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To retrieve the Type of Service (TOS) value on a TCP socket in a C program, you can use the getsockopt
function. The getsockopt
function allows you to read socket options. For retrieving the TOS value, you’ll use the IP_TOS
option with getsockopt
.
Here’s an example of how to do this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
int main() {
int sockfd;
int tos;
socklen_t optlen;
struct sockaddr_in server_addr;
// Create a socket
sockfd = socket(AF_INET, SOCK_STREAM, 0);
if (sockfd < 0) {
perror("socket");
exit(EXIT_FAILURE);
}
// Initialize server address structure
memset(&server_addr, 0, sizeof(server_addr));
server_addr.sin_family = AF_INET;
server_addr.sin_port = htons(8080);
server_addr.sin_addr.s_addr = inet_addr("127.0.0.1");
// Connect to the server
if (connect(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
perror("connect");
close(sockfd);
exit(EXIT_FAILURE);
}
// Retrieve the TOS value
optlen = sizeof(tos);
if (getsockopt(sockfd, IPPROTO_IP, IP_TOS, &tos, &optlen) < 0) {
perror("getsockopt");
close(sockfd);
exit(EXIT_FAILURE);
}
printf("TOS value: %d\n", tos);
// Close the socket
close(sockfd);
return 0;
}
Explanation
- Creating a Socket:
- The
socket
function creates a TCP socket.
2. Setting Up Server Address:
- The
server_addr
structure is initialized with the server’s IP address and port.
3. Connecting to the Server:
- The
connect
function establishes a connection to the server.
4. Retrieving the TOS Value:
getsockopt
is used to retrieve the TOS value.- The arguments for
getsockopt
include the socket descriptor (sockfd
), the protocol level (IPPROTO_IP
), the option name (IP_TOS
), a pointer to store the TOS value (&tos
), and the size of the option value (&optlen
).
5. Printing the TOS Value:
- The retrieved TOS value is printed to the console.
6. Closing the Socket:
- The
close
function closes the socket to free resources.
Important Notes
- Error Handling: Always check for errors when calling socket functions and handle them appropriately.
- Permissions: Depending on the system configuration, retrieving or setting the TOS value may require elevated permissions.
- Compatibility: Ensure that your environment supports the required socket options and functions.
This example demonstrates how to retrieve the TOS value for a TCP socket in a C program, which can be useful for debugging or monitoring network traffic characteristics.