This section provides details about the precise syntax of conversion
specifications that can appear in a printf
template
string.
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 section 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
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
print a 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:
printf
function 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 part of the format must be an integer 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.
ARGMAX
is the maximum value allowed for the
specification of an 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 section Definition of sysconf
.
Some system have a quite low limit such as @math{9} for System V
systems. The GNU C library has no real limit.
int
.
If the value is negative, this means to set the `-' flag (see
below) and to use the absolute value as the field width.
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 section `Declaring Attributes of Functions' in Using GNU CC, for more information.
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