_TRUNCATE
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The latest version of this topic can be found at _TRUNCATE.
Specifies string truncation behavior.
Syntax
#include <stdlib.h>
Remarks
_TRUNCATE enables truncation behavior when passed as the count parameter to these functions:
strncpy_s, _strncpy_s_l, wcsncpy_s, _wcsncpy_s_l, _mbsncpy_s, _mbsncpy_s_l
strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l
_snprintf_s, _snprintf_s_l, _snwprintf_s, _snwprintf_s_l
vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwprintf_s_l
If the destination buffer is too small to hold the entire string, the normal behavior of these functions is to treat it as an error situation (see Parameter Validation). However, if string truncation is enabled by passing _TRUNCATE, these functions will copy only as much of the string as will fit, leaving the destination buffer null-terminated, and return successfully.
String truncation changes the return values of the affected functions. The following functions return 0 if no truncation occurs, or STRUNCATE if truncation does occur:
strncpy_s, _strncpy_s_l, wcsncpy_s, _wcsncpy_s_l, _mbsncpy_s, _mbsncpy_s_l
strncat_s, _strncat_s_l, wcsncat_s, _wcsncat_s_l, _mbsncat_s, _mbsncat_s_l
The following functions return the number of characters copied if no truncation occurs, or -1 if truncation does occur (matching the behavior of the original snprintf functions):
_snprintf_s, _snprintf_s_l, _snwprintf_s, _snwprintf_s_l
vsnprintf_s, _vsnprintf_s, _vsnprintf_s_l, _vsnwprintf_s, _vsnwprintf_s_l
Example
// crt_truncate.c
#include <stdlib.h>
#include <errno.h>
int main()
{
char src[] = "1234567890";
char dst[5];
errno_t err = strncpy_s(dst, _countof(dst), src, _TRUNCATE);
if ( err == STRUNCATE )
printf( "truncation occurred!\n" );
printf( "'%s'\n", dst );
}
truncation occurred!
'1234'