STR34-C. Do not truncate strings while copying

Alternative functions that limit the number of bytes copied are often recommended to mitigate buffer overflow vulnerabilities. For example:

• strncpy() instead of strcpy()
• strncat() instead of strcat()
• fgets() instead of gets()
• snprintf() instead of sprintf()

These functions truncate strings that exceed the specified limits. Additionally, some functions such as strncpy() do not guarantee that the resulting string is null-terminated [[STR33-C]].

Truncation results in a loss of data and, in some cases, leads to software vulnerabilities.

Exception

An exception to this rule applies if the intent of the programmer was to intentionally truncate the null-terminated byte string. To be compliant with this standard, this intent must be clearly stated in comments.

Priority: P2 Level: L3

Truncating strings can lead to a loss of data and, in some cases, exploitable vulnerabilities.

References

• ISO/IEC 9899-1999 Section 7.21 String handling <string.h>
• Seacord 05a Chapter 2 Strings
• ISO/IEC TR 24731-2006