What happens when I use the wrong format specifier?

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what happens when I use the wrong format specifier in C?

Generally speaking, undefined behaviour.*

However, recall that printf is a variadic function, and that the arguments to variadic functions undergo the default argument promotions. So for instance, a char is promoted to an int. So in practice, these will both give the same results:

char x = 'A';
printf("%c\n", x);

int y = 'A';
printf("%c\n", y);

whereas this is undefined behaviour:

long z = 'A';
printf("%c\n", z);

* See for example section 7.19.6.1 p9 of the C99 standard:

If any argument is
not the correct type for the corresponding conversion specification, the behavior is
undefined.

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