c - 打印 : Is this safe?

Question

我只是想知道这个表达式是否安全:

int main (void)
{
  char my_tab[256];

  memset(my_tab,0x61,sizeof(my_tab));

  printf("Is it safe ? : %.256s",my_tab); /* is it safe ? */
}

Answer 1

是的，您将打印出 256 个字符，仅此而已。

来自 C11 标准 7.21.6。 p4:

An optional precision that gives the minimum number of digits to appear for the d, i, o, u, x, and X conversions, the number of digits to appear after the decimal-point character for a, A, e, E, f, and F conversions, the maximum number of significant digits for the g and G conversions, or the maximum number of bytes to be written for s conversions. The precision takes the form of a period (.) followed either by an asterisk * (described later) or by an optional decimal integer; if only the period is specified, the precision is taken as zero. If a precision appears with any other conversion specifier, the behavior is undefined.

7.21.6.1。 p8:

s : If no l length modifier is present, the argument shall be a pointer to the initial element of an array of character type. Characters from the array are written up to (but not including) the terminating null character. If the precision is specified, no more than that many bytes are written. If the precision is not specified or is greater than the size of the array, the array shall contain a null character.