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5.3.2 Use of uninitialised values

For example:

Conditional jump or move depends on uninitialised value(s)
   at 0x402DFA94: _IO_vfprintf (_itoa.h:49)
   by 0x402E8476: _IO_printf (printf.c:36)
   by 0x8048472: main (tests/manuel1.c:8)

An uninitialised-value use error is reported when your program uses a value which hasn't been initialised--in other words, is undefined. Here, the undefined value is used somewhere inside the printf() machinery of the C library. This error was reported when running the following small program:

int main()
{
  int x;
  printf ("x = %d\n", x);
}

It is important to understand that your program can copy around junk (uninitialised) data as much as it likes. Memcheck observes this and keeps track of the data, but does not complain. A complaint is issued only when your program attempts to make use of uninitialised data. In this example, x is uninitialised. Memcheck observes the value being passed to ‘_IO_printf’ and thence to ‘_IO_vfprintf’, but makes no comment. However, ‘_IO_vfprintf’ has to examine the value of x so it can turn it into the corresponding ASCII string, and it is at this point that Memcheck complains.

Sources of uninitialised data tend to be:

• Local variables in procedures which have not been initialised, as in the example above.
• The contents of malloc'd blocks, before you write something there. In C++, the new operator is a wrapper round malloc, so if you create an object with new, its fields will be uninitialised until you (or the constructor) fill them in.