getenv() causes segfault in recv() loop

MK27 · 11-26-2008, 10:08 AM

I've written a recv loop function that looks like this:

Code:

int Recept (int sock, char *buffer) {
        int count=0;
        char *ptr=buffer;
        if ((buffer=malloc(1))==NULL) return -2;
        while (recv(sock,ptr,1,0)>0) {
                printf("bytes=%6d\t%c\n",count,ptr[0]);      // for debugging
                if ((ptr[0]=='\n') && (count > 0) && (buffer[count]=='\r')) {
                        buffer[count]='\0';
                        puts ("Recept() done.");
                        return count;} 
                ptr++; count++;
                buffer=realloc(buffer,count+1);
                if (buffer==NULL) return -3;
        }
        return -1; 
}

It reaches the end (but sometimes not) of a GET request (eg, "...</html>") and then segfaults before "Recept () done" appears -- apparently in getenv() from libc.so.6. I'm almost positive it worked fine yesturday, although I have changed the code which accounts for the connection on sock.

vart · 11-26-2008, 10:11 AM

your ptr pointer points into some address that has nothing to do with storage allocated with malloc/realloc... so you writing to some random address...

And I do not see a reason to pass buffer as a parameter - you change its value in the function anyway and the calling function does not know about it... - so it has no feed back anyway

MK27 · 11-26-2008, 04:25 PM

Quote:

Originally Posted by vart

your ptr pointer points into some address that has nothing to do with storage allocated with malloc/realloc... so you writing to some random address...

I was surprised by this but you're right. Anyway, I changed it to use a buffer with a fixed size:

Code:

int Recept (int sock, char *buffer, int size) {
        int count=0;
        char *ptr=buffer;
        while (recv(sock,ptr,1,0)>0) {
                if (ptr[0]=='\n') {
                        if (buffer[count-1]=='\r') buffer[count-1]='\0'; 
                        else buffer[count]='\0';
                        return count;}
                ptr++; count++;
                if (count>=size) return -2; 
        }
        perror("Recept() -- recv");
        return -1; 
}

and the strange thing that happens now is this: I use it looped in one function for the reply to a HEAD request. No problems. Then I send GET on the same item (a binary) so I want to skip thru the head:

Code:

for (i=o;i<10;i++)  {
      tmp=Recept(sock,buffer,4096);         // exactly the same line as in the other function
      printf("%d %s\n",tmp,buffer);                   // just to check
}

At every iteration including the first one, I get:

Recept() -- recv: Illegal seek
-1

I've googled around for why recv would return this error and no one seems to have sorted it out, but I still have my fingers crossed

It would seem that the recv here doesn't work at all, because the next thing is I want the binary and I have Content-Length, so I just do recv(sock,buffer,Content-Length,0), which does work except it includes the ten lines of the head I wanted to skip -- ie, Recept did nothing but fail.

brewbuck · 12-05-2008, 02:04 PM

Quote:

Originally Posted by MK27

Recept() -- recv: Illegal seek
-1

I've googled around for why recv would return this error and no one seems to have sorted it out, but I still have my fingers crossed

recv() returns no such error. You are improperly calling perror() when the call to recv() did not set an error code. recv() can return 0 if the socket is disconnected. Instead of treating this as a normal event you are treating it as an error and you are printing whatever garbage error code happens to be sitting in errno at the moment.

The error which sits in errno is only a valid error if the last call to a library function return -1.

11-26-2008, 10:08 AM	#1
MK27 critical genius Join Date: Jul 2008 Location: SE Queens Posts: 5,166	getenv() causes segfault in recv() loop I've written a recv loop function that looks like this: Code: int Recept (int sock, char buffer) { int count=0; char ptr=buffer; if ((buffer=malloc(1))==NULL) return -2; while (recv(sock,ptr,1,0)>0) { printf("bytes=%6d\t%c\n",count,ptr[0]); // for debugging if ((ptr[0]=='\n') && (count > 0) && (buffer[count]=='\r')) { buffer[count]='\0'; puts ("Recept() done."); return count;} ptr++; count++; buffer=realloc(buffer,count+1); if (buffer==NULL) return -3; } return -1; } It reaches the end (but sometimes not) of a GET request (eg, "...</html>") and then segfaults before "Recept () done" appears -- apparently in getenv() from libc.so.6. I'm almost positive it worked fine yesturday, although I have changed the code which accounts for the connection on sock. __________________ "A man can't just sit around." -- Larry Walters
MK27 is offline

11-26-2008, 10:11 AM	#2
vart CSharpener Join Date: Oct 2006 Posts: 5,319	your ptr pointer points into some address that has nothing to do with storage allocated with malloc/realloc... so you writing to some random address... And I do not see a reason to pass buffer as a parameter - you change its value in the function anyway and the calling function does not know about it... - so it has no feed back anyway __________________ If I have eight hours for cutting wood, I spend six sharpening my axe.
vart is online now

Thread Tools
Show Printable Version Email this Page
Display Modes
Linear Mode Switch to Hybrid Mode Switch to Threaded Mode

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Visual Studio Express / Windows SDK?	cyberfish	C++ Programming	23	01-22-2009 02:13 AM
Personal Program that is making me go wtf?	Submeg	C Programming	20	06-27-2006 12:13 AM
A somewhat bizzare problem!!! - WHILE LOOP	bobthebullet990	C Programming	3	03-31-2006 07:19 AM
when a while loop will stop ?	blue_gene	C Programming	13	04-20-2004 03:45 PM
QUERY_STRING in CGI with a C/C++ Program	stovellpaul	C++ Programming	5	10-03-2002 12:51 AM