Example code using libcap to set CAP_NET_BIND_SERVICE (binding to port 81) to the effective capability set.

captest.c

/* 
  Example code using libcap to set CAP_NET_BIND_SERVICE 
  (binding to port 81) to the effective capability set.

  For more info, see the full blog post at
   https://k3a.me/linux-capabilities-in-a-nutshell/
*/

#include <stdlib.h>
#include <stdio.h>
#include <sys/capability.h>

#include <linux/capability.h>

/* === from <linux/capability.h>:

#define _LINUX_CAPABILITY_VERSION_1  0x19980330
#define _LINUX_CAPABILITY_U32S_1     1

#define _LINUX_CAPABILITY_VERSION_2  0x20071026  // deprecated - use v3
#define _LINUX_CAPABILITY_U32S_2     2

#define _LINUX_CAPABILITY_VERSION_3  0x20080522
#define _LINUX_CAPABILITY_U32S_3     2

typedef struct __user_cap_header_struct {
	__u32 version;
	int pid;
} __user *cap_user_header_t;

typedef struct __user_cap_data_struct {
        __u32 effective;
        __u32 permitted;
        __u32 inheritable;
} __user *cap_user_data_t;
*/

/* === from libcap/libcap.h
struct _cap_struct {
    struct __user_cap_header_struct head;
    struct __user_cap_data_struct set;
};
*/

#define handle_error(msg)   \
    do                      \
    {                       \
        perror(msg);        \
        exit(EXIT_FAILURE); \
    } while (0)

void hex_dump(const void *data, size_t size)
{
    char ascii[17];
    size_t i, j;
    ascii[16] = '\0';
    for (i = 0; i < size; ++i)
    {
        printf("%02X ", ((unsigned char *)data)[i]);
        if (((unsigned char *)data)[i] >= ' ' && ((unsigned char *)data)[i] <= '~')
        {
            ascii[i % 16] = ((unsigned char *)data)[i];
        }
        else
        {
            ascii[i % 16] = '.';
        }
        if ((i + 1) % 8 == 0 || i + 1 == size)
        {
            printf(" ");
            if ((i + 1) % 16 == 0)
            {
                printf("|  %s \n", ascii);
            }
            else if (i + 1 == size)
            {
                ascii[(i + 1) % 16] = '\0';
                if ((i + 1) % 16 <= 8)
                {
                    printf(" ");
                }
                for (j = (i + 1) % 16; j < 16; ++j)
                {
                    printf("   ");
                }
                printf("|  %s \n", ascii);
            }
        }
    }
}

#include <strings.h>
#include <netdb.h>
#include <netinet/in.h>

static void test_bind_socket()
{
    int sockfd, newsockfd, portno = 81, clilen;
    char buffer[256];
    struct sockaddr_in serv_addr, cli_addr;
    int n;

    /* First call to socket() function */
    sockfd = socket(AF_INET, SOCK_STREAM, 0);

    if (sockfd < 0)
        handle_error("socket");

    /* Initialize socket structure */
    bzero((char *)&serv_addr, sizeof(serv_addr));

    serv_addr.sin_family = AF_INET;
    serv_addr.sin_addr.s_addr = INADDR_ANY;
    serv_addr.sin_port = htons(portno);

    /* Now bind the host address using bind() call.*/
    if (bind(sockfd, (struct sockaddr *)&serv_addr, sizeof(serv_addr)) < 0)
        handle_error("bind");
    else 
        printf("bind successful! :)\n");
}

static void dump_caps(cap_t caps)
{
    unsigned capsz = sizeof(struct __user_cap_header_struct);

    cap_user_header_t caphead = (cap_user_header_t)caps;

    if (caphead->version == _LINUX_CAPABILITY_VERSION_1)
        capsz += sizeof(struct __user_cap_data_struct);
    else
        capsz += 2 * sizeof(struct __user_cap_data_struct);

    hex_dump(caps, capsz);
}

int main(int argc, char *argv[])
{
    cap_t caps;
    char *txt_caps;

    // get caps currently set to the process
    caps = cap_get_proc();
    if (caps == NULL)
        handle_error("cap_get_proc");

    // dump caps as binary binary
    dump_caps(caps);

    // convert capability to text representation
    txt_caps = cap_to_text(caps, NULL);
    if (txt_caps == NULL)
        handle_error("cap_to_text");

    printf("Current process capabilities (+set): %s\n", txt_caps);

    // free the allocated capability text memory
    if (cap_free(txt_caps) != 0)
        handle_error("cap_free");

    // set NET_BIND_SERVICE to effective set
    // (requires NET_BIND_SERVICE already in the permitted set,
    // otherwise fails with EPERM. One of the ways to set permitted
    // set is by setting file capabilities:
    //   sudo setcap cap_net_bind_service+p ./captest
    // )
    cap_value_t cap_list[1];
    cap_list[0] = CAP_NET_BIND_SERVICE;
    if (cap_set_flag(caps, CAP_EFFECTIVE, 1, cap_list, CAP_SET) == -1)
        handle_error("cap_set_flag");

    if (cap_set_proc(caps) != 0)
        handle_error("cap_set_proc");

    // try to bind privileged port 81
    test_bind_socket();

    // free the allocated caps
    if (cap_free(caps) == -1)
        handle_error("cap_free");

    exit(EXIT_SUCCESS);
}

how it build?

gcc -lcap captest.c -o captest

See the full article https://k3a.me/linux-capabilities-in-a-nutshell/

Thanks