Why does using the wrong format specifier in C crash my program on Windows 7?

My program is as follows;

#include <stdio.h>
#include <string.h>

int main()
{
        char string[] = "Gentlemen start your engines!";
        printf("That string is %s characters long.\r\n", strlen(string));
        return 0;
}

I'm compiling under gcc, and although it doesn't give me any errors the 开发者_运维问答program crashes every time I run it. The code seems to be fine from examples I've seen. It'd be great to know if I'm doing anything wrong.

Thanks.

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Using incorrect format specifier in printf() invokes Undefined Behaviour. Correct format specifier should be %zu (not %d) because the return type of strlen() is size_t

Note: Length modifier z in %zu represents an integer of length same as size_t

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You have wrong format specifier. %s is used for strings but you are passing size_t (strlen(string)). Using incorrect format specifier in printf() invokes undefined behaviour. Use %zu instead because the return type of strlen() is size_t.

So change

 printf("That string is %s characters long.\r\n", strlen(string));

to:

 printf("That string is %zu characters long.\r\n", strlen(string));

Since you are using gcc have a look here for more info what can be passed to printf

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 printf("That string is %d characters long.\r\n", strlen(string));

instead:

 printf("That string is %s characters long.\r\n", strlen(string));

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You have a problem here printf("That string is %s characters long.\r\n", strlen(string));

put

printf("That string is %d characters long.\r\n", strlen(string)); %d because you want to printthe length of str (strlen returns number)

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Program crashes because formatting routine tries to access a string at address 0x0000001D which is the result of strlen() where is nothing like a string and likely there's no acessible memory at all.