Strtok() returning NULL randomly - c

Question

I am trying to create a client that communicate with a server by sending 2 types of messages:
The word QUIT that communicate to the server to close the connection.
An operation with the following syntax: operator first_operand second_operand. For example: + 3 3, - 5 6 etc. (the operands must be positive integers, and there must be only 2 operands).
If the server receive an operation, it executes it and returns the result to the client. The problem is that the first operation I send returns the right result, while the following ones work randomly (sometimes they return the right result, other times the function strtok() doesn't get the second operand and returns NULL...).
This is code of the client that process the message written by the user in the prompt and that scan the message to check if the operation is written with the correct syntax (WARNING: the code is written in an extremely unprofessional and unclean way).
The code part that creates the problem is inside the while(1).
#define MAXLENGTH 256
int main (int argc, char *argv[]) {
int simpleSocket = 0;
int simplePort = 0;
int returnStatus = 0;
char first[10], second[10];
char* operator;
char buffer[MAXLENGTH] = "";
char message[50];
char terminationCommand[] = "QUIT\n";
char space[2] = " ";
struct sockaddr_in simpleServer;
if (3 != argc) {
fprintf(stderr, "Usage: %s <server> <port>\n", argv[0]);
exit(1);
}
/* create a streaming socket */
simpleSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (simpleSocket == -1) {
fprintf(stderr, "Could not create a socket!\n");
exit(1);
} else {
fprintf(stderr, "Socket created!\n");
}
/* retrieve the port number for connecting */
simplePort = atoi(argv[2]);
/* setup the address structure */
/* use the IP address sent as an argument for the server address */
//bzero(&simpleServer, sizeof(simpleServer));
memset(&simpleServer, '\0', sizeof(simpleServer));
simpleServer.sin_family = AF_INET;
//inet_addr(argv[2], &simpleServer.sin_addr.s_addr);
simpleServer.sin_addr.s_addr=inet_addr(argv[1]);
simpleServer.sin_port = htons(simplePort);
/* connect to the address and port with our socket */
returnStatus = connect(simpleSocket, (struct sockaddr *)&simpleServer, sizeof(simpleServer));
if (returnStatus == 0) {
fprintf(stderr, "Connect successful!\n\n");
} else {
fprintf(stderr, "Could not connect to address!\n");
close(simpleSocket);
exit(1);
}
/* get the message from the server */
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", &buffer[3]);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
memset(&buffer, '\0', sizeof(buffer));
printf("You can execute 2 commands:\n");
printf("1. Operations ( +, -, *, /, % ) with the following syntax: operator + first operand + second operand.\n");
printf("Example: + 5 2 \n");
printf("2. Termination of the connection with the following syntax: QUIT + press Enter.\n");
while(1) {
printf("\nEnter a command:\n");
fgets(message, 1000, stdin);
// the if with the termination command works fine
if (strcmp(message, terminationCommand) == 0) {
if (send(simpleSocket, message, strlen(message), 0) < 0) {
printf("Send failed.");
return 1;
}
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", &buffer[4]);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
close(simpleSocket);
exit(1);
}
operator = strtok(message, space);
if (strcmp(operator, "+") == 0 || strcmp(operator, "-") == 0 || strcmp(operator, "/") == 0 || strcmp(operator, "%") == 0 || strcmp(operator, "*") == 0) {
char *first_operand = strtok(NULL, space);
if (first_operand != NULL) {
if (strcmp(first_operand, "ANS") == 0)
strcpy(first, "ANS");
else
strcpy(first, first_operand);
printf("%s\n", operator);
printf("%s\n", first);
char *second_operand = strtok(NULL, space);
printf("%s\n", second_operand);
if (second_operand != NULL && strtok(NULL, space) == NULL && (atoi(first) > 0 || strcmp(first, "ANS") == 0)) {
if (strcmp(second_operand, "ANS\n") == 0)
strcpy(second, "ANS");
else {
strcpy(second, second_operand);
}
if (atoi(second) > 0 || strcmp(second, "ANS") == 0) {
printf("OK\n");
char operation[] = "";
strcat(operation, operator);
strcat(operation, " ");
strcat(operation, first);
strcat(operation, " ");
strcat(operation, second);
if (send(simpleSocket, operation, strlen(operation), 0) < 0) {
printf("Send failed.");
return 1;
}
returnStatus = read(simpleSocket, buffer, sizeof(buffer));
if (returnStatus > 0) {
printf("%s\n", buffer);
} else {
fprintf(stderr, "Return Status = %d \n", returnStatus);
}
}
}
}
}
// after everything I reset the buffers I use to memorize the message and the elements of the message
memset(&buffer, '\0', sizeof(buffer));
memset(&first, '\0', sizeof(first));
memset(&second, '\0', sizeof(second));
memset(&message, '\0', sizeof(message));
memset(operator, '\0', sizeof(operator));
}
}
Can someone tell me why the second strtok() acts weird 90% of the times? What am I doing wrong?

Answer 1

There are multiple issues in you program:
You send newline terminated messages and you assume on the other end the read will return exactly the bytes sent by the other party, which is an incorrect assumption for the TCP/IP communications, only the order of bytes received is guaranteed, but the messages can be split on the way and received in chunks different from the sending sequence. You should instead read the socket into a buffer and only handle it once you receive a newline.
In your case, there is another problem which is more pressing: the buffer into which you read the data is not null terminated, so you should not pass it to standard C functions such as strtok().