11.7 Formatting strings (printf-like)

Formatted output can be made with format:

Builtin: format (format-string, …)

Works much like the C function printf. The first argument format-string can contain ‘%’ specifications which are satisfied by additional arguments, and the expansion of format is the formatted string.

The macro format is recognized only with parameters.

Its use is best described by a few examples:

define(`foo', `The brown fox jumped over the lazy dog')
⇒
format(`The string "%s" uses %d characters', foo, len(foo))
⇒The string "The brown fox jumped over the lazy dog" uses 38 characters
format(`%*.*d', `-1', `-1', `1')
⇒1
format(`%.0f', `56789.9876')
⇒56790
len(format(`%-*X', `5000', `1'))
⇒5000
ifelse(format(`%010F', `infinity'), `       INF', `success',
       format(`%010F', `infinity'), `  INFINITY', `success',
       format(`%010F', `infinity'))
⇒success
ifelse(format(`%.1A', `1.999'), `0X1.0P+1', `success',
       format(`%.1A', `1.999'), `0X2.0P+0', `success',
       format(`%.1A', `1.999'))
⇒success
format(`%g', `0xa.P+1')
⇒20

Using the forloop macro defined earlier (see Forloop), this example shows how format can be used to produce tabular output.

$ m4 -I examples
include(`forloop.m4')
⇒
forloop(`i', `1', `10', `format(`%6d squared is %10d
', i, eval(i**2))')
⇒     1 squared is          1
⇒     2 squared is          4
⇒     3 squared is          9
⇒     4 squared is         16
⇒     5 squared is         25
⇒     6 squared is         36
⇒     7 squared is         49
⇒     8 squared is         64
⇒     9 squared is         81
⇒    10 squared is        100
⇒

The builtin format is modeled after the ANSI C ‘printf’ function, and supports these ‘%’ specifiers: ‘c’, ‘s’, ‘d’, ‘o’, ‘x’, ‘X’, ‘u’, ‘a’, ‘A’, ‘e’, ‘E’, ‘f’, ‘F’, ‘g’, ‘G’, and ‘%’; it supports field widths and precisions, and the flags ‘+’, ‘-’, ‘ ’, ‘0’, ‘#’, and ‘'’. For integer specifiers, the width modifiers ‘hh’, ‘h’, and ‘l’ are recognized, and for floating point specifiers, the width modifier ‘l’ is recognized. Items not yet supported include positional arguments, the ‘n’, ‘p’, ‘S’, and ‘C’ specifiers, the ‘z’, ‘t’, ‘j’, ‘L’ and ‘ll’ modifiers, and any platform extensions available in the native printf. For more details on the functioning of printf, see the C Library Manual, or the POSIX specification (for example, ‘%a’ is supported even on platforms that haven’t yet implemented C99 hexadecimal floating point output natively).

Unrecognized specifiers result in a warning. It is anticipated that a future release of GNU m4 will support more specifiers, and give better warnings when various problems such as overflow are encountered. Likewise, escape sequences are not yet recognized.

format(`%p', `0')
error→m4:stdin:1: Warning: unrecognized specifier in `%p'
⇒