C - does read() add a '\0'?

Question

Does it have to? I've always been fuzzy on this sort of stuff, but if I have something like:

char buf[256];
read(fd, buf, 256);

write(fd2, buf, 256);

Is there potential for error here, other than the cases where those functions return -1?

If it were to only read 40 characters, would it put a \0 after it? (And would write recognize that \0 and stop? Also, if it were to read 256 characters, is there a \0 after those 256?

Answer 1

does read() add a '\0'?

No, it doesn't. It just reads.

From read()'s documentation:

The read() function shall attempt to read nbyte bytes from the file associated with the open file descriptor, fildes, into the buffer pointed to by buf.

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Is there potential for error here, other than the cases where those functions return -1?

read() might return 0 indicating end-of-file.

If reading (also from a socket descriptor) read() not necessarily reads as much bytes as it was told to do. So in this context do not just test the outcome of read against -1, but also compare it against the number of bytes the function was told to read.

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A general note:

Functions do what is documented (at least for proper implementations of the C language). Both your assumptions (autonomously set a 0-termination, detect the latter) are not documented.

Answer 2

Does it have to?

Not unless the data that is successfully read from the file contains a '\0'...

Is there potential for error here, other than the cases where those functions return -1?

Yes. read returns the actual number of bytes read (or a negative value to indicate failure). If you choose to write more than that number of bytes into your other file, then you are writing potential garbage.

Answer 3

No.

Consider reading binary data (eg. a photo from a file): adding extra bytes would corrupt the data.

Answer 4

From the man page:

Synopsis

#include <unistd.h>
ssize_t read(int fd, void *buf, size_t count);

That is void *, not char *, because read() reads bytes, not characters. It reads zero bytes as well as any other value, and since blocks of bytes (as opposed to strings) aren't terminated, read() doesn't.