Different system architectures use different byte ordering, either little endian (least significant byte first) or big endian (most significant byte first). IA-32 is an example of an architecture that implements little endian byte ordering. In contrast, PowerPC and most Network Protocols (including TCP and IP) use big endian.
When transferring data between systems of different endianness, the programmer must take care to reverse the byte ordering before interpreting the data.
ntohs() can be used to transfer between network byte ordering (big endian) and the host's byte ordering. On big endian systems, these functions do nothing. They may also be implemented as macros rather than functions.
Noncompliant Code Example
In this noncompliant code example, the programmer tries to read an unsigned 32-bit integer off a previously connected network socket.
It is important to know the sizes of your data types lest they be different on architectures that are accessible over the network. Hence, we transfer a
uint32_t rather than an
int. For more information, see FIO09-C. Be careful with binary data when transferring data across systems.
This program prints out the number received from the socket using an incorrect byte ordering. For example, if the value 4 is sent from a big endian machine, and the receiving system is little endian, the value 536,870,912 is read. This problem can be corrected by sending and receiving using network byte ordering.
In this compliant solution, the programmer uses the
ntohl() function to convert the integer from network byte order to host byte ordering:
ntohl() function (network to host long) translates a
uint32_t value into the host byte ordering from the network byte ordering. This function is always appropriate to use because its implementation depends on the specific system's byte ordering. Consequently, on a big endian architecture,
ntohl() does nothing.
The reciprocal function
htonl() (host to network long) should be used before sending any data to another system over network protocols.
htonl()are not part of the C Standard and are consequently not guaranteed to be portable to non-POSIX systems.
- The POSIX implementations of
htonl()take arguments of types
uint32_tand can be found in the header file
- The Windows implementations use
unsigned longand can be found in the header file
- Other variants of
htont(), such as
htonll(), may exist on some systems.
If the programmer is careless, this bug is likely. However, it will immediately break the program by printing the incorrect result and therefore should be caught by the programmer during the early stages of debugging and testing. Recognizing a value as in reversed byte ordering, however, can be difficult depending on the type and magnitude of the data.
|Axivion Bauhaus Suite|
Use the correct byte ordering when transferring data between systems
|CERT C: Rule POS39-C||Checks for missing byte reordering when transferring data (rule fully covered)|
|[Open Group 2004]||htonl, htons, ntohl, ntohs—Convert Values between Host and Network Byte Order|