Re: [PATCH] read/write: documentation of limits v3

[Jakub Wilk <jwilk@xxxxxxxxx>]

* Shawn Landden <shawn@xxxxxxx>, 2019-05-06, 08:06:
> --- a/man2/write.2
> +++ b/man2/write.2
> @@ -190,10 +190,18 @@ flag, and either the address specified in
> .IR buf ,
> the value specified in
> .IR count ,
> or the file offset is not suitably aligned.
> .TP
> +.B EINVAL
> +.\" MAX_RW_COUNT in include/linux/fs.h
> +The write amount is greater than
> +.B MAX_RW_COUNT,
> +which is
> +.B INT_MAX
> +rounded down to the page size (INT_MAX & ~PAGE_MASK).
> +.TP

I can't reproduce this. For me, write() behaves as it is documented in 
another part of this man page:

"On Linux, write() (and similar system calls) will transfer at most 
0x7ffff000 (2,147,479,552) bytes, returning the number of bytes actually 
transferred. (This is true on both 32-bit and 64-bit systems.)"

I've attached the program that I used for testing.

--
Jakub Wilk

#include <fcntl.h>
#include <limits.h>
#include <stddef.h>
#include <stdio.h>
#include <sys/mman.h>
#include <unistd.h>

int main(int argc, char **argv)
{
	int fd = open("/dev/zero", O_RDWR);
	if (fd < 0) {
		perror("/dev/zero");
		return 1;
	}
	void *mem = mmap(NULL, INT_MAX, PROT_READ, MAP_PRIVATE, fd, 0);
	if (mem == MAP_FAILED) {
		perror("mmap()");
		return 1;
	}
	int rc = write(fd, mem, INT_MAX);
	if (rc < 0) {
		perror("write()");
		return 1;
	}
	printf("0x%x bytes written\n", rc);
}