Is the "NULL" or "\0"-sign, of a Null-terminated string, stored in a file?

Question

If i want to store a Null-terminated string into a file, and the file will only containing that string, is the "\0" or "NULL"-character stored in the file (before the "EOF" (End of File)-sign)?

Furthermore: Is the result depended from the operation system and so on the compiler, on which i will compile the source code on?

Answer 1

You might be able to write null characters to a text file, but you almost certainly don't want to.

A string (defined as "a contiguous sequence of characters terminated by and including the first null character") is an in-memory data format.

A text stream consists of a sequence of lines:

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined.

A string may or may not contain a single line of text. If it represents a line of text, it may or may not include the terminating new-line '\n' character (you'll need to keep track of that yourself).

If you have a sequence of strings in memory, the usual way to write them to a text file is to write the contents of each string, not including the terminating null character, to the file, adding a new-line character if necessary. Functions like fprintf and fputs assume their arguments are strings, so they take care of omitting the '\0'.

You can write a null character to a text stream, but it's implementation-defined what will actually be written to the file. You can write a null character, or any byte value, to a binary stream -- but then you can't safely use string functions (strlen() et al, or even fgets() and fputs()) on data written to or read from the stream. (And in practice, most systems allow null characters to be written to and read from text files -- though a number of standard library functions assume that text files contain only printable characters.)

Answer 2

'\0' is not a printing character so if you use an io stream in text mode, then whether it will be preserved when you write it to a file through such a stream is implementation-dependent.

7.21.2p2

A text stream is an ordered sequence of characters composed into lines, each line consisting of zero or more characters plus a terminating new-line character. Whether the last line requires a terminating new-line character is implementation-defined. Characters may have to be added, altered, or deleted on input and output to conform to differing conventions for representing text in the host environment. Thus, there need not be a one- to-one correspondence between the characters in a stream and those in the external representation. Data read in from a text stream will necessarily compare equal to the data that were earlier written out to that stream only if: the data consist only of printing characters and the control characters horizontal tab and new-line; no new-line character is immediately preceded by space characters; and the last character is a new-line character. Whether space characters that are written out immediately before a new-line character appear when read in is implementation-defined.

If you write the '\0' to a file through a binary stream (one opened with e.g., fopen("file","wb")), e.g., with fputc('\0',f) or fwrite("",1,1,f), you should be able to get it back.

Answer 3

No, the functions that write a string to a file will not include the terminating null. You can write a null to a file using a function that takes a byte count, but that doesn't make sense because there's no corresponding read function.