Invalid argument from sendto in ansi C when trying to implement UDP

Question

I know there are a few similar question on SO, but I feel like everyone has a different issue that is causing this. So I'm posting my particular case.

I'm working on the raspberry pi, using raspbian, a debian derivative. I'm coding in ansi C. This is a server that sends some data to a udp client on another end through port 500.

#include <sys/socket.h>
#include <netinet/in.h>
#include <stdio.h>
int main()
{
   int sockfd;
   struct sockaddr_in servaddr,cliaddr;
   socklen_t clilen;
   char buffer[256];

   sockfd=socket(AF_INET,SOCK_DGRAM,0);

   bzero(&servaddr,sizeof(servaddr));
   servaddr.sin_family = AF_INET;
   servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
   servaddr.sin_port=htons(500);
   bind(sockfd,(struct sockaddr *)&servaddr,sizeof(servaddr));
   sprintf(buffer,"hi this is a test \n");
   if(sendto(sockfd,buffer,sizeof(buffer),0,(struct sockaddr *)&cliaddr,sizeof(cliaddr))==-1)
   { error("fail:")
    };

   return 0;
}

But this just gives me the error "invalid arguments", I can't figure out what could possibly be wrong. If the type of argument I supply to sendto is incorrect, this would fail on compile. But the type is correct and this compiles, only at runtime does it fail.

It's not related to the type of the arguments. `sockfd` for example can be any integer even if that integer doesn't refer to a socket. Thus, your code will compile but you'll get an error at runtime. — Mihai Maruseac, Sep 18 '13 at 17:24

Mihai Maruseac · Answer 1 · 2013-09-18T17:34:54.790

2

You have no initialization for the cliaddr structure. Thus, its contents are random. Your code compiles fine because you have a struct sockaddr_in where it should be but the compiler cannot check if you have properly initialized it.

On the other hand, at runtime when sendto is invoked the glibc or the kernel finds out that that random data in the cliaddr structure doesn't look like a valid address and throws up an invalid argument error.

Solution: initialize the cliaddr variable before using it.

Edit: You also need to properly initialize the socket, the third argument there shouldn't be 0.

Second edit, read me: For resources I recommend the extraordinary guide of Beej. For your exact problem this subchapter can help.

edited Sep 18 '13 at 17:34

answered Sep 18 '13 at 17:28

Mihai Maruseac

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how do I initialize the cliaddr struct? I've been trying to find a good sample code and it seems the current one has completely led me astray. – mugetsu Sep 18 '13 at 17:33
See the links in the second edit. It is a good tutorial and you even have examples there. – Mihai Maruseac Sep 18 '13 at 17:36

Invalid argument from sendto in ansi C when trying to implement UDP

1 Answers1