buffer));
returnStatus = read(simpleChildSocket, buffer, sizeof(buffer));
if(returnStatus <= 0)
fprintf(stderr, "Status: %d\n", returnStatus);
for(i = 0; i < (int)strlen(buffer); i++){
if(isdigit(buffer[i]))
flag = 1;
else if(isspace(buffer[i]))
continue;
else{
flag = 0;
break;
}
}
if(flag == 0){
memset(buffer, '\0', sizeof(buffer));
snprintf(buffer, sizeof(buffer), BOLD RED "ER" RESET " Devi inserire solo numeri\n");
write(simpleChildSocket, buffer, strlen(buffer));
}
}while(flag != 1);
token = strtok(buffer, " ");
digit = atoi(token);
if(digit != 0){
printf("Token #%d: %d\n", k, digit);
k++;
}
else
printf("Ricevuto %d\n", digit);
numberSample += digit;
array = realloc(array, numberSample * sizeof(int));
for(i = 0; (tmpValue = strtok(NULL, " "))!= NULL; i++, j++){
array[j] = atoi(tmpValue);
}
}while(digit != 0);
for(i = 0; i < numberSample; i++)
printf("%d: %d\n", i, array[i]);
printf("\n\n<%d>\n\n", numberSample);
free(array);
close(simpleChildSocket);
}
Instead this is a portion of the code of the client:
/* get the message from the server */
memset(buffer, '\0', sizeof(buffer));
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if(returnStatus > 0)
printf("%s", buffer);
else
fprintf(stderr, "Return Status: %d\n", returnStatus);
/* start client operations */
do{
memset(buffer, '\0', sizeof(buffer));
fgets(buffer, sizeof(buffer), stdin);
/*fflush(stdout);*/
write(simpleSocket, buffer, strlen(buffer));
memset(string, '\0', sizeof(string));
returnStatus = read(simpleSocket, string, sizeof(string));
if(returnStatus > 0)
printf("%s\n", string);
else
fprintf(stderr, "Return Status: %d\n", returnStatus);
}while(atoi(&buffer[0]) != 0);
The problem is that when I send data for the first time to the server, the latter goes into segmentation fault. The problem is caused by the client's read, which must take the error if the user attempts to send letters or symbols instead of the numbers. How can I fix the problem?