This section describes miscellaneous conversions for printf.
The ‘%c’ conversion prints a single character. In case there is no
‘l’ modifier the int argument is first converted to an
unsigned char. Then, if used in a wide stream function, the
character is converted into the corresponding wide character. The
‘-’ flag can be used to specify left-justification in the field,
but no other flags are defined, and no precision or type modifier can be
given. For example:
printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');
prints ‘hello’.
If there is an ‘l’ modifier present the argument is expected to be
of type wint_t. If used in a multibyte function the wide
character is converted into a multibyte character before being added to
the output. In this case more than one output byte can be produced.
The ‘%s’ conversion prints a string. If no ‘l’ modifier is
present the corresponding argument must be of type char * (or
const char *). If used in a wide stream function the string is
first converted to a wide character string. A precision can be
specified to indicate the maximum number of characters to write;
otherwise characters in the string up to but not including the
terminating null character are written to the output stream. The
‘-’ flag can be used to specify left-justification in the field,
but no other flags or type modifiers are defined for this conversion.
For example:
printf ("%3s%-6s", "no", "where");
prints ‘ nowhere ’.
If there is an ‘l’ modifier present, the argument is expected to
be of type wchar_t (or const wchar_t *).
If you accidentally pass a null pointer as the argument for a ‘%s’ conversion, the GNU C Library prints it as ‘(null)’. We think this is more useful than crashing. But it’s not good practice to pass a null argument intentionally.
The ‘%m’ conversion prints the string corresponding to the error
code in errno. See Error Messages. Thus:
fprintf (stderr, "can't open `%s': %m\n", filename);
is equivalent to:
fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));
The ‘%m’ conversion can be used with the ‘#’ flag to print an
error constant, as provided by strerrorname_np. Both ‘%m’
and ‘%#m’ are GNU C Library extensions.
The ‘%p’ conversion prints a pointer value. The corresponding
argument must be of type void *. In practice, you can use any
type of pointer.
In the GNU C Library, non-null pointers are printed as unsigned integers, as if a ‘%#x’ conversion were used. Null pointers print as ‘(nil)’. (Pointers might print differently in other systems.)
For example:
printf ("%p", "testing");
prints ‘0x’ followed by a hexadecimal number—the address of the
string constant "testing". It does not print the word
‘testing’.
You can supply the ‘-’ flag with the ‘%p’ conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.
The ‘%n’ conversion is unlike any of the other output conversions.
It uses an argument which must be a pointer to an int, but
instead of printing anything it stores the number of characters printed
so far by this call at that location. The ‘h’ and ‘l’ type
modifiers are permitted to specify that the argument is of type
short int * or long int * instead of int *, but no
flags, field width, or precision are permitted.
For example,
int nchar;
printf ("%d %s%n\n", 3, "bears", &nchar);
prints:
3 bears
and sets nchar to 7, because ‘3 bears’ is seven
characters.
The ‘%%’ conversion prints a literal ‘%’ character. This conversion doesn’t use an argument, and no flags, field width, precision, or type modifiers are permitted.