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    <p>You ask:</p> <blockquote> <p>Is there anything I should know about using strtok on a malloced string?</p> </blockquote> <p>There are a number of things to be aware of. First, <code>strtok()</code> modifies the string as it processes it, inserting nulls (<code>'\0'</code>) where the delimiters are found. This is not a problem with allocated memory (that's modifiable!); it is a problem if you try passing a constant string to <code>strtok()</code>.</p> <p>Second, you must have as many calls to <code>free()</code> as you do to <code>malloc()</code> and <code>calloc()</code> (but <code>realloc()</code> can mess with the counting).</p> <blockquote> <p>In my code I have (in general terms)</p> <pre><code> char* line=getline(); Parse(dest,line); free(line); </code></pre> </blockquote> <p>Unless <code>Parse()</code> allocates the space it keeps, you cannot use the <code>dest</code> structure (or, more precisely, the pointers into the line within the <code>dest</code> structure) after the call to <code>free()</code>. The <code>free()</code> releases the space that was allocated by <code>getline()</code> and any use of the pointers after that yields undefined behaviour. Note that undefined behaviour includes the option of 'appearing to work, but only by coincidence'.</p> <blockquote> <p>where getline() is a function that returns a char * to some malloced memory, and Parse(dest, line) is a function that does parsing online, storing the results in dest (which has been partially filled earlier, from other information).</p> <p>Parse() calls strtok() a a variable number of times on line, and does some validation. Each token (a pointer to what is returned by strtok()) is put into a queue 'til I know how many I have.</p> </blockquote> <p>Note that the pointers returned by <code>strtok()</code> are all pointers into the single chunk of space allocated by <code>getline()</code>. You have not described any extra memory allocation.</p> <blockquote> <p>They are then copied onto a malloc'd char** in dest.</p> </blockquote> <p>This sounds as if you copy the pointers from <code>strtok()</code> into an array of pointers, but you do not attend to copying the data that those pointers are pointing at.</p> <blockquote> <p>Now free(line) and a function that free's each part of the char*[] in dest, both come up on valgrind as:</p> <pre><code>"Address 0x5179450 is 8 bytes inside a block of size 38 free'd" </code></pre> <p>or something similar.</p> </blockquote> <p>The first <code>free()</code> of the '<code>char *[]</code>' part of <code>dest</code> probably has a pointer to <code>line</code> and therefore frees the whole block of memory. All the subsequent frees on the parts of <code>dest</code> are trying to free an address not returned by <code>malloc()</code>, and <code>valgrind</code> is trying to tell you that. The <code>free(line)</code> operation then fails because the first <code>free()</code> of the pointers in <code>dest</code> already freed that space.</p> <blockquote> <p>I'm considering refactoring my code [to] store a copy of them [...].</p> </blockquote> <p>The refactoring proposed is probably sensible; the function <code>strdup()</code> already mentioned by others will do the job neatly and reliably.</p> <p>Note that after refactoring, you will still need to release line, but you will not release any of the pointers returned by <code>strtok()</code>. They are just pointers into the space managed by (identified by) <code>line</code> and will all be released when you release <code>line</code>.</p> <p>Note that you will need to free each of the separately allocated (<code>strdup()</code>'d) strings as well as the array of character pointers that are accessed via <code>dest</code>.</p> <p>Alternatively, do not free line immediately after calling <code>Parse()</code>. Have <code>dest</code> record the allocated pointer (<code>line</code>) and free that when it frees the array of pointers. You still do not release the pointers returned by <code>strtok()</code>, though.</p>
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