Actually, you can use a literal 0 anyplace you would use NULL.
Section 6.3.2.3p3 of the C standard states:
An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
And section 7.19p3 states:
The macros are:
NULLwhich expands to an implementation-defined null pointer constant
So 0 qualifies as a null pointer constant, as does (void *)0 and NULL. The use of NULL is preferred however as it makes it more evident to the reader that a null pointer is being used and not the integer value 0.
How do I print out the null value in a string?
Why is there a NULL in the C language? - Stack Overflow
string - Writing NULL char to file in C - Stack Overflow
Old compilers and NULL
I learned in C that a string ends with a null value, "\0". How do I print out this null value in C?
I tried doing this by scanning the string "paint". However, it doesn't seem to work -
```
#include <stdio.h>
int main() {
char name[100];
scanf("%s", name);
printf("The name is %c", name[5]);
}
```
This is my output -
```
paint
The name is some weird symbol looking like 0
Process finished with exit code 0
```
Actually, you can use a literal 0 anyplace you would use NULL.
Section 6.3.2.3p3 of the C standard states:
An integer constant expression with the value 0, or such an expression cast to type
void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.
And section 7.19p3 states:
The macros are:
NULLwhich expands to an implementation-defined null pointer constant
So 0 qualifies as a null pointer constant, as does (void *)0 and NULL. The use of NULL is preferred however as it makes it more evident to the reader that a null pointer is being used and not the integer value 0.
NULL is used to make it clear it is a pointer type.
Ideally, the C implementation would define NULL as ((void *) 0) or something equivalent, and programmers would always use NULL when they want a null pointer constant.
If this is done, then, when a programmer has, for example, an int *x and accidentally writes *x = NULL;, then the compiler can recognize that a mistake has been made, because the left side of = has type int, and the right side has type void *, and this is not a proper combination for assignment.
In contrast, if the programmer accidentally writes *x = 0; instead of x = 0;, then the compiler cannot recognize this mistake, because the left side has type int, and the right side has type int, and that is a valid combination.
Thus, when NULL is defined well and is used, mistakes are detected earlier.
In particular answer to your question “Is there a context in which just plain literal 0 would not work exactly the same?”:
- In correct code,
NULLand0may be used interchangeably as null pointer constants. 0will function as an integer (non-pointer) constant, butNULLmight not, depending on how the C implementation defines it.- For the purpose of detecting errors,
NULLand0do not work exactly the same; usingNULLwith a good definition serves to help detect some mistakes that using0does not.
The C standard allows 0 to be used for null pointer constants for historic reasons. However, this is not beneficial except for allowing previously written code to compile in compilers using current C standards. New code should avoid using 0 as a null pointer constant.
Don't use fprintf() to write binary data, of course it's going to interpret its formatting string as a string. That's what it does!
Use fwrite(), and open your file in binary mode with "wb".
You can use sizeof to compute the size of the array, no need to hardcode the value:
FILE *picFile = fopen("pic.bmp", "wb");
if(picFile != NULL)
fwrite(bmp1, sizeof bmp1, 1, picFile);
fclose(picFile);
This works because it's in the same scope as the array declaration of bmp1.
The function fprintf() and its relatives are used to format some information and produce a string then write its characters1 into a file or put it on screen or store it into a given array of characters.
Use function fwrite() to write binary data; this function does not interpret the data you give it in any way and just writes the number of bytes you specify into the file.
Try this:
FILE *picFile = fopen("pic.bmp","w");
fwrite(bmp1, sizeof(bmp1), 1, picFile);
fclose(picFile);
(your call to fprintf() was erroneous, anyway)
1
The functions sprintf() and snprintf() (they put the generated string into a provided buffer of characters) copy the entire generated string onto their destination buffer, including the null terminating character.
The functions fprintf() (writes the string into a file) and printf() (puts the string on screen) do not put the null terminating character of the generated string into the output stream.
(Thanks @chux for pointing out that the C strings include the null terminating character.)