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    <p>Like everyone else has pointed out, the problem is that <code>dest</code> hasn't been initialized and is pointing to a random location that contains something important. You have several choices of how to deal with this:</p> <ol> <li>Allocate the <code>dest</code> buffer dynamically and return that pointer value, which the caller is responsible for freeing;</li> <li>Assign <code>dest</code> to point to <code>src</code> and modify the value in place (in which case you'll have to change the declaration of <code>a</code> in main() from <code>char *a = "hello world";</code> to <code>char a[] = "hello world";</code>, otherwise you'll be trying to modify the contents of a string literal, which is undefined);</li> <li>Pass the destination buffer as a separate argument.</li> </ol> <p>Option 1 -- allocate the target buffer dynamically:</p> <pre><code>char *to_upper(char *src) { char *dest = malloc(strlen(src) + 1); ... } </code></pre> <p>Option 2 -- have dest point to src and modify the string in place:</p> <pre><code>int main(void) { char a[] = "hello world"; ... } char *to_upper(char *src) { char *dest = src; ... } </code></pre> <p>Option 3 -- have main() pass the target buffer as an argument:</p> <pre><code>int main(void) { char *a = "hello world"; char *b = malloc(strlen(a) + 1); // or char b[12]; ... printf("Uppercase becomes %s\n", to_upper(a,b)); ... free(b); // omit if b is statically allocated return 0; } char *to_upper(char *src, char *dest) { ... return dest; } </code></pre> <p>Of the three, I prefer the third option; you're not modifying the input (so it doesn't matter whether a is an array of char or a pointer to a string literal) and you're not splitting memory management responsibilities between functions (i.e., main() is solely responsible for allocating and freeing the destination buffer). </p> <p>I realize you're trying to familiarize yourself with how pointers work and some other low-level details, but bear in mind that <code>a[i]</code> is easier to read and follow than <code>*(a+i)</code>. Also, there are number of functions in the standard library such as <code>islower()</code> and <code>toupper()</code> that don't rely on specific encodings (such as ASCII):</p> <pre><code>#include &lt;ctype.h&gt; ... if (islower(src[i]) dest[i] = toupper(src[i]); </code></pre>
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