Question

Hi I have a simple browser that sends a request to yahoo.com which the server responds telling me 200 Ok and that it will send me a gzip file using encoded-transfer:chunked. Thats fine and all but when I run my program and continuously call recv(), I eventually run into a Bus Error. I’m not sure what that means at this point. Also I’m unclear how to read the header of the packet to tell me how many bytes it will send since the compressed file is in machine code. In this question I have included right bellow my code as well as the output that I see in terminal.

#include <stdio.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>
#include <arpa/inet.h>

#define MAXDATASIZE 500;

int main(int argc, char *argv[]){
struct addrinfo serverSide,*serverInfo;
int mySock, status;
char buf[501],ipstr[INET6_ADDRSTRLEN];

memset(&serverSide, 0, sizeof serverSide);
serverSide.ai_family = AF_UNSPEC;
serverSide.ai_socktype = SOCK_STREAM;

if(getaddrinfo("www.yahoo.com","80",&serverSide,&serverInfo)==0){

}

mySock = socket(serverInfo->ai_family, serverInfo->ai_socktype, serverInfo->ai_protocol);
connect(mySock, serverInfo->ai_addr, serverInfo->ai_addrlen);

char msg[500] = "GET http://www.yahoo.com HTTP/1.1\r\n";
strcat(msg,"Host: www.yahoo.com:80\r\n");
strcat(msg,"User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:7.0.1) Gecko/20100101 Firefox/7.0.1\r\n");
strcat(msg,"Accept: text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8\r\n");
strcat(msg,"Accept-Language: en-us,en;q=0.5\r\n");
strcat(msg,"Accept-Encoding: gzip, deflate\r\n");
strcat(msg,"Accept-Charset: ISO-8859-1,utf-8;q=0.7,*;q=0.7\r\n");
strcat(msg,"Connection: keep-alive\r\n\r\n");

// I want to keep the code simple so I just call recv enough times to see what has been
// written to my buffer. If I could read the packet length in the header then I would 
// code in a while loop to adjust for that.
if((status = send(mySock,msg,strlen(msg),0))== -1){
printf("request not sent %d\n",status);
perror("\n");
}else{
if((status = recv(mySock, buf, 1500, 0))== -1){
printf("recieved byte error");
}else{
printf("%s",buf);
}
if((status = recv(mySock, buf, 1500, 0))== -1){
printf("recieved byte error");
}else{
printf("%s",buf);
}
if((status = recv(mySock, buf, 1500, 0))== -1){
printf("recieved byte error");
}else{
printf("%s",buf);
}
if((status = recv(mySock, buf, 1500, 0))== -1){
printf("recieved byte error");
}else{
printf("%s",buf);
}
}
close(mySock);
freeaddrinfo(serverInfo);
return 0;
}

Bellow is the output of my code. The server responds with the following…

HTTP/1.1 200 OK Date: Sat, 21 Jan 2012 08:53:09 GMT Set-Cookie:
B=4peq6lh7hkv7l&b=3&s=u4; expires=Tue, 21-Jan-2014 20:00:00 GMT;
path=/; domain=.yahoo.com P3P:
policyref=”http://info.yahoo.com/w3c/p3p.xml”, CP=”CAO DSP COR CUR ADM
DEV TAI PSA PSD IVAi IVDi CONi TELo OTPi OUR DELi SAMi OTRi UNRi PUBi
IND PHY ONL UNI PUR FIN COM NAV INT DEM CNT STA POL HEA PRE LOC GOV”
Cache-Control: private X-Frame-Options: SAMEORIGIN Set-Cookie:
IU=deleted; expires=Fri, 21-Jan-2011 08:53:09 GMT; path=/;
domain=.yahoo.com Set-Cookie: PH=deleted; expires=Fri, 21-Jan-2011
08:53:09 GMT; path=/; domain=.yahoo.com Set-Cookie:
fpc=d=2BrdHmSMUw00.1uwnK1w8hHJcKnQt3UjRGxvUnBVIn0e6eAyRyd96eAPIN33Jne3IWoEE8r8eAk9xF0ExLsN5JJJmANZRlEBg8hpcDJ1GD7Gd50uZeP1H0_Wbf_mc.LJ45tDfhhwjR1BSedjT7AeGszK321i_gS34xKNuHlH2niKnP1lFG8y3aztEQsOkQHUu1w3zxk-&v=2;
expires=Sun, 20-Jan-2013 08:53:10 GMT; path=/; domain=www.yahoo.com
Set-Cookie: CH=deleted; expires=Fri, 21-Jan-2011 08:53:09 GMT; path=/;
domain=www.yahoo.com Set-Cookie:
CH=AgBPGnwQAA9AEAAA3RAAKY0QAAMPEAAYcxAAKN0QAD5XEAASNBAAJeoQACI8;
expires=Mon, 20-Feb-2012 08:53:09 GMT; path=/; domain=.yahoo.com
Set-Cookie:
fpt=d=nZFZlZHXecyEB3UWO0p6uOQtHkIdLjY734kqWOMHwg8Yq1e0sUpJBiMgYN63uaJ0.zEBsZIbeO93bfF0mXfQ8CtJYxAuet8CIS5PYmNMWfBDUU6ew8pXSI6cY3aX7Nk743wzRxbuCoBZGqvOM0eGhQMFOQ7BrCYBAZsAIYAOqwHKu7sNvbkPN7r0T7ncKpu5bX8LWPGngHHS97cCbNgpbgzOh13nP1m6.9cw7oPeudXdRjfzxsDUYqq2LvQJdECPWmccPhDEYfAoIl.Cfc8T8w_5.zrvoWG5kJr_T0megV4GtcTB324ZS2zkf0pi1GiMGYVHxGNh01c04XJnjk5q0ek_Xg�ogaa5oZoyhkMYQLI66nMCt1yAiz1jXmq2MTBxHqtbxBkoJaVsJt0YQMdpkkxwpRpXZHUgur.K9kY4j3vgyG1j44CQPlNsh7mBMxNe5nLdCMjFMy0ufmocRYowg38kMiK9hpFqkFnwYZSZPMGM4wAK5wVvFrwn1phkY3OXr76z5OC5tjNeq3Q–&v=1;
path=/; domain=www.yahoo.com Set-Cookie: fpps=deleted; expires=Fri,
21-Jan-2011 08:53:09 GMT; path=/; domain=www.yahoo.com Set-Cookie:
fpc_s=d=33OMFTGMUw2Z.oeMhzS8DGMizJ2NFSc4VT5QjW6pM.25xOqOYy5nr_kK5C83.tjkxzIiFvBITPc7P5YBehviyRS3piAAliJxvHQHDlbZYOlAdvgPXzJ4zGghf.xEL6Rb6rVdP90xlOc_njpBqlqOcwyMeq0ZqBy2VXuNrIxiD9QjUUsJyfVJ4miF1frSXQOI5Z8MVIErFoBjW3jTQANFu_CayJrjp6RRDc.YG5DCn04SFk3hKALdzVPSyIoawxmicCoWFfN7dvyge8jRoeQXHey.IALpAtcCPlY4eX.teLb.GO0yQxUN5HzgR9I-&v=2;
path=/; domain=www.yahoo.com Vary: Accept-Encoding Content-Type:
text/html;charset=utf-8 Content-Encoding: gzip Age: 2
Transfer-Encoding: chunked Connection: keep-alive Server: YTS/1.20.0

000088e ���I�HG8Ae��t�>#%xc�c#���a��e9��*d��S��@�~}�ZF��~x�
6�S8�#ӉO�i:i%��?���a��9rxJ�n���<Q����_�-�A����!D{�{ޱN���f*/h��ΰ ���Ea�h��E��2UY�
2�7�����dL>U���^W˳g��H�Q�>���~����iiΟ��@:���R�L�b��K���=�#
Ғ�5���|'�J��eo��{� Bus error

Answer 1

char buf[501];
...
recv(mySock, buf, 1500, 0)
                  ^^^^

If recv ever actually reads more than 501 bytes, you’ll overflow your buffer, which results in undefined behavior. Make sure you never read more than 501 bytes, or increase your buffer’s size.

Also the printfs are not safe. There is nothing that guarantees the input will be proper, 0-terminated C strings. (You could use write and pass the length recv returned.)