I come late to the party but...
_snprintf was written before snprintf was defined in C99 and Microsoft's C compilers prior to Visual Studio 2015 have been largely stuck at C89.
DO NOT replace snprintf with _snprintf with #define for any reason.
Use _snprintf_s instead if you lack C99's snprintf. You should avoid using _snprintf in any context where you don't control the number of formatted bytes that will be placed in the buffer.
_snprintf will fail to terminate a string with a '\0' character if the length of the formatted data string is greater or equal to the count argument to the function.
In C99 snprintf will safely terminate C strings if the formatted string exceeds the size of the buffer but Microsoft's _snprintf will not terminate the string.
Writing this will leave the string array unterminated:
#include <stdio.h>
int main()
{
char arry[10];
_snprintf(arry, sizeof arry,
"Something much greater than 10 characters.");
printf("%s\n", arry);
return 0;
}
If you don't have snprintf then writing this will be safe:
#include <stdio.h>
int main()
{
char arry[10];
_snprintf_s(arry, sizeof arry, sizeof arry-1,
"Something much greater than 10 characters.");
printf("%s\n", arry);
return 0;
}
This is why Microsoft's compilers issue the C4996 warning for functions like _snprintf.
Here is unified sample of code you can run in VS2015 that tests all three functions:
#include <stdio.h>
int main()
{
char array[10];
int len;
char *string = "Something much greater than 10 characters.";
len = snprintf(array, sizeof array, "%s", string);
if (len > sizeof array)
puts("Error! Possible unterminated string.");
printf("snprintf returned %d and array contains: ", len);
printf("%s\n\n", array);
len = _snprintf_s(array, sizeof array, sizeof array-1, "%s", string);
if (len > sizeof array)
puts("Error! Possible unterminated string.");
printf("_snprintf_s returned %d and array contains: ", len);
printf("%s\n\n", array);
len = _snprintf(array, sizeof array, "%s", string);
if (len > sizeof array)
puts("Error! Actual unterminated string!");
printf("_snprintf returned %d and array contains: ", len);
printf("%s\n\n", array);
return 0;
}
Output from a sample debugging run:
Error! Possible unterminated string.
snprintf returned 42 and array contains: Something
Error! Possible unterminated string.
_snprintf_s returned -1 and array contains: Something
Error! Actual unterminated string!
_snprintf returned -1 and array contains: Something ╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠ ╠╠╠╠╠╠╠╠╠╠
╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠(₧÷⌂
