The "sizeof operator with string" bug when use command line in "output window"

There's no such thing as a string data type in C. The standard library supports a type, which happens to be built on the types "char" and "pointer to char". There's also a "string literal" useful for initializing arrays of char. But "string" is not really a first-class data type in the language.

sizeof returns the size (in units of the size of a char, which is not necessarily one byte, though that's extremely common) of the type of the expression to which it is applied. Note that it's technically an operator, not a function, even though almost everyone writes their code as though it were a function. The parens are there from habit, but also so you don't have to worry about the precedence of sizeof relative to the other operators in your intended expression.

sizeof("a") returns the size of a pointer to char. Depending on the memory model, this might actually be one byte long. A pointer-to-data or pointer-to-pdata is one byte. Pointers to xdata are two. "Generic" pointers (to unknown memory type, so they're type-tagged) or pointers to far memory are three bytes long.

sizeof('a') is the size of the type of 'a'. 'a' is more formally known as an "integer character constant", and -- oddly enough -- has type "int" and not type "char". So, sizeof('a') is the same as sizeof(int), which is 2 for Keil C51.

If you want to count the number of characters in a string, use the library function strlen().

"sizeof("a") returns the size of a pointer to char."

From C99 6.3.2.1 par. 3:

"Except when it is the operand of the sizeof operator or the unary & operator, or is a string literal used to initialize an array, an expression that has type "array of type" is converted to an expression with type "pointer to type" that points to the initial element of the array object and is not an lvalue."

For example:

t.c:
#include <stdio.h>

int main(void)
{
    printf("sizeof(\"1234\") = %u\n", (unsigned)sizeof("1234"));
    printf("sizeof \"1234\"  = %u\n", (unsigned)sizeof "1234" );
    return 0;
}

$ gcc -Wall -ansi -pedantic t.c

$ ./a
sizeof("1234") = 5
sizeof "1234"  = 5

So a string literal has type "array of char" and not "pointer to char"? Thanks for the correction.

"So a string literal has type "array of char" and not "pointer to char"?"

Right. I sometimes use sizeof with a string literal argument in the following context to size a buffer not too big, but just right, along with providing some bit of documentation as to how something will end up being formatted.

char buf[sizeof "Temp: -ttt  Hum: hh%"];

sprintf(buf, "Temp: %4d  Hum: %2u%%", (int)temperature, (unsigned)humidity);

What I want to say is "sizeof operator with string or char in command line (which in the uvision3's debug mode)returns wrong value"
you can try it in uvision3.

int main(void)
{
    printf("sizeof(\"1234\") = %u\n", (unsigned)sizeof("1234"));
}

bulid an exe file,The program works fine.
BUT IN DEBUG mode ( when you press "start debug session" buttom,click the "command" option which in the "outputwindow" and key in such as "sizeof("1234")" )it would return 4