This question already has answers here:
Why does printf("%f",0); give undefined behavior?
(10 answers)
Closed 2 years ago.
printf("%f", 1.0); //prints 1.0
but
printf("%f", 1); // prints 0.0
How did the conversion happen?
printf("%f", 1); causes undefined behavior, because double is expected, but you passed an int. There is no explanation of why it prints 0.0, because the behavior is undefined.
As per the below #Eric Postpischil's comment different.
The first double argument (or float argument, which will be promoted to double if used with the ... part of a function) is put in %xmm0. The first “normal integer class” type of argument would go into %rdi. For printf, though, that pointer to the format string is the first argument of that class, so it does into %rdi. That means the first int argument passed goes into the second register for that class, which is %rsi. So, with printf("%f", 1);, printf looks for the floating-point value in %xmm0, but the caller puts an int 1 in %rsi
Not every compiler behaves like this, some actually print 1.0. But when instruct printf to print a double value, you must pass it a double value, not an integer. You can always use a type cast:
printf("%f", (double)1);
The question is not about printf function itself, the question is if the compiler is smart enough. If your compiler is not smart enough, then it treats printf as just a normal function call and does not know anything about the syntax of arguments for this function. So it just puts a string and an integer number on the stack and calls the function. The printf function takes the first argument and starts to parse it as a format string. When it sees format specifier %f it attempts to interpret the corresponding part of the memory at the stack as a floating point number. It has no way to know that compilator pushed int value there before. So printf does it best to interpret the memory as a floating point number. The result is platform dependent, i.e. on endiness and float/int sizes, and also includes randomness, because you'll most probably hit some garbage on the stack. The transformation done by printf in this case can be seen also like this:
int i = 1; // Integer variable
int* pi = &i; // Pointer to i
float* pf = (float*)pi; // Reinterpret the pointer as floating point number address
float f = *pf; // Get the floating point from this address
printf("%f\n", f);
The thing here printf() will except to receive float based on the format you passed in, to print int as float in printf() you have to cast it
printf("%f", (float)1);
or
printf("%f",(double)1);
because C will treat the variables passed to printf() based on their types and memory representation and you pass the wrong value it will result in undefined behavior.
Related
void main()
{
clrscr();
float f = 3.3;
/* In printf() I intentionaly put %d format specifier to see
what type of output I may get */
printf("value of variable a is: %d", f);
getch();
}
In effect, %d tells printf to look in a certain place for an integer argument. But you passed a float argument, which is put in a different place. The C standard does not specify what happens when you do this. In this case, it may be there was a zero in the place printf looked for an integer argument, so it printed “0”. In other circumstances, something different may happen.
Using an invalid format specifier to printf invokes undefined behavior. This is specified in section 7.21.6.1p9 of the C standard:
If a conversion specification is invalid, the behavior is
undefined.282) If any argument is not the correct type for the
corresponding conversion specification, the behavior is undefined.
What this means is that you can't reliably predict what the output of the program will be. For example, the same code on my system prints -1554224520 as the value.
As to what's most likely happening, the %d format specifier is looking for an int as a parameter. Assuming that an int is passed on the stack and that an int is 4 bytes long, the printf function looks at the next 4 bytes on the stack for the value given. Many implementations don't pass floating point values on the stack but in registers instead, so it instead sees whatever garbage values happen to be there. Even if a float is passed on the stack, a float and an int have very different representations, so printing the bytes of a float as an int will most likely not give you the same value.
Let's look at a different example for a moment. Suppose I write
#include <string.h>
char buf[10];
float f = 3.3;
memset(buf, 'x', f);
The third argument to memset is supposed to be an integer (actually a value of type size_t) telling memset how many characters of buf to set to 'x'. But I passed a float value instead. What happens? Well, the compiler knows that the third argument is supposed to be an integer, so it automatically performs the appropriate conversion, and the code ends up setting the first three (three point zero) characters of buf to 'x'.
(Significantly, the way the compiler knew that the third argument of memset was supposed to be an integer was based on the prototype function declaration for memset which is part of the header <string.h>.)
Now, when you called
printf("value of variable f is: %d", f);
you might think the same thing happens. You passed a float, but %d expects an int, so an automatic conversion will happen, right?
Wrong. Let me say that again: Wrong.
The perhaps surprising fact is, printf is different. printf is special. The compiler can't necessarily know what the right types of the arguments passed to printf are supposed to be, because it depends on the details of the %-specifiers buried in the format string. So there are no automatic conversions to just the right type. It's your job to make sure that the types of the arguments you actually pass are exactly right for the format specifiers. If they don't match, the compiler does not automatically perform corresponding conversions. If they don't match, what happens is that you get crazy, wrong results.
(What does the prototype function declaration for printf look like? It literally looks like this: extern int printf(const char *, ...);. Those three dots ... indicate a variable-length argument list or "varargs", they tell the compiler it can't know how many more arguments there are, or what their types are supposed to be. So the compiler performs a few basic conversions -- such as upconverting types char and short int to int, and float to double -- and leaves it at that.)
I said "The compiler can't necessarily know what the right types of the arguments passed to printf are supposed to be", but these days, good compilers go the extra mile and try to figure it out anyway, if they can. They still won't perform automatic conversions (they're not really allowed to, by the rules of the language), but they can at least warn you. For example, I tried your code under two different compilers. Both said something along the lines of warning: format specifies type 'int' but the argument has type 'float'. If your compiler isn't giving you warnings like these, I encourage you to find out if those warnings can be enabled, or consider switching to a better compiler.
Try
printf("... %f",f);
That's how you print float numbers.
Maybe you only want to print x digits of f, eg.:
printf("... %.3f" f);
That will print your float number with 3 digits after the dot.
Please read through this list:
%c - Character
%d or %i - Signed decimal integer
%e - Scientific notation (mantissa/exponent) using e character
%E - Scientific notation (mantissa/exponent) using E character
%f - Decimal floating point
%g - Uses the shorter of %e or %f
%G - Uses the shorter of %E or %f
%o - Signed octal
%s - String of characters
%u - Unsigned decimal integer
%x - Unsigned hexadecimal integer
%X - Unsigned hexadecimal integer (capital letters)
%p - Pointer address
%n - Nothing printed
The code is printing a 0, because you are using the format tag %d, which represents Signed decimal integer (http://devdocs.io).
Could you please try
void main() {
clrscr();
float f=3.3;
/* In printf() I intentionaly put %d format specifier to see what type of output I may get */
printf("value of variable a is: %f",f);
getch();
}
This question already has answers here:
What happens when I use the wrong format specifier?
(2 answers)
Correct format specifier to print pointer or address?
(5 answers)
Closed 4 years ago.
As you can see in the program, The first output is 6356744 and the second output is 0060FF08, why is it different? Is the %d typecasting it into an integer, if so, how?
#include<stdio.h>
int main()
{
int *a;
int b = 7;
a = &b;
printf(" The value of a = %d",a);
printf("\n The value of a= %p",a);
}
Printing a pointer with %d is formally undefined behavior, meaning anything can happen, including a program crash. Your program will for example likely break when you compile it as a 64 bit application, where int is 32 bits but a pointer is likely 64 bits. Therefore, always use %p and never anything else when printing a pointer.
There is no implicit conversion taking place - the printf family of functions doesn't have that kind of intelligence - it doesn't know the type passed. With the format specifier, you tell the function which type it is getting. And if you lie to printf and say "I'll give you an int" and then give it a pointer, you unleash bugs. This makes the printf family of functions very dangerous in general.
(The only implicit conversion that take place in printf is when you pass small integer types or float, in which case the "default argument promotions" take place and promote the parameter either to int or double. This is not the case here, however.)
In this specific case, you happened to get the decimal representation of 0x0060FF08, which is by no means guaranteed.
Pedantically, you should also cast the pointer to type (void*) since this is what %p expects.
%p prints a pointer and it's not necessarily hexadecimal, or even a number
If format specifiers do not match the datatype of the provided parameter, you yield undefined behaviour. %d expects an integral value, such that when you pass a pointer value, you get undefined behaviour (cf., for example, cppreference.com-printf):
...If any argument after default argument promotions is not the type
expected by the corresponding conversion specifier, or if there are
fewer arguments than required by format, the behavior is undefined.
The (only) correct format specifier for printing pointer values is %p, usually printing the address in hex format.
One of the undefined behaviours is that %d takes the pointer value as an 32/64 bit integral value and hence prints decimals, which - if you printed it using the correct %p-format - corresponds to the hex-value of the address (yet in decimal format).
%d prints an int, in decimal.
%p prints a pointer (strictly speaking, a void * pointer) in an implementation-defined way, typically in hexadecimal.
On a machine where ints and pointers have different sizes, trying to print a pointer using %d will typically give a meaningless result. For example, on a lot of machines these days, ints are 32 bits while pointers are 64 bits. So if you try to print a pointer using %d, what you might get is half the pointer value, in decimal.
(Strictly speaking, trying to print a pointer using %d is undefined, no matter what the relative sizes are.)
Bottom line, use the right printf specifier for the job. Use %d to print ints. Use %p to print pointers. Don't try to mix 'n' match.
I tried to print character as a float in printf and got output 0. What is the reason for this.
Also:
char c='z';
printf("%f %X",c,c);
is giving some weird output for hexadecimal while output is correct when I do this:
printf("%X",c);
why is it so?
The printf() function is a variadic function, which means that you can pass a variable number of arguments of unspecified types to it. This also means that the compiler doesn't know what type of arguments the function expects, and so it cannot convert the arguments to the correct types. (Modern compilers can warn you if you get the arguments wrong to printf, if you invoke it with enough warning flags.)
For historical reasons, you can not pass an integer argument of smaller rank than int, or a floating type of smaller rank than double to a variadic function. A float will be converted to double and a char will be converted to int (or unsigned int on bizarre implementations) through a process called the default argument promotions.
When printf parses its parameters (arguments are passed to a function, parameters are what the function receives), it retrieves them using whatever method is appropriate for the type specified by the format string. The "%f" specifier expects a double. The "%X" specifier expects an unsigned int.
If you pass an int and printf tries to retrieve a double, you invoke undefined behaviour.
If you pass an int and printf tries to retrieve an unsigned int, you invoke undefined behaviour.
Undefined behaviour may include (but is not limited to) printing strange values, crashing your program or (the most insidious of them all) doing exactly what you expect.
Source: n1570 (The final public draft of the current C standard)
You need to use a cast operator like this:
char c = 'z';
printf("%f %X", (float)c, c);
or
printf("%f %X", (double)c, c);
In Xcode, if I do not do this, I get the warning:
Format specifies specifies 'double' but the argument has type 'char', and the output is 0.000000.
I tried to print character as a float in printf and got output 0. What is the reason for this.
The question is, what value did you expect to see? Why would you expect something other than 0?
The short answer to your question is that the behavior of printf is undefined if the type of the argument doesn't match the conversion specifier. The %f conversion specifier expects its corresponding argument to have type double; if it isn't, all bets are off, and the exact output will vary.
To understand the floating point issue, consider reading: http://en.wikipedia.org/wiki/IEEE_floating_point
As for hexadecimal, let me guess.. the output was something like... 99?
This is because of encodings.. the machine has to represent information in some format, and usually that format entails either giving meanings to certain bits in a number, or having a table of symbols to numbers, or both
Floating points are sometimes represented as a (sign,mantissa,exponent) triplet all packed in a 32 or 64 bit number - characters are sometimes represented in a format named ASCII, which establishes which number corresponds to each character you type
Because printf, like any function that work with varargs, eg: int foobar(const char fmt, ...) {} tries to interpret its parameter to certain type.
If you say "%f", then pass c (as a char), then printf will try to read a float.
You can read more here: var_arg (even if this is C++, it still applies).
This question already has answers here:
float to int unexpected behaviour
(6 answers)
Closed 6 years ago.
here are part of my code.
float a = 12.5;
printf("%d\n", a);
printf("%d\n", (int)a);
printf("%d\n", *(int *)&a);
when I compile in windows, I got:
0
12
1094713344
and then, I compile in linux, I got:
-1437851864
12
1094713344
-1437851864 will be changed every time I excuted it.
my question is: in how does the "printf" function works in linux
It works very well, but why are you passing the wrong sort of data to it? The %d specifier expects and int, but you're passing something else. Bad idea.
If float and int are differently sized across the varargs barrier, this is undefined behavior. And since float is typically promoted to double with varargs calls, if your int is smaller than your double this will break.
In short, this is really bad and broken code. Don't do this.
To print a floating point number in C, you should do:
float a = 12.5;
printf("%f\n", a);
As has been mentioned, passing arguments with types not matching the format string invokes undefined behaviour, so the language standard doesn't place any restrictions on what
float a = 12.5;
printf("%d\n", a);
actually does.
To find out what it does, you'd need to analyse your implementation, or at least the assembly the compiler produced for that code.
A common way for translating that code is to pass the promoted (to double) float argument in a floating point register and tell printf how many arguments are passed in floating point registers. But since the format tells printf to look for an int, it doesn't look in a floating point register for it, but in another register. So the printed value would be whatever happened to be in that register when printf was called.
int main()
{
int x;
float y;
char c;
x = -4443;
y = 24.25;
c = 'M';
printf("\nThe value of integer variable x is %f", (float)x);
printf("\nThe value of float variable y is %d", y);
printf("\nThe value of character variable c is %f\n",c);
return 0;
}
Output:
The value of integer variable x is -4443.000000
The value of float variable y is 0
The value of character variable c is 24.250000
Why am I not getting the expected output?
But when I am using external casting I am getting expected output which is:
The value of integer variable x is -4443.000000
The value of float variable y is 24
The value of character variable c is 77.000000
why i am not getting the expected output ?
Short answer: Because your expectations are wrong.
You're instructing the compiler to read an integer from where y is. Which is wrong. Format specifier don't tell the compiler to do casts, just what type to expect, and trust you to provide the right type.
The behaviour can be due to the fact that, for example, a float is stored in 8 bytes. The high-order bytes will be 0 in this case. But an int is stored in 4 bytes. So you tell the compiler read the int from where y is, it reads the first 4 bytes, which are 0, and prints 0...
EDIT: As John pointed out in the comments, this is UB, which means that anything can happen:
7.21.6.1/9
If a conversion specification is invalid, the behavior is undefined.282) If any argument is not the correct type for the corresponding conversion specification, the behavior is undefined.
Many computing platforms pass different types of arguments in different ways. On some platforms, floating-point arguments are passed in special floating-point registers. On most platforms, integer arguments are passed in general processor registers. Large arguments, such as structures, are stored somewhere in memory, and a pointer is passed instead (invisibly to the C source code). Once the few registers available for arguments are used, the remaining arguments are typically passed on the stack.
When you call printf, the compiler does not match the arguments you pass to the conversion specifiers in the format string. (Except that a good compiler will check and issue a warning if the types do not match.) In order to operate, the printf routine reads the format string and, when it finds a conversion specifier, it reads data from where the corresponding argument should be. If you specify “%d” but pass a float, the printf routine may read data from a general processor register, but the float value is in a floating-point register. Therefore, the value that is printed will be whatever data happened to be in the general processor register.
Similarly, when you specify “%f” but pass an integer, the printf routine may read from a floating-point register, but the integer value is in a general processor register.
The compiler will not convert printf arguments to the target type and might not warn you about the mismatches. You must match the conversion specifiers in the format string to the argument types.
Bonus: Here are documents describing how arguments are passed to subroutines on one platform (Mac OS X).
You cannot format a char as a float "%f", use "%c" or "%d" instead. I find that http://www.cplusplus.com/reference/clibrary/cstdio/printf/ is a good reference.
The format specifiers and the types of the arguments don't match, which I believe causes undefined behavior. printf doesn't do casting for you, so you have to explicitly cast the arguments.