Cannot pass char * array

[Xx r3negade]

EDIT: Title is a misnomer, it's not an array it's a pointer

I have a struct that I use to pass arguments to a function referenced by pthread_create(). It looks like this:

struct __tryConnect_args
{
    int sock;
    char * hoststr;
    unsigned int port;
    int proto;
};
 

My problem is with the char * hoststr member of the struct. Take a look at this code:

char * hostString;
hostString = dnsLookup(argv[1], AF_INET); /* prototype : char * dnsLookup(char *, int) */
 
printf("Connecting to %s\n", hostString);
 
for (i = 0; i < maxConns; i++)
{
        connArgs[i].sock = sock;
        connArgs[i].hoststr = hostString;
        connArgs[i].proto = PF_INET;
        connArgs[i].port = port;
 
        pthread_create( &connThreads[i],
                                 NULL,
                                 &__tryConnect,
                                 (void *) &connArgs[i] );
}
 

When __tryConnect() is executed, "sock", "proto", and "port" are passed correctly, but "hoststr" becomes gibberish.

__tryConnect() looks like this:

void * __tryConnect(void * args)
{
    struct __tryConnect_args * argsStruct;
    argsStruct = (struct __tryConnect_args *) args;
    printf("args: hoststr: %s\nport: %d\n", argsStruct->hoststr, argsStruct->port);
}
 

The program's output looks like this:

Getting address info for google.com
Connecting to 209.85.171.100
args: hoststr: �        �@
port: 80
 

What is happening with hoststr?

[JosAH]

What happens if you make a copy of the char* string right after dnsLookup() has returned it to you? It seems as if the buffer returned by that function is reused for other purposes ...

kind regards,

Jos

[Xx r3negade]

OK, that works. I don't know what's happening with hoststr, though...the only two times that it's used in the program are the ones shown in the code. Whatever, it works though. Thanks.

EDIT: On a side note, if I replace the code in dnsLookup() so it looks like this:

char * dnsLookup(char * hoststr, unsigned int proto)
{
   return "testing";
}
 

everything works fine, without having to copy the character buffer. So the problem lies in my dnsLookup() function...

[donbock]

@Xx r3negade
Nothing happens to hoststr (ie, the pointer). The problem is with the string being pointed at.

Function dnsLookup apparently constructs the string in a static buffer and returns a pointer to that buffer. Apparently some other function (perhaps pthread_connect) uses that same buffer as a scratchpad, thereby corrupting the string that you expect hoststr to point to.

You should always be suspicious of functions that return a pointer: what is the lifetime of the buffer being pointed to? There are two ways you can be burned: an interposed function call might corrupt the buffer; or "simultaneous" calls to the same function by two threads might leave both threads pointing to the same buffer.

Best is when a function copies such information into a buffer you provide as an argument; second best is for you to copy information from a "built-in" buffer into one of your own; third best is a function that mallocs the necessary buffer (but don't forget to free it!).