Printf format string maximum width value (padding) %(??)d%n

user_11077979 0
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What is the maximum width value I can put in the %d format specifier? For example:

int c=0;   
printf("%1234567899d%n",0,&c);
printf("%d",c);

When I use large values the written value of c is 0. Why is that?

printf c
2 Answers

Although it's not stated explicitly, the type of the field width is most likely an int. This is because if the field width is given as * then an int argument is expected.

Section 7.21.6.1p5 of the C standard regarding the fprintf function (any by extension printf) states the following regarding field witdh:

As noted above, a field width, or precision, or both, may be indicated by an asterisk. In this case, an int argument supplies the field width or precision. The arguments specifying field width, or precision, or both, shall appear (in that order) before the argument (if any) to be converted. A negative field width argument is taken as a - flag followed by a positive field width. A neg ative precision argument is taken as if the precision were omitted.

I tested this on CentOS 7 and Ubuntu 18, and the largest width I could specify was 2147483614 which is 33 less than the max value for a signed 32 bit int. If I use anything larger the first printf prints nothing and c remains 0.

Go generally speaking, the largest value you can expect for c would be INT_MAX, however the exact value will vary based on the implementation.

user_16871190

What is the maximum width value I can put in the %d format specifier?

The limit may be as small as 4095.

Aside from issues with specifying a width outside the positive int range, long output can incur environmental limits.

Environmental limits (fprintf)
The number of characters that can be produced by any single conversion shall be at least 4095. C17dr § 7.21.6.1 15

As I see this, an attempt to "%(some large value)d" print more than 4K (or some greater implementation limit) characters is UB.

Also for output to a text file:

Environmental limits
An implementation shall support text files with lines containing at least 254 characters, including the terminating new-line character. The value of the macro BUFSIZ shall be at least 256. C17dr § 7.21.2 9

Although stdout with printf() is not a file, with re-directed output and freopen(), printf() may be subject to the above constraint.

Then printing an int with many digits can form a line exceeding 254 and some weak file system may not handle long lines.

Such a small 254 limit for a line length I have never seen, so I only include it as a reference, yet do not see it as a modern true limitation.

user_24103590
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