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POrealloc after free problem ("realloc: invalid next size")
 

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  1. POrealloc after free problem ("realloc: invalid next size")
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    <p>I've got a strange problem with realloc, here's my code (relevant parts only, too big to post the full code here):</p> <pre><code>char * print_bar(struct bar *ptr) { char *buf = NULL; char *buftmp = NULL; size_t size = 60; int count = 0; if (ptr) { while (count == 0 || count+4 &gt;= size) { buftmp = (char *) realloc((void *) buf, size * sizeof (char)); if (buftmp == NULL) { if (buf != NULL) free(buf); exit(EXIT_FAILURE); } buf = buftmp; count = snprintf(buf, size, "%04d-%02d-%02d\t%02d:%02d:00\t%d\t%.2f\t%.2f\t%.2f\t%.2f\t%d", ptr-&gt;year, ptr-&gt;month, ptr-&gt;day, ptr-&gt;hour, ptr-&gt;minute, ptr-&gt;timeframe, ptr-&gt;open, ptr-&gt;high, ptr-&gt;low, ptr-&gt;close, ptr-&gt;volume ); size += 4; } } return buf; } char * print_historico(short timeframe) { struct barlist *tmp = get_barlist(timeframe); struct bar *ptr; char * result = NULL; char * resulttmp = NULL; char * buf; int len; if (tmp) { ptr = tmp-&gt;first; while (ptr) { buf = print_bar(ptr); len = (result != NULL) ? strlen(result)+strlen(buf)+1 : strlen(buf)+1; resulttmp = (char *)realloc((void *)result, len); if (resulttmp == NULL) { if (result != NULL) free(result); exit (EXIT_FAILURE); } result = resulttmp; strncat(result, buf, strlen(buf)); free(buf); ptr = ptr-&gt;next; } } return result; } </code></pre> <p>In my main function i've got the following loop:</p> <pre><code>for (i = 1; i &lt;= 27; i++) { historico = print_historico(i); if (historico != NULL) { puts(historico); free(historico); } } </code></pre> <p>If i compile and run it fails with "realloc(): invalid next size: 0x0000000001704f60". If i run with the debugger i see it finishes the first iteration of the main loop ok, freeing the 'historico' variable. When it executes "print_historico" with i=2 it fails on the second iteration of the "while(ptr)" loop.</p> <p>I can't find a reason, any clue? I've also tried to make a small program to isolate the problem, but i wasn't able.</p>
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    1. USroirodriguez
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    1. CO1704f60 is 24MB... seems quite large to me... is that what you're expecting?
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      1. USforsvarir
    2. COWhat is the first function actually trying to do? It appears to be repeatedly trying to print a bunch of text into a buffer, expanding the buffer when it finds out it is too small. This is probably not a good approach--you should probably start with a size that is very likely to fit all the text, and double it if it doesn't.
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      1. USJohn Zwinck
    3. CO@forsvarir: Not only does the code eventually try to allocate 24 MB, I think it gets there by allocating 60 bytes then 64 then 68 all the way up. I guess it frees the too-small buffers along the way, but this seems pretty weird.
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      1. POrealloc after free problem ("realloc: invalid next size")
      1. USJohn Zwinck
 

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