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POThe valgrind reports error when printing allocated strings
 

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  1. POThe valgrind reports error when printing allocated strings
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    <p>The code is here:</p> <pre><code>#include &lt;stdio.h&gt; #include &lt;stdlib.h&gt; #include &lt;string.h&gt; int main() { char* buf = malloc(3); strcpy(buf, "hi"); printf("%s\n", buf); free(buf); } </code></pre> <p>It's compiled with:</p> <pre><code>gcc a.c &amp;&amp; valgrind ./a.out </code></pre> <p>The error message is here:</p> <pre><code>==1421== Memcheck, a memory error detector ==1421== Copyright (C) 2002-2010, and GNU GPL'd, by Julian Seward et al. ==1421== Using Valgrind-3.6.1 and LibVEX; rerun with -h for copyright info ==1421== Command: ./a.out ==1421== ==1421== Invalid read of size 8 ==1421== at 0x4EA96C1: ??? (in /lib/libc-2.14.1.so) ==1421== by 0x4E92D3B: puts (in /lib/libc-2.14.1.so) ==1421== by 0x4005BB: main (in /home/peter/a.out) ==1421== Address 0x51b4040 is 0 bytes inside a block of size 3 alloc'd ==1421== at 0x4C2740D: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so) ==1421== by 0x400595: main (in /home/peter/a.out) ==1421== hi ==1421== ==1421== HEAP SUMMARY: ==1421== in use at exit: 0 bytes in 0 blocks ==1421== total heap usage: 1 allocs, 1 frees, 3 bytes allocated ==1421== ==1421== All heap blocks were freed -- no leaks are possible ==1421== ==1421== For counts of detected and suppressed errors, rerun with: -v ==1421== ERROR SUMMARY: 2 errors from 1 contexts (suppressed: 6 from 6) </code></pre> <p>It is also very strange that valgrind reports no more errors if I use the following (just one more space):</p> <pre><code>printf("%s \n", buf); </code></pre> <p>Would anyone please help me?</p>
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    1. CO[You should **never** cast the result of `malloc` in C](http://c-faq.com/malloc/mallocnocast.html)
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    2. COBut I am using the g++ compiler and without the cast the error would be something like 'cannot convert from void* to char*'. I have some other reasons for why I am not using 'new' instead of 'malloc'.
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    3. COYour Valgrind output doesn't match the code you've posted. It shows `main()` calling `puts()`, but your posted code uses `printf()`. I don't think the compiler does that optimization automatically. Of course, you didn't specify if you built the program with optimization enabled.
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