Valgrind throwing unexplainable error in thread - c

Question

I'm having trouble identifying why valgrind is throwing this error:
==82185== Thread 2:
==82185== Use of uninitialised value of size 8
==82185== at 0x401B9A: proc_outconnection_thread (station.c:401)
==82185== by 0x4E3CDF4: start_thread (in /usr/lib64/libpthread-2.17.so)
==82185== by 0x51471AC: clone (in /usr/lib64/libc-2.17.so)
==82185==
the pass im sending is 'this'
==82185== Use of uninitialised value of size 8
==82185== at 0x401BCA: proc_outconnection_thread (station.c:403)
==82185== by 0x4E3CDF4: start_thread (in /usr/lib64/libpthread-2.17.so)
==82185== by 0x51471AC: clone (in /usr/lib64/libc-2.17.so)
==82185==
As a bit of background information, the program i'm trying to create in C simulates a train station that uses TCP connections as "trains". I'm trying to get the program to use threads in order to both listen for and try and connect to other stations (other instances of the program).
The problem seems to exist when passing an internal data struct to a thread creation function via an argument struct that contains a pointer to the internal data struct. This way each thread has a pointer to the programs internal data.
In my efforts of testing, the file is compiled with
gcc -pthread -g -o station station.c -Wall -pedantic -std=gnu99
To produce my error, begin an instance of station with valgrind ./station tom authf logfile 3329 127.0.1.1
and then begin another instance with valgrind ./station tim authf logfile 3328 127.0.1.1
Due to an if statement in main, the station name with tim will attempt to connect to tom, and tom will create a socket and listen for tims attempt to connect. The connection seems to be successful however for some reason I'm also unable to flush the connection to send anything between, which i have a feeling may be because of what Valgrind is telling me.
What's so strange is that when a thread is created for the connection on tom's instance, no errors in valgrind are thrown despite having a very similar procedure for creating the thread (the same arguments are passed through the argument pointer and the same assignments are made).
Could it be a false positive for tim's end, or am I doing something severely wrong here?

Answer 1

Your problem is passing pointer on local variable into thread function. Simplest workaround is declare this variable as static or global, but this is not good if there are several threads use that variable.
Its better to allocate needed memory size for structure, initialize and pass this into thread function:
ArgStruct *argStruct = malloc(sizeof(ArgStruct));
if(argStruct == NULL) {
fprintf(stderr, "Cant alloc memory!\n");
exit(98);
}
argStruct->internalStruct = internal;
argStruct->clientCon = fdopen(fd, "r+");
pthread_create(&threadId, NULL, proc_outconnection_thread, (void *)argStruct);
Also, don't forget to free this memory (at the end of proc_outconnection_thread() for example).

Answer 2

Track the value of your internal data structure back to where it comes from and you will see that it originates from a struct object that is not initialized. You later assign values to some of the fields, but not to all.
Always initialize struct objects, and at the same time watch that you have a convention that makes it clear what default initialization (as if done with 0) means for the type.
If, one day, you really have a performance bottleneck because your compiler doesn't optimize an unused initialization, think of it again and do it differently. Here, because you are launching threads and do other complicated stuff, the difference will never be measurable.