printf fromat

%[flags][min field width][precision][length]conversion specifier
  -----  ---------------  ---------  ------ -------------------
   \             #,*        .#, .*     /             \
    \                                 /               \
   #,0,-,+, ,',I                 hh,h,l,ll,j,z,L    c,d,u,x,X,e,f,g,s,p,%
   -------------                 ---------------    -----------------------
   # | Alternate,                 hh | char,           c | unsigned char,
   0 | zero pad,                   h | short,          d | signed int,
   - | left align,                 l | long,           u | unsigned int,
   + | explicit + - sign,         ll | long long,      x | unsigned hex int,
     | space for + sign,           j | [u]intmax_t,    X | unsigned HEX int,
   ' | locale thousands grouping,  z | size_t,         e | [-]d.ddde±dd double,
   I | Use locale's alt digits     t | ptrdiff_t,      E | [-]d.dddE±dd double,
                                   L | long double,  ---------=====
   if no precision   => 6 decimal places            /  f | [-]d.ddd double,
   if precision = 0  => 0 decimal places      _____/   g | e|f as appropriate,
   if precision = #  => # decimal places               G | E|F as appropriate,
   if flag = #       => always show decimal point      s | string,
                                             ..............------
                                               /          p | pointer,
   if precision      => max field width    /           % | %

Examples of common combinations:

format

output

printf("%08X",32_bit_var);

0000ABCD

printf("%lu",32_bit_var);

43981

printf("%'d",32_bit_var);

43,981

printf("%10s","string");

string

printf("%*s",10,"string");

string

printf("%-10s","string");

string

printf("%-10.10s","truncateiftoolong");

truncateif

REF: http://www.pixelbeat.org/programming/gcc/format_specs.html