Sending a single unsigned char through a socket using UDP in C -

i trying send single unsigned char through buffer. using buffer of size 2

unsigned char temp_buf [2]; temp_buf [0]= (unsigned char) 0xff; temp_buf [1]= null; 

and sendto functions looks this:

if (sendto(fd, temp_buf, sizeof (temp_buf), 0, (struct sockaddr *)&remaddr, addrlen) < 0)         perror("sendto"); 

it compiles no issues, @ run time error:

sendto: invalid argument

which means there wrong buffer im using. suspected issue might because im using siezeof changed strlen(temp_buf) still no luck!

edit: trying make question simpler not including whole code here is, sorry that!

#include <stdlib.h> #include <stdio.h> #include <string.h> #include <netdb.h> #include <sys/socket.h> #include <arpa/inet.h> #include "port.h"  #define bufsize 2048  int main(int argc, char **argv) {     struct sockaddr_in myaddr;  /* our address */     struct sockaddr_in remaddr; /* remote address */     socklen_t addrlen = sizeof(remaddr);        /* length of addresses */     int recvlen;            /* # bytes received */     int fd;             /* our socket */     int msgcnt = 0;         /* count # of messages received */     unsigned char buf[bufsize]; /* receive buffer */       /* create udp socket */      if ((fd = socket(af_inet, sock_dgram, 0)) < 0) {         perror("cannot create socket\n");         return 0;     }      /* bind socket valid ip address , specific port */      memset((char *)&myaddr, 0, sizeof(myaddr));     myaddr.sin_family = af_inet;     myaddr.sin_addr.s_addr = htonl(inaddr_any);     myaddr.sin_port = htons(service_port);      if (bind(fd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {         perror("bind failed");         return 0;     }      /* loop, receiving data , printing received */            printf("waiting on port %d\n", service_port);          //recvfrom(fd, buf, bufsize, 0, (struct sockaddr *)&remaddr, &addrlen);          //buf [0] = 0xff;         unsigned char temp_buf [2];         temp_buf [0]= (unsigned char) 0xff;         temp_buf [1]= '\0';          if (sendto(fd, temp_buf, sizeof (temp_buf), 0, (struct sockaddr *)&remaddr, addrlen) < 0)             perror("sendto");         else             printf("%s \n", "communication established");    } 

the contents of remaddr uninitialized. in other words, you're not telling sendto send data.

you need populate struct ip , port wish send to.

if uncomment call recvfrom , subsequently packet other service, remaddr gets populated ip/port sent packet, can use send packet back. without that, need fill in remaddr.