How to get the string size in bytes?

Question

As the title implies, my question is how to get the size of a string in C. Is it good to use sizeof if I've declared it (the string) in a function without malloc in it? Or, if I've declared it as a pointer? What if I initialized it with malloc? I would like to have an exhaustive response.

Answer 1

You can use strlen. Size is determined by the terminating null-character, so passed string should be valid.

If you want to get size of memory buffer, that contains your string, and you have pointer to it:

• If it is dynamic array(created with malloc), it is impossible to get it size, since compiler doesn't know what pointer is pointing at. (check this)
• If it is static array, you can use sizeof to get its size.

If you are confused about difference between dynamic and static arrays, check this.

Answer 2

Use strlen to get the length of a null-terminated string.

sizeof returns the length of the array not the string. If it's a pointer (char *s), not an array (char s[]), it won't work, since it will return the size of the pointer (usually 4 bytes on 32-bit systems). I believe an array will be passed or returned as a pointer, so you'd lose the ability to use sizeof to check the size of the array.

So, only if the string spans the entire array (e.g. char s[] = "stuff"), would using sizeof for a statically defined array return what you want (and be faster as it wouldn't need to loop through to find the null-terminator) (if the last character is a null-terminator, you will need to subtract 1). If it doesn't span the entire array, it won't return what you want.

An alternative to all this is actually storing the size of the string.

Answer 3

While sizeof works for this specific type of string:

char str[] = "content";
int charcount = sizeof str - 1; // -1 to exclude terminating '\0'

It does not work if str is pointer (sizeof returns size of pointer, usually 4 or 8) or array with specified length (sizeof will return the byte count matching specified length, which for char type are same).

Just use strlen().

Answer 4

If you use sizeof()then a char *str and char str[] will return different answers. char str[] will return the length of the string(including the string terminator) while char *str will return the size of the pointer(differs as per compiler).

Answer 5

I like to use:

(strlen(string) + 1 ) * sizeof(char)

This will give you the buffer size in bytes. You can use this with snprintf() may help:

const char* message = "%s, World!";
char* string = (char*)malloc((strlen(message)+1))*sizeof(char));
snprintf(string, (strlen(message)+1))*sizeof(char), message, "Hello");

Cheers! Function: size_t strlen (const char *s)

Answer 6

There are two ways of finding the string size bytes:

1st Solution:

# include <iostream>
# include <cctype>
# include <cstring>
using namespace std;

int main()
{
    char str[] = {"A lonely day."};
    cout<<"The string bytes for str[] is: "<<strlen(str);
    return 0;
}

2nd Solution:

# include <iostream>
# include <cstring>
using namespace std;

int main()
{
    char str[] = {"A lonely day."};
    cout<<"The string bytes for str[] is: "<<sizeof(str);
    return 0;
}

Both solution produces different outputs. I will explain it to you after you read these.

The 1st solution uses strlen and based on cplusplus.com,

The length of a C string is determined by the terminating null-character: A C string is as long as the number of characters between the beginning of the string and the terminating null character (without including the terminating null character itself).

That can explain why does the 1st Solution prints out the correct string size bytes when the 2nd Solution prints the wrong string size bytes. But if you still don't understand, then continue reading.

The 2nd Solution uses sizeof to find out the string size bytes. Based on this SO answer, it says (modified it):

sizeof("f") must return 2 string size bytes, one for the 'f' and one for the terminating '\0' (terminating null-character).

That is why the output is string size bytes 14. One for the whole string and one for '\0'.

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Conclusion:

To get the correct answer for 2nd Solution, you must do sizeof(str)-1.

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References:

1. Sizeof string literal
2. https://cplusplus.com/reference/cstring/strlen/?kw=strlen