The first optional field of the format specification is flags. A flag directive is a character that justifies output and prints signs, blanks, decimal points, and octal and hexadecimal prefixes. More than one flag directive may appear in a format specification.
| Flag | Meaning | Default |
| – | Left align the result within the given field width. | Right align. |
| + | Prefix the output value with a sign (+ or –) if the output value is of a signed type. | Sign appears only for negative signed values (–). |
| 0 | If width is prefixed with 0, zeros are added until the minimum width is reached. If 0 and – appear, the 0 is ignored. If 0 is specified with an integer format (i, u, x, X, o, d) the 0 is ignored. | No padding. |
| blank (' ') | Prefix the output value with a blank if the output value is signed and positive; the blank is ignored if both the blank and + flags appear. | No blank appears. |
| # | When used with the o, x, or X format, the # flag prefixes any nonzero output value with 0, 0x, or 0X, respectively. | No blank appears. |
| When used with the e, E, or f format, the # flag forces the output value to contain a decimal point in all cases. | Decimal point appears only if digits follow it. | |
| When used with the g or G format, the # flag forces the output value to contain a decimal point in all cases and prevents the truncation of trailing zeros. Ignored when used with c, d, i, u, or s. |
Decimal point appears only if digits follow it. Trailing zeros are truncated. |