This section provides details about the precise syntax of conversion
specifications that can appear in a
Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see Character Set Handling) are permitted in a template string.
The conversion specifications in a
printf template string have
the general form:
% [ param-no $] flags width [ . precision ] type conversion
% [ param-no $] flags width . * [ param-no $] type conversion
For example, in the conversion specifier ‘%-10.8ld’, the ‘-’
is a flag, ‘10’ specifies the field width, the precision is
‘8’, the letter ‘l’ is a type modifier, and ‘d’ specifies
the conversion style. (This particular type specifier says to
long int argument in decimal notation, with a minimum of
8 digits left-justified in a field at least 10 characters wide.)
In more detail, output conversion specifications consist of an initial ‘%’ character followed in sequence by:
printffunction are assigned to the formats in the order of appearance in the format string. But in some situations (such as message translation) this is not desirable and this extension allows an explicit parameter to be specified.
The param-no parts of the format must be integers in the range of 1 to the maximum number of arguments present to the function call. Some implementations limit this number to a certainly upper bound. The exact limit can be retrieved by the following constant.
The value of
NL_ARGMAX is the maximum value allowed for the
specification of a positional parameter in a
printf call. The
actual value in effect at runtime can be retrieved by using
sysconf using the
_SC_NL_ARGMAX parameter see Sysconf Definition.
Some system have a quite low limit such as 9 for System V systems. The GNU C Library has no real limit.
If any of the formats has a specification for the parameter position all of them in the format string shall have one. Otherwise the behavior is undefined.
You can also specify a field width of ‘*’. This means that the
next argument in the argument list (before the actual value to be
printed) is used as the field width. The value must be an
If the value is negative, this means to set the ‘-’ flag (see
below) and to use the absolute value as the field width.
You can also specify a precision of ‘*’. This means that the next
argument in the argument list (before the actual value to be printed) is
used as the precision. The value must be an
int, and is ignored
if it is negative. If you specify ‘*’ for both the field width and
precision, the field width argument precedes the precision argument.
Other C library versions may not recognize this syntax.
int, but you can specify ‘h’, ‘l’, or ‘L’ for other integer types.)
The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.
With the ‘-Wformat’ option, the GNU C compiler checks calls to
printf and related functions. It examines the format string and
verifies that the correct number and types of arguments are supplied.
There is also a GNU C syntax to tell the compiler that a function you
write uses a
printf-style format string.
See Declaring Attributes of Functions in Using GNU CC, for more information.