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cformat-specifiers

What happens when I use the wrong format specifier?

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

For example:

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

x = 65;
printf("%c\n", x);
printf("%d\n", x);

x = 128;
printf("%d\n", x);
Solution

  • 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.