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 Topic: [Apache 2.4] Segmentation fault on writting a file |
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DoHITB
Joined: 16 Feb 2017
Posts: 8
Location: Catalonia, Lleida
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 Posted: Fri 23 Apr '21 21:03 Post subject: [Apache 2.4] Segmentation fault on writting a file |
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Hello:
I'm working on a custom module that writes a information file (just to check if working as desired).
My main function looks like
| Code: |
static int myhandler(request_rec *r){
int mci = 0;
int isRef = 0;
const char *ref;
FILE *tmp;
tmp = fopen("/var/www/conf/my-file.log", "w");
if(tmp == 0){
ap_rprintf(r, " tmp is null ");
return OK;
}
fprintf(tmp, "Getting data...\n");
ref = apr_table_get(r->headers_in, "custom-header");
//If there's not custom-header, just end
if(ref == NULL){
fprintf(tmp, "No custom-header\n");
fclose(tmp);
return OK;
}
fprintf(tmp, "custom-header: %s\n", ref);
fclose(tmp);
return OK;
}
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So, if I access "http://my-web..../somepage", file "/var/www/conf/my-file.log" contains information, but if I access same URL via AJAX, it writes nothing (not even the first line), and Apache log shows a "segmentation fault (11)".
I did "chown www-data:www-data /var/www/conf" and "chmod 777 /var/www/conf" to allow file writting.
Also, as I said ,when accessing via URL it writes, so maybe there's something I'm missing...
May anyone please help?
Thanks! |
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tangent
Moderator
Joined: 16 Aug 2020
Posts: 348
Location: UK
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| Posted: Fri 23 Apr '21 22:47 Post subject: |
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You've only provided a snippet of your module code (for example, myhandler() defines mci and isRef variables, which aren't used), so it's challenging to say what's causing your segmentation fault (indicating your module is accessing unallocated memory).
I'd also question that you've chosen standard C file handling functions rather than apr ones such as apr_file_open(), etc., which will cope with concurrency.
Is the file handling core to your module function, or just an aid to debugging? |
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DoHITB
Joined: 16 Feb 2017
Posts: 8
Location: Catalonia, Lleida
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| Posted: Sun 25 Apr '21 14:20 Post subject: |
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Hello @tangent
Thanks for replying. I was using standard C functions as I only left the file for a test (I needed to trace some info without putting it on screen).
Also, didn't know about apr_file_open, so I will take a look if I need to concurrently open the same file 
About the segfault, I already figured it out (I was commenting code lines to see where the error was). I thought that the error was on the write because the output file shows empty; but the error was way after (I suposse the write was cached, so that's why I saw an empty file). The problem was obvious once I move forward to the write itself...
Thanks for letting me know about apr functions! I'm a bit new to this world and every ounce of knowledge is good |
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James Blond
Moderator
Joined: 19 Jan 2006
Posts: 7371
Location: Germany, Next to Hamburg
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