Date: Tue, 30 Nov 1999 23:44:31 -0500 From: "Brian J. McGovern" <mcgovern@spoon.beta.com> To: hackers@freebsd.org Subject: writing to an mmap()'ed region requires read access? Message-ID: <199912010444.XAA17911@spoon.beta.com>
next in thread | raw e-mail | index | archive | help
I was just playing with mmap() really for the first time, and I noticed
something a bit odd. When you mmap() a file for writing, it also appears
to require that you give it read permissions, else it dies on a signal 10.
Any reason for this? (I'm using 3.3-STABLE (11/30/99) with the following
program)
-Brian
#include <sys/types.h>
#include <sys/mman.h>
#include <stdlib.h>
#include <unistd.h>
#include <fcntl.h>
void main(int argc, char *argv[])
{
int f1 = 0, /* File descriptor for source */
f2 = 0; /* File descriptor for target */
off_t size = 0, /* Size of source/mmap'ed regions */
len = 0; /* Size of destination (for test) */
unsigned char *b1 = NULL, /* MMAP of source */
*b2 = NULL; /* MMAP of destination */
if (argc < 3) /* Too few arguments? */
{
printf("Usage: mcopy <f1> <f2>\n");
exit(1);
}
f1 = open(argv[1],O_RDWR); /* Open src */
f2 = open(argv[2],O_RDWR | O_CREAT);/* Open dst */
if (f1 < 0) /* Error in src? */
{
printf("Couldn't open file %s\n",argv[1]);
exit(2);
}
if (f2 < 0) /* Error in dst? */
{
printf("Couldn't open file %s\n",argv[2]);
exit(3);
}
printf("Both files opened\n");
size = lseek(f1,0,SEEK_END); /* How big is src? */
printf("Size is %d\n",size);
if (lseek(f2,size - 1,SEEK_SET) != (size - 1)) /* Make dst 1 byte shorter */
{
printf("Couldn't seek in output file to location %d\n",size - 1);
exit(4);
}
write(f2,"\0",1); /* Write single byte, thereby creating a "hole" in the file.
This allows u to mmap the entire region
by only writing 1 byte */
len = lseek(f2,0,SEEK_END); /* Make sure our destination is now as big
as our source. This is not required */
printf("output file should now by %d bytes long\n",len);
/* MMAP the source */
b1 = (unsigned char *)mmap((void *)NULL, (size_t)size, PROT_READ, MAP_SHARED, f1, 0);
if (b1 == MAP_FAILED)
{
perror("mmap input file");
exit(5);
}
printf("mmap of file 1 complete. Address is %p.\n",b1);
/* MMAP the destination. Is read access really required? */
b2 = (unsigned char *)mmap((void *)NULL, (size_t)size, PROT_READ | PROT_WRITE, MAP_SHARED, f2, 0);
if (b2 == MAP_FAILED)
{
perror("mmap input file");
exit(5);
}
printf("mmap of file 2 complete. Address is %p.\n",b2);
printf("Performing bcopy\n");
/* Copy the files */
memcpy(b2,b1,size);
printf("Unmapping files\n");
/* Unmap/close them */
munmap(b1,(size_t)size);
munmap(b2,(size_t)size);
printf("Closing descriptors\n");
close(f1);
close(f2);
}
To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?199912010444.XAA17911>