source: rtems-docs/networking/using_networking_rtems_app.rst @ d389819

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Using Networking in an RTEMS Application

Makefile changes

Including the required managers

The FreeBSD networking code requires several RTEMS managers in the application: .. code:: c

MANAGERS = io event semaphore

Increasing the size of the heap

The networking tasks allocate a lot of memory. For most applications the heap should be at least 256 kbytes. The amount of memory set aside for the heap can be adjusted by setting the CFLAGS_LD definition as shown below: .. code:: c

CFLAGS_LD += -Wl,--defsym -Wl,HeapSize=0x80000

This sets aside 512 kbytes of memory for the heap.

System Configuration

The networking tasks allocate some RTEMS objects. These must be accounted for in the application configuration table. The following lists the requirements.

TASKS
One network task plus a receive and transmit task for each device.
SEMAPHORES
One network semaphore plus one syslog mutex semaphore if the application uses openlog/syslog.
EVENTS
The network stack uses RTEMS_EVENT_24 and RTEMS_EVENT_25. This has no effect on the application configuration, but application tasks which call the network functions should not use these events for other purposes.

Initialization

Additional include files

The source file which declares the network configuration structures and calls the network initialization function must include .. code:: c

#include <rtems/rtems_bsdnet.h>

Network Configuration

The network configuration is specified by declaring and initializing the rtems_bsdnet_config structure. .. code:: c

struct rtems_bsdnet_config { /* * This entry points to the head of the ifconfig chain. */ struct rtems_bsdnet_ifconfig *ifconfig; /* * This entry should be rtems_bsdnet_do_bootp if BOOTP * is being used to configure the network, and NULL * if BOOTP is not being used. */ void (*bootp)(void); /* * The remaining items can be initialized to 0, in * which case the default value will be used. */ rtems_task_priority network_task_priority; /* 100 */ unsigned long mbuf_bytecount; /* 64 kbytes */ unsigned long mbuf_cluster_bytecount; /* 128 kbytes */ char *hostname; /* BOOTP */ char *domainname; /* BOOTP */ char *gateway; /* BOOTP */ char *log_host; /* BOOTP */ char *name_server[3]; /* BOOTP */ char *ntp_server[3]; /* BOOTP */ unsigned long sb_efficiency; /* 2 */ /* UDP TX: 9216 bytes */ unsigned long udp_tx_buf_size; /* UDP RX: 40 * (1024 + sizeof(struct sockaddr_in)) */ unsigned long udp_rx_buf_size; /* TCP TX: 16 * 1024 bytes */ unsigned long tcp_tx_buf_size; /* TCP TX: 16 * 1024 bytes */ unsigned long tcp_rx_buf_size; /* Default Network Tasks CPU Affinity */ #ifdef RTEMS_SMP const cpu_set_t *network_task_cpuset; size_t network_task_cpuset_size; #endif };

The structure entries are described in the following table. If your application uses BOOTP/DHCP to obtain network configuration information and if you are happy with the default values described below, you need to provide only the first two entries in this structure.

struct rtems_bsdnet_ifconfig \*ifconfig

A pointer to the first configuration structure of the first network device. This structure is described in the following section. You must provide a value for this entry since there is no default value for it.

void (\*bootp)(void)

This entry should be set to rtems_bsdnet_do_bootp if your application by default uses the BOOTP/DHCP client protocol to obtain network configuration information. It should be set to NULL if your application does not use BOOTP/DHCP. You can also use rtems_bsdnet_do_bootp_rootfs to have a set of standard files created with the information return by the BOOTP/DHCP protocol. The IP address is added to :file:`/etc/hosts` with the host name and domain returned. If no host name or domain is returned``me.mydomain`` is used. The BOOTP/DHCP server's address is also added to :file:`/etc/hosts`. The domain name server listed in the BOOTP/DHCP information are added to :file:`/etc/resolv.conf`. A``search`` record is also added if a domain is returned. The files are created if they do not exist. The default rtems_bsdnet_do_bootp and``rtems_bsdnet_do_bootp_rootfs`` handlers will loop for-ever waiting for a BOOTP/DHCP server to respond. If an error is detected such as not valid interface or valid hardware address the target will reboot allowing any hardware reset to correct itself. You can provide your own custom handler which allows you to perform an initialization that meets your specific system requirements. For example you could try BOOTP/DHCP then enter a configuration tool if no server is found allowing the user to switch to a static configuration.

???
int network_task_priority
The priority at which the network task and network device receive and transmit tasks will run. If a value of 0 is specified the tasks will run at priority 100.
unsigned long mbuf_bytecount
The number of bytes to allocate from the heap for use as mbufs. If a value of 0 is specified, 64 kbytes will be allocated.
unsigned long mbuf_cluster_bytecount
The number of bytes to allocate from the heap for use as mbuf clusters. If a value of 0 is specified, 128 kbytes will be allocated.
char \*hostname
The host name of the system. If this, or any of the following, entries are NULL the value may be obtained from a BOOTP/DHCP server.
char \*domainname
The name of the Internet domain to which the system belongs.
char \*gateway
The Internet host number of the network gateway machine, specified in 'dotted decimal' (129.128.4.1) form.
char \*log_host
The Internet host number of the machine to which syslog messages will be sent.
char \*name_server[3]
The Internet host numbers of up to three machines to be used as Internet Domain Name Servers.
char \*ntp_server[3]
The Internet host numbers of up to three machines to be used as Network Time Protocol (NTP) Servers.
unsigned long sb_efficiency
This is the first of five configuration parameters related to the amount of memory each socket may consume for buffers. The TCP/IP stack reserves buffers (e.g. mbufs) for each open socket. The TCP/IP stack has different limits for the transmit and receive buffers associated with each TCP and UDP socket. By tuning these parameters, the application developer can make trade-offs between memory consumption and performance. The default parameters favor performance over memory consumption. Seehttp://www.rtems.org/ml/rtems-users/2004/february/msg00200.html for more details but note that after the RTEMS 4.8 release series, the sb_efficiency default was changed from 8 to 2. The user should also be aware of the SO_SNDBUF and SO_RCVBUF IO control operations. These can be used to specify the send and receive buffer sizes for a specific socket. There is no standard IO control to change the sb_efficiency factor. The sb_efficiency parameter is a buffering factor used in the implementation of the TCP/IP stack. The default is 2 which indicates double buffering. When allocating memory for each socket, this number is multiplied by the buffer sizes for that socket.

unsigned long udp_tx_buf_size

This configuration parameter specifies the maximum amount of buffer memory which may be used for UDP sockets to transmit with. The default size is 9216 bytes which corresponds to the maximum datagram size.

unsigned long udp_rx_buf_size

This configuration parameter specifies the maximum amount of buffer memory which may be used for UDP sockets to receive into. The default size is the following length in bytes:

40 * (1024 + sizeof(struct sockaddr_in)

unsigned long tcp_tx_buf_size

This configuration parameter specifies the maximum amount of buffer memory which may be used for TCP sockets to transmit with. The default size is sixteen kilobytes.

unsigned long tcp_rx_buf_size

This configuration parameter specifies the maximum amount of buffer memory which may be used for TCP sockets to receive into. The default size is sixteen kilobytes.

const cpu_set_t \*network_task_cpuset

This configuration parameter specifies the CPU affinity of the network task. If set to 0 the network task can be scheduled on any CPU. Only available in SMP configurations.

size_t network_task_cpuset_size

This configuration parameter specifies the size of the``network_task_cpuset`` used. Only available in SMP configurations.

In addition, the following fields in the rtems_bsdnet_ifconfig are of interest.

int port
The I/O port number (ex: 0x240) on which the external Ethernet can be accessed.
int irno
The interrupt number of the external Ethernet controller.
int bpar
The address of the shared memory on the external Ethernet controller.

Network device configuration

Network devices are specified and configured by declaring and initializing a``struct rtems_bsdnet_ifconfig`` structure for each network device.

The structure entries are described in the following table. An application which uses a single network interface, gets network configuration information from a BOOTP/DHCP server, and uses the default values for all driver parameters needs to initialize only the first two entries in the structure.

char \*name
The full name of the network device. This name consists of the driver name and the unit number (e.g. "scc1"). The bsp.h include file usually defines RTEMS_BSP_NETWORK_DRIVER_NAME as the name of the primary (or only) network driver.
int (\*attach)(struct rtems_bsdnet_ifconfig \*conf)
The address of the driver attach function. The network initialization function calls this function to configure the driver and attach it to the network stack. The bsp.h include file usually defines RTEMS_BSP_NETWORK_DRIVER_ATTACH as the name of the attach function of the primary (or only) network driver.
struct rtems_bsdnet_ifconfig \*next
A pointer to the network device configuration structure for the next network interface, or NULL if this is the configuration structure of the last network interface.
char \*ip_address
The Internet address of the device, specified in 'dotted decimal' (129.128.4.2) form, or NULL if the device configuration information is being obtained from a BOOTP/DHCP server.
char \*ip_netmask
The Internet inetwork mask of the device, specified in 'dotted decimal' (255.255.255.0) form, or NULL if the device configuration information is being obtained from a BOOTP/DHCP server.
void \*hardware_address
The hardware address of the device, or NULL if the driver is to obtain the hardware address in some other way (usually by reading it from the device or from the bootstrap ROM).
int ignore_broadcast
Zero if the device is to accept broadcast packets, non-zero if the device is to ignore broadcast packets.
int mtu
The maximum transmission unit of the device, or zero if the driver is to choose a default value (typically 1500 for Ethernet devices).
int rbuf_count
The number of receive buffers to use, or zero if the driver is to choose a default value
int xbuf_count
The number of transmit buffers to use, or zero if the driver is to choose a default value Keep in mind that some network devices may use 4 or more transmit descriptors for a single transmit buffer.

A complete network configuration specification can be as simple as the one shown in the following example. This configuration uses a single network interface, gets network configuration information from a BOOTP/DHCP server, and uses the default values for all driver parameters. .. code:: c

static struct rtems_bsdnet_ifconfig netdriver_config = { RTEMS_BSP_NETWORK_DRIVER_NAME, RTEMS_BSP_NETWORK_DRIVER_ATTACH }; struct rtems_bsdnet_config rtems_bsdnet_config = { &netdriver_config, rtems_bsdnet_do_bootp, };

Network initialization

The networking tasks must be started before any network I/O operations can be performed. This is done by calling:

rtems_bsdnet_initialize_network ();

This function is declared in rtems/rtems_bsdnet.h. t returns 0 on success and -1 on failure with an error code in errno. It is not possible to undo the effects of a partial initialization, though, so the function can be called only once irregardless of the return code. Consequently, if the condition for the failure can be corrected, the system must be reset to permit another network initialization attempt.

Application Programming Interface

The RTEMS network package provides almost a complete set of BSD network services. The network functions work like their BSD counterparts with the following exceptions:

  • A given socket can be read or written by only one task at a time.

  • The select function only works for file descriptors associated with sockets.

  • You must call openlog before calling any of the syslog functions.

  • Some of the network functions are not thread-safe. For example the following functions return a pointer to a static buffer which remains valid only until the next call:

    gethostbyaddr

    gethostbyname

    inet_ntoa

    (inet_ntop is thread-safe, though).

  • The RTEMS network package gathers statistics.

  • Addition of a mechanism to "tap onto" an interface and monitor every packet received and transmitted.

  • Addition of SO_SNDWAKEUP and SO_RCVWAKEUP socket options.

Some of the new features are discussed in more detail in the following sections.

Network Statistics

There are a number of functions to print statistics gathered by the network stack. These function are declared in rtems/rtems_bsdnet.h.

rtems_bsdnet_show_if_stats
Display statistics gathered by network interfaces.
rtems_bsdnet_show_ip_stats
Display IP packet statistics.
rtems_bsdnet_show_icmp_stats
Display ICMP packet statistics.
rtems_bsdnet_show_tcp_stats
Display TCP packet statistics.
rtems_bsdnet_show_udp_stats
Display UDP packet statistics.
rtems_bsdnet_show_mbuf_stats
Display mbuf statistics.
rtems_bsdnet_show_inet_routes
Display the routing table.

Tapping Into an Interface

RTEMS add two new ioctls to the BSD networking code: SIOCSIFTAP and SIOCGIFTAP. These may be used to set and get a*tap function*. The tap function will be called for every Ethernet packet received by the interface.

These are called like other interface ioctls, such as SIOCSIFADDR. When setting the tap function with SIOCSIFTAP, set the ifr_tap field of the ifreq struct to the tap function. When retrieving the tap function with SIOCGIFTAP, the current tap function will be returned in the ifr_tap field. To stop tapping packets, call SIOCSIFTAP with a ifr_tap field of 0.

The tap function is called like this: .. code:: c

int tap (struct ifnet *, struct ether_header *, struct mbuf *)

The tap function should return 1 if the packet was fully handled, in which case the caller will simply discard the mbuf. The tap function should return 0 if the packet should be passed up to the higher networking layers.

The tap function is called with the network semaphore locked. It must not make any calls on the application levels of the networking level itself. It is safe to call other non-networking RTEMS functions.

Socket Options

RTEMS adds two new SOL_SOCKET level options for setsockopt and``getsockopt``: SO_SNDWAKEUP and SO_RCVWAKEUP. For both, the option value should point to a sockwakeup structure. The sockwakeup structure has the following fields: .. code:: c

void (*sw_pfn) (struct socket *, caddr_t); caddr_t sw_arg;

These options are used to set a callback function to be called when, for example, there is data available from the socket (SO_RCVWAKEUP) and when there is space available to accept data written to the socket (SO_SNDWAKEUP).

If setsockopt is called with the SO_RCVWAKEUP option, and the``sw_pfn`` field is not zero, then when there is data available to be read from the socket, the function pointed to by the sw_pfn field will be called. A pointer to the socket structure will be passed as the first argument to the function. The sw_arg field set by the``SO_RCVWAKEUP`` call will be passed as the second argument to the function.

If setsockopt is called with the SO_SNDWAKEUP function, and the sw_pfn field is not zero, then when there is space available to accept data written to the socket, the function pointed to by the sw_pfn field will be called. The arguments passed to the function will be as with``SO_SNDWAKEUP``.

When the function is called, the network semaphore will be locked and the callback function runs in the context of the networking task. The function must be careful not to call any networking functions. It is OK to call an RTEMS function; for example, it is OK to send an RTEMS event.

The purpose of these callback functions is to permit a more efficient alternative to the select call when dealing with a large number of sockets.

The callbacks are called by the same criteria that the select function uses for indicating "ready" sockets. In Stevens Unix Network Programming on page 153-154 in the section "Under what Conditions Is a Descriptor Ready?" you will find the definitive list of conditions for readable and writable that also determine when the functions are called.

When the number of received bytes equals or exceeds the socket receive buffer "low water mark" (default 1 byte) you get a readable callback. If there are 100 bytes in the receive buffer and you only read 1, you will not immediately get another callback. However, you will get another callback after you read the remaining 99 bytes and at least 1 more byte arrives. Using a non-blocking socket you should probably read until it produces error EWOULDBLOCK and then allow the readable callback to tell you when more data has arrived. (Condition 1.a.)

For sending, when the socket is connected and the free space becomes at or above the "low water mark" for the send buffer (default 4096 bytes) you will receive a writable callback. You don't get continuous callbacks if you don't write anything. Using a non-blocking write socket, you can then call write until it returns a value less than the amount of data requested to be sent or it produces error EWOULDBLOCK (indicating buffer full and no longer writable). When this happens you can try the write again, but it is often better to go do other things and let the writable callback tell you when space is available to send again. You only get a writable callback when the free space transitions to above the "low water mark" and not every time you write to a non-full send buffer. (Condition 2.a.)

The remaining conditions enumerated by Stevens handle the fact that sockets become readable and/or writable when connects, disconnects and errors occur, not just when data is received or sent. For example, when a server "listening" socket becomes readable it indicates that a client has connected and accept can be called without blocking, not that network data was received (Condition 1.c).

Adding an IP Alias

The following code snippet adds an IP alias: .. code:: c

void addAlias(const char *pName, const char *pAddr, const char *pMask) { struct ifaliasreq aliasreq; struct sockaddr_in *in; /* initialize alias request */ memset(&aliasreq, 0, sizeof(aliasreq)); sprintf(aliasreq.ifra_name, pName); /* initialize alias address */ in = (struct sockaddr_in *)&aliasreq.ifra_addr; in->sin_family = AF_INET; in->sin_len = sizeof(aliasreq.ifra_addr); in->sin_addr.s_addr = inet_addr(pAddr); /* initialize alias mask */ in = (struct sockaddr_in *)&aliasreq.ifra_mask; in->sin_family = AF_INET; in->sin_len = sizeof(aliasreq.ifra_mask); in->sin_addr.s_addr = inet_addr(pMask); /* call to setup the alias */ rtems_bsdnet_ifconfig(pName, SIOCAIFADDR, &aliasreq); }

Thanks to Mike Seirs for this example code.

Adding a Default Route

The function provided in this section is functionally equivalent to the command route add default gw yyy.yyy.yyy.yyy: .. code:: c

void mon_ifconfig(int argc, char *argv[], unsigned32 command_arg, bool verbose) { struct sockaddr_in ipaddr; struct sockaddr_in dstaddr; struct sockaddr_in netmask; struct sockaddr_in broadcast; char *iface; int f_ip = 0; int f_ptp = 0; int f_netmask = 0; int f_up = 0; int f_down = 0; int f_bcast = 0; int cur_idx; int rc; int flags; bzero((void*) &ipaddr, sizeof(ipaddr)); bzero((void*) &dstaddr, sizeof(dstaddr)); bzero((void*) &netmask, sizeof(netmask)); bzero((void*) &broadcast, sizeof(broadcast)); ipaddr.sin_len = sizeof(ipaddr); ipaddr.sin_family = AF_INET; dstaddr.sin_len = sizeof(dstaddr); dstaddr.sin_family = AF_INET; netmask.sin_len = sizeof(netmask); netmask.sin_family = AF_INET; broadcast.sin_len = sizeof(broadcast); broadcast.sin_family = AF_INET; cur_idx = 0; if (argc <= 1) { /* display all interfaces */ iface = NULL; cur_idx += 1; } else { iface = argv[1]; if (isdigit(*argv[2])) { if (inet_pton(AF_INET, argv[2], &ipaddr.sin_addr) < 0) { printf("bad ip address: %s\n", argv[2]); return; } f_ip = 1; cur_idx += 3; } else { cur_idx += 2; } } if ((f_down !=0) && (f_ip != 0)) { f_up = 1; } while(argc > cur_idx) { if (strcmp(argv[cur_idx], "up") == 0) { f_up = 1; if (f_down != 0) { printf("Can't make interface up and down\n"); } } else if(strcmp(argv[cur_idx], "down") == 0) { f_down = 1; if (f_up != 0) { printf("Can't make interface up and down\n"); } } else if(strcmp(argv[cur_idx], "netmask") == 0) { if ((cur_idx + 1) >= argc) { printf("No netmask address\n"); return; } if (inet_pton(AF_INET, argv[cur_idx+1], &netmask.sin_addr) < 0) { printf("bad netmask: %s\n", argv[cur_idx]); return; } f_netmask = 1; cur_idx += 1; } else if(strcmp(argv[cur_idx], "broadcast") == 0) { if ((cur_idx + 1) >= argc) { printf("No broadcast address\n"); return; } if (inet_pton(AF_INET, argv[cur_idx+1], &broadcast.sin_addr) < 0) { printf("bad broadcast: %s\n", argv[cur_idx]); return; } f_bcast = 1; cur_idx += 1; } else if(strcmp(argv[cur_idx], "pointopoint") == 0) { if ((cur_idx + 1) >= argc) { printf("No pointopoint address\n"); return; } if (inet_pton(AF_INET, argv[cur_idx+1], &dstaddr.sin_addr) < 0) { printf("bad pointopoint: %s\n", argv[cur_idx]); return; } f_ptp = 1; cur_idx += 1; } else { printf("Bad parameter: %s\n", argv[cur_idx]); return; } cur_idx += 1; } printf("ifconfig "); if (iface != NULL) { printf("%s ", iface); if (f_ip != 0) { char str[256]; inet_ntop(AF_INET, &ipaddr.sin_addr, str, 256); printf("%s ", str); } if (f_netmask != 0) { char str[256]; inet_ntop(AF_INET, &netmask.sin_addr, str, 256); printf("netmask %s ", str); } if (f_bcast != 0) { char str[256]; inet_ntop(AF_INET, &broadcast.sin_addr, str, 256); printf("broadcast %s ", str); } if (f_ptp != 0) { char str[256]; inet_ntop(AF_INET, &dstaddr.sin_addr, str, 256); printf("pointopoint %s ", str); } if (f_up != 0) { printf("up\n"); } else if (f_down != 0) { printf("down\n"); } else { printf("\n"); } } if ((iface == NULL) || ((f_ip == 0) && (f_down == 0) && (f_up == 0))) { rtems_bsdnet_show_if_stats(); return; } flags = 0; if (f_netmask) { rc = rtems_bsdnet_ifconfig(iface, SIOCSIFNETMASK, &netmask); if (rc < 0) { printf("Could not set netmask: %s\n", strerror(errno)); return; } } if (f_bcast) { rc = rtems_bsdnet_ifconfig(iface, SIOCSIFBRDADDR, &broadcast); if (rc < 0) { printf("Could not set broadcast: %s\n", strerror(errno)); return; } } if (f_ptp) { rc = rtems_bsdnet_ifconfig(iface, SIOCSIFDSTADDR, &dstaddr); if (rc < 0) { printf("Could not set destination address: %s\n", strerror(errno)); return; } flags |= IFF_POINTOPOINT; } /* This must come _after_ setting the netmask, broadcast addresses */ if (f_ip) { rc = rtems_bsdnet_ifconfig(iface, SIOCSIFADDR, &ipaddr); if (rc < 0) { printf("Could not set IP address: %s\n", strerror(errno)); return; } } if (f_up != 0) { flags |= IFF_UP; } if (f_down != 0) { printf("Warning: taking interfaces down is not supported\n"); } rc = rtems_bsdnet_ifconfig(iface, SIOCSIFFLAGS, &flags); if (rc < 0) { printf("Could not set interface flags: %s\n", strerror(errno)); return; } } void mon_route(int argc, char *argv[], unsigned32 command_arg, bool verbose) { int cmd; struct sockaddr_in dst; struct sockaddr_in gw; struct sockaddr_in netmask; int f_host; int f_gw = 0; int cur_idx; int flags; int rc; memset(&dst, 0, sizeof(dst)); memset(&gw, 0, sizeof(gw)); memset(&netmask, 0, sizeof(netmask)); dst.sin_len = sizeof(dst); dst.sin_family = AF_INET; dst.sin_addr.s_addr = inet_addr("0.0.0.0"); gw.sin_len = sizeof(gw); gw.sin_family = AF_INET; gw.sin_addr.s_addr = inet_addr("0.0.0.0"); netmask.sin_len = sizeof(netmask); netmask.sin_family = AF_INET; netmask.sin_addr.s_addr = inet_addr("255.255.255.0"); if (argc < 2) { rtems_bsdnet_show_inet_routes(); return; } if (strcmp(argv[1], "add") == 0) { cmd = RTM_ADD; } else if (strcmp(argv[1], "del") == 0) { cmd = RTM_DELETE; } else { printf("invalid command: %s\n", argv[1]); printf("\tit should be 'add' or 'del'\n"); return; } if (argc < 3) { printf("not enough arguments\n"); return; } if (strcmp(argv[2], "-host") == 0) { f_host = 1; } else if (strcmp(argv[2], "-net") == 0) { f_host = 0; } else { printf("Invalid type: %s\n", argv[1]); printf("\tit should be '-host' or '-net'\n"); return; } if (argc < 4) { printf("not enough arguments\n"); return; } inet_pton(AF_INET, argv[3], &dst.sin_addr); cur_idx = 4; while(cur_idx < argc) { if (strcmp(argv[cur_idx], "gw") == 0) { if ((cur_idx +1) >= argc) { printf("no gateway address\n"); return; } f_gw = 1; inet_pton(AF_INET, argv[cur_idx + 1], &gw.sin_addr); cur_idx += 1; } else if(strcmp(argv[cur_idx], "netmask") == 0) { if ((cur_idx +1) >= argc) { printf("no netmask address\n"); return; } f_gw = 1; inet_pton(AF_INET, argv[cur_idx + 1], &netmask.sin_addr); cur_idx += 1; } else { printf("Unknown argument\n"); return; } cur_idx += 1; } flags = RTF_STATIC; if (f_gw != 0) { flags |= RTF_GATEWAY; } if (f_host != 0) { flags |= RTF_HOST; } rc = rtems_bsdnet_rtrequest(cmd, &dst, &gw, &netmask, flags, NULL); if (rc < 0) { printf("Error adding route\n"); } }

Thanks to Jay Monkman for this example code.

Time Synchronization Using NTP

int rtems_bsdnet_synchronize_ntp (int interval, rtems_task_priority priority);

If the interval argument is 0 the routine synchronizes the RTEMS time-of-day clock with the first NTP server in the rtems_bsdnet_ntpserve array and returns. The priority argument is ignored.

If the interval argument is greater than 0, the routine also starts an RTEMS task at the specified priority and polls the NTP server every 'interval' seconds. NOTE: This mode of operation has not yet been implemented.

On successful synchronization of the RTEMS time-of-day clock the routine returns 0. If an error occurs a message is printed and the routine returns -1 with an error code in errno. There is no timeout - if there is no response from an NTP server the routine will wait forever.

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