尝试通过模块在apache中强制自定义输出

Question

I'm trying to create an apache filter that eventually creates a custom output based on the incoming status code.

I can easily retrieve the status code in the below function after everything in relation to the request is processed using the result from f->r->status .

I want to be able to do it without looping through existing data to change it which is what all other filters do. What I want to do is ditch the existing data and replace it with my own.

In the code below, I'm trying to achieve the following HTTP header:

HTTP/1.1 777 Lucky

And I'm trying to achieve this data:

Testing

Instead, the apache log reports a segmentation fault.

Is there a way to replace the output or will I have to loop through the input and blank each segment out and replace one segment with my own data?

    static apr_status_t OUTF(ap_filter_t *f,apr_bucket_brigade *inbb){
      conn_rec* c=f->r->connection;
      apr_brigade_cleanup(inbb);
      apr_bucket_brigade* mybrigade=apr_brigade_create(f->r->pool,c->bucket_alloc);
      int len=1000;
      char* buf=apr_bucket_alloc(len,c->bucket_alloc);
      strcpy(buf,"HTTP/1.1 777 Lucky\r\n\r\nTesting");
      apr_bucket* mybucket=apr_bucket_heap_create(buf,len,apr_bucket_free,c->bucket_alloc);
      APR_BRIGADE_INSERT_TAIL(mybrigade,mybucket);
      return ap_pass_brigade(f->next,mybrigade);
    }

PS: The filter has been activated and registered so this code does get executed every time I request a URL and somewhere in the above code, I receive a segmentation fault.

Answer 1

You're doing apr_bucket_alloc() and then treating the result of that function as a char * and trying to strcpy() something into it. That's not right. apr_bucket_alloc() doesn't create a char * . It's also not the right way to create a bucket.

What you want is apr_bucket_pool_create() . (It's in apr_buckets.h , with enough documentation to figure out how to call it.)

If we assume you have your string like so:

char *str = "HTTP/1.1 777 Lucky\r\n\r\nTesting";

Then you can do this:

apr_bucket *b = apr_bucket_pool_create(str, strlen(str)+1,
    f->r->pool, f->r->connection->bucket_alloc);

Now you have your bucket and you can APR_BRIGADE_INSERT_TAIL() .

But: you have another problem. There is no guarantee that your filter function will be called only once for a request. It may be called several times, each time with just a chunk of the output from the request. You need to keep state so you know if you're being called a second (or more) time on the same request.

The f->ctx pointer is provided for this purpose. You can put anything you want in it.

typedef struct {
    int c;
    // anything you want to preserve from one call to the next
} filter_ctx;

Within your filter function:

filter_ctx *ctx = f->ctx;
if (ctx == NULL) {
    ctx = f->ctx = apr_palloc(f->r->pool, sizeof(filter_ctx));
    ctx->c = 1;
} else {
    ctx->c += 1;
}

Now you have a counter telling you how many times you've been called. So, you can make sure you only do your thing on the first call. (Obviously you can accomplish this any way you like; using a counter is just an example.) If you want to be safe, check your incoming buckets for an EOS bucket and NULL out your f->ctx pointer when you see one. Since you're allocating from the request pool, you probably don't need to do that.