What does write() write if null terminator is already reached?

Question

For write(fd[1], string, size) - what would happen if string is shorter than size?

I looked up the man page but it doesn't clearly specify that situation. I know that for read, it would simply stop there and read whatever string is, but it's certainly not the case for write. So what is write doing? The return value is still size so is it appending null terminator? Why doesn't it just stop like read.

Answer 1

When you call write(), the system assumes you are writing generic data to some file - it doesn't care that you have a string. A null-terminated string is seen as a bunch of non-zero bytes followed by a zero byte - the system will keep writing out until it's written size bytes.

Thus, specifying size which is longer than your string could be dangerous. It's likely that the system is reading data beyond the end of the string out your file, probably filled with garbage data.

Answer 2

write will write size bytes of data starting at string. If you define string to be an array shorter than size it will have undefined behaviour. But in you previous question the char *line = "apple"; contains 6 characters (i.e. a, p, p, l, e and the null character).

So it is best to write the with the value of size set to the correct value

Answer 3

write(int fildes, const void *buf, size_t nbyte) does not write null terminated strings. It writes the content of a buffer. If there are any null characters in the buffer they will be written as well.

read(int fildes, void *buf, size_t nbyte) also pays no attention to null characters. It reads a number of bytes into the given buffer, up to a maximum of nbyte. It does not add any null terminating bytes.

These are low level routines, designed for reading and writing arbitrary data.

Answer 4

The write call outputs a buffer of the given size. It does not attempt to interpret the data in the buffer. That is, you give it a pointer to a memory location and a number of bytes to write (the length) then, as long as those memory locations exist in a legal portion of your program's data, it will copy those bytes to the output file descriptor.

Unlike the string manipulation routines write, and read for that matter, ignore null bytes, that is bytes with the value zero. read does pay attention to the EOF character and, on certain devices, will only read that amount of data available at the time, perhaps returning less data than requested, but they operate on raw bytes without interpreting them as "strings".

If you attempt to write more data than the buffer contains, it may or may not work depending on the position of the memory. At best the behavior is undefined. At worst you'll get a segment fault and your program will crash.