In C, what is the effect of a negative number in a field width specifier?

Concept: In C's printf-family functions, a field width specifies the minimum number of characters printed for a value, with the remaining space filled by padding. A positive width right-justifies the value by default (padding added on the left). A negative width — which can only arise when the width is supplied via the * width argument — is defined by the C standard to be interpreted as the - (left-justify) flag combined with the absolute value of that width: it forces left-justified output, with padding added on the right instead.

Application: Trace a concrete case:

1. Consider printf("[%5d]", 7) — the width 5 is positive, so the default (right-justified) alignment applies: it prints [ 7].

2. Now consider printf("[%*d]", -5, 7) — the width argument passed is -5, a negative number.

3. Per the format-specifier rule, that negative sign is stripped off to get an actual width of 5, and the - (left-justify) flag is activated because the supplied width was negative.

4. The output therefore becomes [7 ] — left-justified, with the padding spaces moved to the right of the value.

Cross-check: This matches the ISO C standard (WG14 N1570 §7.21.6.1) and the POSIX printf specification, both of which state that a negative width argument is taken as the - flag with a positive width. It also rules out the other options: printf's format specifiers have no centering capability at all (ruling out center-alignment), and the field width never changes the sign of the printed value — only its alignment and padding (ruling out the "displayed as negative numbers" option).

Result: A negative field width specifier causes the value to be displayed left-justified.