CODEWITHSUNDEEP
ctcp
Martha Pears
Martha Pears

Reputation: 149

Getting uninitialized value error in TCP code

I have a TCP code, but it keeps giving me error in the server code.

When I run it using valgrind, I get the following in my server terminal:

    ==12370== Use of uninitialised value of size 8
==12370==    at 0x4E6B506: ____strtol_l_internal (in /lib64/libc-2.18.so)
==12370==    by 0x4E687DF: atoi (in /lib64/libc-2.18.so)
==12370==    by 0x400F1D: main (in /mnt/castor/seas_home/x/xyz/TCP/serv)
==12370== 
==12370== Invalid read of size 1
==12370==    at 0x4E6B506: ____strtol_l_internal (in /lib64/libc-2.18.so)
==12370==    by 0x4E687DF: atoi (in /lib64/libc-2.18.so)
==12370==    by 0x400F1D: main (in /mnt/castor/seas_home/a/xyz/TCP/serv)
==12370==  Address 0xffdfffac00000 is not stack'd, malloc'd or (recently) free'd
==12370== 
==12370== 
==12370== Process terminating with default action of signal 11 (SIGSEGV)
==12370==  General Protection Fault
==12370==    at 0x4E6B506: ____strtol_l_internal (in /lib64/libc-2.18.so)
==12370==    by 0x4E687DF: atoi (in /lib64/libc-2.18.so)
==12370==    by 0x400F1D: main (in /mnt/castor/seas_home/a/xyz/TCP/serv)
==12370== 
==12370== HEAP SUMMARY:
==12370==     in use at exit: 0 bytes in 0 blocks
==12370==   total heap usage: 0 allocs, 0 frees, 0 bytes allocated
==12370== 
==12370== All heap blocks were freed -- no leaks are possible
==12370== 
==12370== For counts of detected and suppressed errors, rerun with: -v
==12370== Use --track-origins=yes to see where uninitialised values come from
==12370== ERROR SUMMARY: 2 errors from 2 contexts (suppressed: 2 from 2)

But I can't figure out how to fix it. Could anyone tell me how to fix this?

Upvotes: 0

Views: 161

Answers (2)

user3629249
user3629249

Reputation: 16540

there are several basic problems with the server code.
here is just one that needs correction.

the server code is using the 'listen'/'accept' socket 
to write to the client.  
However, the accept function returns a NEW socket
That new socket is the one to use to communicate with the client.

here is an excerpt from the man page about the accept() function.

   The  accept()  system  call  is used with connection-based socket types
   (SOCK_STREAM,  SOCK_SEQPACKET).   It  extracts  the  first   connection
   request  on  the queue of pending connections for the listening socket,
   sockfd, creates a new connected socket, and returns a new file descrip‐
   tor  referring  to that socket.  The newly created socket is not in the
   listening state.  The original socket  sockfd  is  unaffected  by  this
   call.

   The  argument  sockfd is a socket that has been created with socket(2),
   bound to a local address with bind(2), and is listening for connections
   after a listen(2).

the key information is:

"creates a new connected socket, and returns a new file descrip‐
   tor  referring  to that socket."

suggest reading the manual 
for system functions that are called within your code

so the variable 'recsize' is actually a socket for communication with the client.

Upvotes: 0

wimh
wimh

Reputation: 15232

This is the important part of your code:

char *words[10];
char buffer[1024];
memset(buffer, 0, sizeof buffer);
nwords = getwords(buffer, words, 10);
option = atoi(words[0]);

Because you call getwords with an empty buffer, it will return 0 and not set anything in words, so words[0] is also uninitialised. You don't check the return value of getwords, but you should. If nwords==0, you should not use words[0].

Upvotes: 1

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