CODEWITHSUNDEEP
cstringchar
barndog
barndog

Reputation: 7173

Char* array not retaining value C

For my networking class, we're building a bittorrent client based off the UDP protocol, which is pretty cool but I'm having a ton of trouble with C strings for some reasons.

The first time I receive a packet, I do:

if(server_data == NULL){
    server_data = malloc(one_block.total_blocks*sizeof(char*));
    int i;
    for(i = 0; i < one_block.total_blocks; i++){
        server_data[i] = malloc(sizeof(char*));
        server_data[i] = "";
    }
}

Here, server_data is a char** and one_block is struct that holds packet information and the payload.

Next I do:

server_data[one_block.which_block] = one_block.payload;
blocks_rcv++;
if(blocks_rcv == one_block.total_blocks-1)
    done = TRUE; //macro
if(done){
    int i;
    for(i = 0; i < one_block.total_blocks; i++){
        printf("%s", server_data[i];
    }
}

All seems well and dandy but for whatever insane reason when I print the contents of server_data before all the packets are received, I see different data from each packet. Afterwards I set done = TRUE and go into that for loop, every spot in the array contains the same string value.

I have no idea why this is happening and I really want to understand how from the beginning of the post to the end, the contents of the array change, even though I verify them through every iteration of the loop that reads in one packet at a time.

Upvotes: 0

Views: 156

Answers (2)

jboyd
jboyd

Reputation: 16

There are a couple of issues going on here:

1) First, server_data, if it's declared as a char**, may or may not be null off the bat, unless you declare it so. I'm not sure if you initialized it to NULL or not. It's a good idea to explicitly initialize it to NULL.

2) I'm not sure from what's going on if you intend for each item of the array server_data to hold a char* (in other words, a reference to a string), or for the array to be a string itself. Is one_block.payload a string, or a set of pointers to strings?

I ran your code with some test values and I'm personally not getting any problems with unexpected values...I think the issue may be in how the struct that holds your payload data is set up. Could you show us your one_block struct? What type of variable/array is one_block.payload?

Upvotes: 0

Some programmer dude
Some programmer dude

Reputation: 409176

This line is the problem:

server_data[i] = "";

It overwrites the allocated pointer, with a pointer to the string literal. And as string literals can't be modified, if you later copy into this pointer, you experience undefined behavior.

If you want to make sure the string is empty, either use calloc, set the first character to '\0', or use strcpy to copy in the new string.

Upvotes: 3

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