Scanf Printf double variable C

Question

Let s say I have this following bit of code in C   double var  scanf   lf    amp var   printf   lf   var   printf   f   var     It reads from stdin variable  var  and then prints twice in stdout  var   I understand that s how you read a double variable from stdin  but my questions are    Why can you print a double with  lf  Why can you print a double with  f  Which one is better and correct to use

User · Accepted Answer

For variable argument functions like printf and scanf  the arguments are promoted  for example  any smaller integer types are promoted to int  float is promoted to double  scanf takes parameters of pointers  so the promotion rule takes no effect  It must use  f for float  and  lf for double   printf will never see a float argument  float is always promoted to double  The format specifier is  f  But C99 also says  lf is the same as  f in printf   C99   7 19 6 1 The fprintf function l  ell  Specifies that a following d  i  o  u  x  or X conversion specifier applies to a long int or unsigned long int argument  that a following n conversion specifier applies to a pointer to a long int argument  that a following c conversion specifier applies to a wint t argument  that a following s conversion specifier applies to a pointer to a wchar t argument  or has no effect on a following a  A  e  E  f  F  g  or G conversion specifier

User · Answer

As far as I read manual pages  scanf says that  l  length modifier indicates  in case of floating points  that the argument is of type double rather than of type float  so you can have  lf  le  lg    As for printing  officially  the manual says that  l  applies only to integer types   So it might be not supported on some systems or by some standards   For instance  I get the following error message when compiling with gcc -Wall -Wextra -pedantic  a c 6 1  warning  ISO C90 does not support the     lf    gnu printf format  -Wformat     So you may want to doublecheck if your standard supports the syntax   To conclude  I would say that you read with   lf  and you print with   f

User · Answer

When a float is passed to printf  it is automatically converted to a double  This is part of the default argument promotions  which apply to functions that have a variable parameter list  containing       largely for historical reasons  Therefore  the    natural    specifier for a float   f  must work with a double argument  So the  f and  lf specifiers for printf are the same  they both take a double value   When scanf is called  pointers are passed  not direct values  A pointer to float is not converted to a pointer to double  this could not work since the pointed-to object cannot change when you change the pointer type   So  for scanf  the argument for  f must be a pointer to float  and the argument for  lf must be a pointer to double

[c] Scanf/Printf double variable C

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