Page History

Different alignments are possible for different types of objects. If the type-checking system is overridden by an explicit cast or the pointer is converted to a void pointer (void *) and then to a different type, the alignment of an object may be changed.

According to C99, Section 6.3.2.3 p7 (see also undefined behavior 22 of Annex J):, p7

A pointer to an object or incomplete type may be converted to a pointer to a different object or incomplete type. If the resulting pointer is not correctly aligned for the pointed-to type, the behavior is undefined.

(See also undefined behavior 22 of Annex J.)

If the misaligned pointer is dereferenced, the program may terminate abnormally. The cast alone may cause a loss of information, even if the value is not dereferenced. For example, the following code is not guaranteed to work conforming C99 implementations, even though no pointers are dereferenced:

...

Many architectures requires that pointers are correctly aligned when accessing objects bigger than a byte. There are, however, many places in system code where you receive unaligned data (e.g., the network stacks) that needs to be copied to a properly aligned memory location such as in this noncompliant code example.

...

Automated Detection

The LDRA tool suite V 7.6.0 can detect violations of this rule.

...

Tool	Version	Checker	Description
Section LDRA tool suite	Include Page c:LDRA_V c:LDRA_V
Section GCC	Include Page c:GCC_V c:GCC_V		Section can detect some violations of this rule when the -Wcast-align flag is used.
Section

...

EDG Front End to Compass/ROSE

...

Section Compass/ROSE			Section can detect violations of this rule. However, it does not flag explicit casts to void * and then back to another pointer type.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

Other Languages

Related Guidelines

This rule appears in the C++ Secure Coding Standard as : EXP36-CPP. Do not convert pointers into more strictly aligned pointer types.

Bibliography

[Walfridsson 032003] Krister Walfridsson. [Aliasing, pointer casts and gcc 3.3|http://mail-index.netbsd.org/tech-kern/2003/08/11/0001.html]. August, 2003.
\[[Bryant 032003|AA. Bibliography#Bryant 03]\]
\[[ISO/IEC 9899:1999|AA. Bibliography#ISO/IEC 9899-1999]\] Section 6.2.5, "Types"
\[[ISO/IEC PDTR 24772|AA. Bibliography#ISO/IEC PDTR 24772]\] "HFC Pointer casting and pointer type changes"
\[[MISRA 042004|AA. Bibliography#MISRA 04]\] Rules 11.2 and 11.3

...