BSP Developer's Guide

VxWorks 5.5
BSP Developer’s Guide
Avoid alloca( ) Function
Many compilers support the alloca( ) function as an extension to the C language.
This normally allocates storage from the stack, as any declared variable would.
Since the storage area is on the stack, this area does not need to be freed, as the
stack is restored upon exiting the function.
While alloca( ) is widely used in code from other OS programming models, it does
not suit VxWorks very well. While other OSs may support automatic stack
expansion and stack checking, VxWorks does not. In embedded programming,
predictable timing and stack usage can be very important. Code for VxWorks
should definitely avoid the use of the alloca( ) function. Allocate the storage
directly on the stack, or use malloc( ) and free( ) if necessary.
Take Care with void Pointer Arithmetic
It is common to use pointer arithmetic on void pointer data types. Because the size
of a void item should be unspecified, the ANSI standard does not allow this
practice. The GNU compiler, however, did allow it and assumed the data size to be
one byte, the same as a char data type.
For example, the following code fragment is faulty:
{
void * pVoid;
pVoid += 1; /* WRONG */
pVoid++; /* WRONG */
pVoid = pVoid + sizeof(char); /* WRONG */
}
The example above is faulty because ANSI pointer arithmetic is based on the size
of the object pointed to. In the case of a pointer to a void, the size is undefined. For
the first faulty statement in the example above, the only correct implementation is
as follows:
{
void * pVoid;
pVoid = (char *)pVoid + 1; /* RIGHT */
(char *)pVoid++; /* WRONG */
(char *)pVoid += 1 /* WRONG */
}
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