Error from recvfrom - c

Question

I try to make a function that sends a icmp packet to another computer and when the other computer sends something back, the function returns a 1, else a 0. But the recvfrom function returns an error: "errno: Resource temporarily unavailable". I work on an Mac OS X, so I don't include headers from the linux kernel. Can anyone help me, cause I'm stuck.
#include "info.h"
char *getip()
{
char buffer[256];
struct hostent *h;
gethostname(buffer, 256);
h = gethostbyname(buffer);
return inet_ntoa(*(struct in_addr *)h->h_addr);
}
int host_alive(char *dst_addr, char *src_addr)
{
struct ip *ippacket;
struct ip *ip_reply;
struct icmp *icmppacket;
struct sockaddr_in connection;
struct timeval tv;
char *packet;
char *buffer;
int optval;
int addrlen;
int size;
int sock = 0;
packet = malloc(sizeof(struct ip) + sizeof(struct icmp));
buffer = malloc(sizeof(struct ip) + sizeof(struct icmp));
check(getuid() == 0, "Root priviliges are needed. Try: sudo ./bin/main");
ippacket = (struct ip *) packet;
icmppacket = (struct icmp *) (packet + sizeof(struct ip));
ippacket->ip_hl = 5;
ippacket->ip_v = 4;
ippacket->ip_tos = 0;
ippacket->ip_len = sizeof(struct ip) + sizeof(struct icmp);
ippacket->ip_id = htons(random());
ippacket->ip_ttl = 255;
ippacket->ip_p = IPPROTO_ICMP;
inet_aton(src_addr, &ippacket->ip_src);
inet_aton(dst_addr, &ippacket->ip_dst);
tv.tv_sec = 5;
tv.tv_usec = 0;
check((sock = socket(AF_INET, SOCK_RAW, IPPROTO_ICMP)) != -1,\
"Failed to create socket");
check(setsockopt(sock, IPPROTO_IP, IP_HDRINCL, &optval, sizeof(int)) != -1,\
"Failed to set the option to the socket.");
check(setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, (const char *)&tv, sizeof(struct timeval)) != -1,\
"Failed to set the option to the socket.");
icmppacket->icmp_type = ICMP_ECHO;
icmppacket->icmp_code = 0;
icmppacket->icmp_id = 0;
icmppacket->icmp_seq = 0;
icmppacket->icmp_cksum = in_cksum((unsigned short *)icmppacket, sizeof(struct icmp));
ippacket->ip_sum = in_cksum((unsigned short *)ippacket, sizeof(struct ip));
connection.sin_family = AF_INET;
connection.sin_addr.s_addr = inet_addr(dst_addr);
sendto(sock, packet, ippacket->ip_len, 0, (struct sockaddr *)&connection,\
sizeof(struct sockaddr));
addrlen = sizeof(connection);
check((size = recvfrom(sock, buffer, sizeof(struct ip) + sizeof(struct icmp), 0,\
(struct sockaddr *)&connection, (socklen_t *)&addrlen)) != -1,\
"Failed to receive a message.");
printf("Received %d byte reply from %s:\n", size , dst_addr);
ip_reply = (struct ip*) buffer;
printf("ID: %d\n", ntohs(ip_reply->ip_id));
printf("TTL: %d\n", ip_reply->ip_ttl);
close(sock);
free(packet);
free(buffer);
return 1;
error:
if (sock)
close(sock);
free(packet);
free(buffer);
return 0;
}
unsigned short in_cksum(unsigned short *addr, int len)
{
int sum = 0;
u_short answer = 0;
u_short *w = addr;
int nleft = len;
while (nleft > 1) {
sum += *w++;
nleft -= 2;
}
if (nleft == 1) {
*(u_char *) (&answer) = *(u_char *) w;
sum += answer;
}
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
answer = ~sum;
return (answer);
}

Answer 1

According to the documentation for recvfrom this is expected if you're using a non-blocking call:
If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see fcntl(2)) in which case the value -1 is returned and the external vari-
able errno set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the
socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt(2).
You can look up errno values in /usr/include/sys/errno.h if you're curious as to what values map to what errors.
If you want this function to block you may need to set the MSG_WAITALL flag which "requests that the operation block until the full request is satisfied".
Normally in low-level UNIX socket code you'd do a select on that socket to wait for a read signal, then call recvfrom to receive the data if and only if that signal triggered. You can also do a non-blocking receive and on an EAGAIN just wait a short period of time before trying again, though that is less efficient.