getservent_r - Linux


getservent_r is a Linux function used to retrieve information about Internet services. It iterates through the /etc/services file to return the service name, port number, and protocol.


int getservent_r(struct servent *result_buf, char *buf, int buflen, struct servent **result);


| Option | Description |
| result_buf | Pointer to a buffer for storing the service information |
| buf | Pointer to a buffer for storing the temporary data |
| buflen | Length of the buf buffer |
| result | Pointer to the result_buf after a successful call |


Example 1: Getting the port number of the "http" service:

#include <netdb.h>

int main() {
    struct servent *result;
    if (getservent_r("http", NULL, 0, &result) == 0) {
        // Success, port number stored in result->s_port
    } else {
        // Error handling
    endservent();  // Release memory allocated by getservent_r
    return 0;

Example 2: Iterating through all services:

#include <netdb.h>

int main() {
    struct servent *result;
    while ( (getservent_r(NULL, NULL, 0, &result)) == 0) {
        printf("Service: %s, Port: %d\n", result->s_name, result->s_port);
    return 0;

Common Issues

  • Invalid service name: Check that the service name is spelled correctly and exists in the /etc/services file.
  • Buffer too small: Make sure the buflen and buf are large enough to hold the service information.
  • Memory leak: Don’t forget to call endservent after using getservent_r to release allocated memory.


getservent_r is commonly used in network programming to resolve service names to port numbers and protocols. It can be integrated with:

  • socket to create sockets and specify service port numbers
  • bind to bind a socket to a specific service port
  • listen to listen for incoming connections on a service port

Related Commands

  • getservbyport: Retrieve service information by port number
  • getservbyname: Retrieve service information by service name
  • /etc/services: The system file containing the service database