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7.8 A dedicated function to format numbers

We have seen that the structure returned by localeconv as well as the values given to nl_langinfo allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts. We have also seen that the underlying rules are quite complex.

Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules.

Function: ssize_t strfmon (char *s, size_t maxsize, const char *format, …)

Preliminary: | MT-Safe locale | AS-Unsafe heap | AC-Unsafe mem | See POSIX Safety Concepts.

The strfmon function is similar to the strftime function in that it takes a buffer, its size, a format string, and values to write into the buffer as text in a form specified by the format string. Like strftime, the function also returns the number of bytes written into the buffer.

There are two differences: strfmon can take more than one argument, and, of course, the format specification is different. Like strftime, the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a ‘%’. Immediately after the ‘%’, you can optionally specify various flags and formatting information before the main formatting character, in a similar way to printf:

  • Immediately following the ‘%’ there can be one or more of the following flags:

    The single byte character f is used for this field as the numeric fill character. By default this character is a space character. Filling with this character is only performed if a left precision is specified. It is not just to fill to the given field width.


    The number is printed without grouping the digits according to the rules of the current locale. By default grouping is enabled.

    +’, ‘(

    At most one of these flags can be used. They select which format to represent the sign of a currency amount. By default, and if ‘+’ is given, the locale equivalent of +/- is used. If ‘(’ is given, negative amounts are enclosed in parentheses. The exact format is determined by the values of the LC_MONETARY category of the locale selected at program runtime.


    The output will not contain the currency symbol.


    The output will be formatted left-justified instead of right-justified if it does not fill the entire field width.

The next part of the specification is an optional field width. If no width is specified 0 is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the ‘-’ flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa.

So far the format looks familiar, being similar to the printf and strftime formats. However, the next two optional fields introduce something new. The first one is a ‘#’ character followed by a decimal digit string. The value of the digit string specifies the number of digit positions to the left of the decimal point (or equivalent). This does not include the grouping character when the ‘^’ flag is not given. If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the ‘=’ flag and by default is a space. For example, if the field width is selected as 6 and the number is 123, the fill character is ‘*’ the result will be ‘***123’.

The second optional field starts with a ‘.’ (period) and consists of another decimal digit string. Its value describes the number of characters printed after the decimal point. The default is selected from the current locale (frac_digits, int_frac_digits, see see Generic Numeric Formatting Parameters). If the exact representation needs more digits than given by the field width, the displayed value is rounded. If the number of fractional digits is selected to be zero, no decimal point is printed.

As a GNU extension, the strfmon implementation in the GNU C Library allows an optional ‘L’ next as a format modifier. If this modifier is given, the argument is expected to be a long double instead of a double value.

Finally, the last component is a format specifier. There are three specifiers defined:


Use the locale’s rules for formatting an international currency value.


Use the locale’s rules for formatting a national currency value.


Place a ‘%’ in the output. There must be no flag, width specifier or modifier given, only ‘%%’ is allowed.

As for printf, the function reads the format string from left to right and uses the values passed to the function following the format string. The values are expected to be either of type double or long double, depending on the presence of the modifier ‘L’. The result is stored in the buffer pointed to by s. At most maxsize characters are stored.

The return value of the function is the number of characters stored in s, including the terminating NULL byte. If the number of characters stored would exceed maxsize, the function returns -1 and the content of the buffer s is unspecified. In this case errno is set to E2BIG.

A few examples should make clear how the function works. It is assumed that all the following pieces of code are executed in a program which uses the USA locale (en_US). The simplest form of the format is this:

strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);

The output produced is


We can notice several things here. First, the widths of the output numbers are different. We have not specified a width in the format string, and so this is no wonder. Second, the third number is printed using thousands separators. The thousands separator for the en_US locale is a comma. The number is also rounded. .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2. Finally, note that the national currency symbol is printed since ‘%n’ was used, not ‘i’. The next example shows how we can align the output.

strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);

The output this time is:

"@    $123.45@   -$567.89@ $12,345.68@"

Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a ‘#’ modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification.

strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
         123.45, -567.89, 12345.678);

The output is

"@ $***123.45@-$***567.89@ $12,456.68@"

Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be 5 and 123.45 has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality.

strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
         123.45, -567.89, 12345.678);

This rather complex format string produces the following output:

"@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"

The most noticeable change is the alternative way of representing negative numbers. In financial circles this is often done using parentheses, and this is what the ‘(’ flag selected. The fill character is now ‘0’. Note that this ‘0’ character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator. Since we used the format specifier ‘i’ instead of ‘n’, the international form of the currency symbol is used. This is a four letter string, in this case "USD ". The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding.

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