Who modified my local variable?

Sasha Pachev sasha at asksasha.com
Mon Jun 12 13:14:43 MDT 2006

 >There's no need to do all kinds of bizarre malloc and free (new and
 >delete) calls when normal C and C++ scoping rules would accomplish the
 >same things and be safer.  Unless copying is a huge concern, my rule of
 >thumb is to always use static allocations (local, on the stack of
 >course) whenever possible.

Stack space could be a concern in a number of cases. But, yes, if you do not 
recurse, it is only a few bytes, and the stack is not restricted too much, stack 
allocation is preferable. It is also much faster than malloc(), and you do not 
have to free.

Now the comment on the code:

void FunctionA(unsigned char *pLen)
    *pLen = 16;
    printf("T2 %d\r\n", *pLen);

void FunctionB(void)
    unsigned char ucLen = 0;
    printf("T1 %d\r\n", ucLen);
    printf("T3 %d\r\n", ucLen);

One problem I see is that your variable is a char (1-byte), but you are printing 
it with %d which should be used for an int (usually 4-byte). With printf this 
could be a problem. printf() is a variable argument number function. It takes 
the first argument, parses it, and then decides how to interpret the rest of 
them. Depending on the implementation of printf() and the compiler, you could 
have some interesting results. Change your char to int, and see if that fixes it.

Another possibility is that your actual code has a stack-smashing bug.

Sasha Pachev
AskSasha Linux Consulting

Running Blog

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