什么时候printf（“％s”，char *）停止打印？

Question

In my class we are writing our own copy of C's malloc() function. To test my code (which can currently allocate space fine) I was using:

char* ptr = my_malloc(6*sizeof(char));
memcpy(ptr, "Hello\n", 6*sizeof(char));
printf("%s", ptr);

The output would typically be this:

Hello
Unprintable character

Some debugging figured that my code wasn't causing this per se, as ptr's memory is as follows:

[24 bytes of meta info][Number of requested bytes][Padding]

So I figured that printf was reaching into the padding, which is just garbage. So I ran a test of: printf("%s", "test\\nd"); and got:

test
d

Which makes me wonder, when DOES printf("%s", char*) stop printing chars?

Answer 1

It stops printing when it reaches a null character ( \\0 ), because %s expects the string to be null terminated (ie, it expects the argument to be a C string).

The string literal "test\\nd" is null terminated (all string literals are null terminated). Your character array ptr is not, however, because you only copy six characters into the buffer ( Hello\\n ), and you do not copy the seventh character--the null terminator.

Answer 2

James is correct about printf stopping when it gets to the null character, and Uri is correct that you need to allocate a 7-character buffer to hold "hello\\n".

Some of the confusion with terminators would be mitigated if you used the usual C idiom for copying a string: strcpy(ptr, "Hello\\n") , rather than memcpy .

Also, by definition, sizeof(char) == 1 in C, so 6*sizeof(char) is redundant

Answer 3

C strings are null terminated (there's a \\0 character at the end), that's how C knows when to stop printing or dealing with the buffer as a string . It is your responsibility to never put a string in a longer space than what you have allocated.

Note that Hello\\n is not a six character string, it is actually a seven character string. You use five for the Hello, one for the newline, and one for the null terminator.

Trying to fit 7 characters into a six character buffer is considered a bug, I am not sure if it is responsible for the problems you are currently having, but it seems like the copying of 6 characters would not copy the null terminator. So I would actually expect your print to go beyond the Hello and into some actual junk.

Answer 4

when it reaches a zero. you need 7 chars.