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

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Non-Compliant Code Example

The standard functions 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 will not be null terminated and any remaining characters are truncated.

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

Compliant Solution (adequate space)

Either the strcpy() or strncpy() function can be used to copy a string and a null character to a destination buffer, provided there is enough space. Care must be taken to ensure that the destination buffer is large enough to hold the string to be copied and the null byte to prevent errors such as data truncation and buffer overflow.

char *string_data;
char a[16];

if (string_data == NULL) {
  /* Handle null pointer error */
}
else if (strlen(string_data) >= sizeof(a)) {
  /* Handle overlong string error */
}
else {
  strcpy(a, string_data);
}

It is assumed that string_data is null terminated.  Otherwise, strlen() will stray into other objects before finding a null byte.

Compliant Solution (TR 24731-1)

The {{strcpy_s()}} function defined in \[[ISO/IEC TR 24731-1-2007|AA. C References#ISO/IEC TR 24731-1-2007]\] provides additional safeguards, including accepting the size of the destination buffer as an additional argument \[[STR00-A. Use TR 24731 for remediation of existing string manipulation code]\].  Also, {{strnlen_s()}} accepts a maximum-length argument for strings that may not be null terminated.

char *string_data;
char a[16];

if (string_data == NULL) {
  /* Handle null pointer error */
}
else if (strnlen_s(string_data, sizeof(a)) >= sizeof(a)) {
  /* Handle overlong string error */
}
else {
  strcpy_s(a, sizeof(a), string_data);
}

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Exceptions

STR03-EX1: The intent of the programmer is to intentionally truncate the null-terminated byte string.

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