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POC Realloc Append String MySQL Connector
 

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  1. POC Realloc Append String MySQL Connector
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    <p>I am attempting to loop through each database result, and have a single field from each row comma separated in a final string. I am aware that this is possible to do in MySQL alone, but I need to run functions on each result so that is not possible with the mysql only method.</p> <p>Every few runs, I will get a segment fault, unwanted characters are added to the output, such as "^H^G" or Hex code (08 and 07) inside of Linux. It compiles properly, I just wish I could have consistent results. This code is inside a thread, but all variables are local to the current thread.</p> <p>Final String should be the userstring char.</p> <p>This is the latest code I've tried to work with.</p> <pre><code>MYSQL dbh; MYSQL *dbh_p; MYSQL_RES *res; MYSQL_ROW row; char *userstring = NULL; char *userstring_r; size_t userstringsize = 0; size_t rowsize = 0; /* Database Connect Code Here [Connects Successfully] */ if(mysql_query(dbh_p,query) == 0) { res = mysql_store_result(dbh_p); if(mysql_num_rows(res) &gt; 0) { while((row = mysql_fetch_row(res))) { rowsize = strlen(row[0]) * sizeof(char); userstringsize += rowsize + 1; userstring_r = (char *) realloc(userstring,userstringsize + 1); printf("%d %d %p\n", rowsize, userstringsize, userstring_r); if(userstring_r != NULL) { /* Where the Issue Is */ userstring = userstring_r; strcat(userstring,row[0]); strcat(userstring,","); }else{ free(userstring); printf("Error With Realloc\n"); mysql_thread_end(); pthread_exit(NULL); } } } mysql_free_result(res); }else{ printf("Query Error: %s\n",mysql_error(&amp;dbh)); } </code></pre> <p>-- EDIT -- Output:</p> <pre><code>9 10 0x7fe5f4014d90 14 25 0x7fe5f4014d90 9 35 0x7fe5f4014d90 11 47 0x7fe5f4014d90 ... (same format and pointer value); *** glibc detected *** ./pthread: realloc(): invalid next size: 0x00007fe5f4014d90 *** </code></pre> <p>This is the result even when I limit the threads to only 1.</p> <p>Thank you for any help, or possible issues. I don't require full code, just tips / hints and possible fixes. I have tried strncat, memcpy, and changing various sizes without any luck.</p>
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    1. USDavid
    1. USDavid
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    1. This table or related slice is empty.
    1. This table or related slice is empty.
    1. This table or related slice is empty.
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      1. POC Realloc Append String MySQL Connector
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    1. COI can't see anything wrong at first glance. Could you add some tracing (i.e. `printf` statements!) to find out what it's doing, for instance `printf("%d %d %p\n", rowsize, userstringsize, userstring_r);` just before the `if` statement to confirm that it's allocating the new string properly.
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      1. USEdmund
    2. COI added the printf line, and updated the output.
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      1. USDavid
    3. COI changed the max threads to 5, and increased userstringsize +2 instead of +1 and it seems to have fixed the issue.
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      1. POC Realloc Append String MySQL Connector
      1. USDavid
 

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