STR34-C. Do not truncate strings

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 function 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.

Non-Compliant Code Example

The standard function strncpy() and strncat() copy a specified number n characters from a source string to a destination array. If there is no null character in the first n characters of the source array the result is not be null-terminated and any remaining charactes are truncated

char *string_data;
char a[16];
...
strncpy(a, string_data, sizeof(a));

Compliant Solution

Truncation resulting from a string copy operation should be treated as an error condition.

char *string_data;
size_t string_len;
char a[16];
...
if (!string_data) {
  if (strlen(a) < sizeof(a)) {
    strcpy(a, string_data);
  }
  else {
    /* handle error condition */
  }
}
else {
  /* handle error condition */
}

Compliant Solution 2

Example using strcpy()

Compliant Solution 3

Example using strncpy_s()

Exception

An exception to this rule applies if the intent of the programmer was to convert a null-terminated byte string to a character array. To be compliant with this standard, this intent must be made clear statement in comments.

References

• ISO/IEC 9899-1999 7.21.2.4 The strncpy function
• ISO/IEC 9899-1999 7.21.3.2 The strncat function
• SAMATE Reference Dataset Test Case ID 000-000-004