coleman@sundae9.DAB.GE.COM (Richard Coleman) (10/01/90)
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I was looking at the system call mmap(). I not sure what
the purpose of this system call is. Could someone help.
Richard Coleman
G.E. Simulation & Control Systems
coleman@sunny.dab.ge.comcpcahil@virtech.uucp (Conor P. Cahill) (10/02/90)
In article <5994@ge-dab.GE.COM> coleman@sundae9.DAB.GE.COM (Richard Coleman) writes: >I was looking at the system call mmap(). I not sure what >the purpose of this system call is. Could someone help. On systems that support it, it allows you to map a portion (or all) of a file into your memory space. This allows you to change the contents of the file by simply assigning to the data area in your mapped area. For example, if mmptr is a pointer to the memory mapped region, memset(mmptr,'\0',512); will clear the first 512 bytes of the file. -- Conor P. Cahill (703)430-9247 Virtual Technologies, Inc., uunet!virtech!cpcahil 46030 Manekin Plaza, Suite 160 Sterling, VA 22170
symon@aesop.cs.unc.edu (James Symon) (10/03/90)
In article <5994@ge-dab.GE.COM>, coleman@sundae9.DAB.GE.COM (Richard Coleman) writes: > > I was looking at the system call mmap(). I not sure what > the purpose of this system call is. Could someone help. > Besides regular files, the file that you map to your program`s data space can be some device. The call to mmap() will use the mmap routine from the device driver. This method is often used with custom hardware to allow direct access to device registers through program variables. With root permission you can memory map directly to bus addresses, at least on Suns, using the various address spaces such as vme24d16. If you have Sun documentation look at the document about writing device drivers. I`ve also seen a book about writing UNIX device drivers that can help you understand mmap(). Jim Symon symon@radonc.unc.edu symon@cs.unc.edu (919) 966-7710
pefv700@perv.pe.utexas.edu (05/10/91)
I'm trying to understand how to use mmap(2) by RTM and am stumped. Of course, the whole concept here is probably escaping me (read the header, I'm no CS major). So does anyone have a short and sweet cat(1)-like example that will broaden my system call knowledge (at least a little)? Chris
pfalstad@phoenix.princeton.edu (Paul Falstad) (05/10/91)
pefv700@perv.pe.utexas.edu wrote: >I'm trying to understand how to use mmap(2) by RTM and am stumped. >So does anyone have a short and sweet cat(1)-like example that will >broaden my system call knowledge (at least a little)? Here's a quick version of cat, without error checking: ----------------------------------------------------------------------------- #include <stdio.h> #include <sys/file.h> #include <sys/mman.h> main(argc,argv) int argc;char **argv; { char *buf; int fd,pgsiz,len,off; pgsiz = getpagesize()*3; fd = open(argv[1],O_RDONLY); len = lseek(fd,0,2); off = 0; buf = NULL; for (;;) { buf = mmap(buf,pgsiz,PROT_READ,MAP_PRIVATE,fd,off); if (len < pgsiz) { write(1,buf,len); exit(0); } write(1,buf,pgsiz); off += pgsiz; len -= pgsiz; } } ----------------------------------------------------------------------------- This dumps the file in blocks of (pagesize*3). I picked this value arbitrarily; the only restriction is that pgsiz must be an integer multiple of the system page size. Note that the first time mmap() is called, buf is 0, so the system will map the file wherever it wants. After that, the system will map the file into the same place as before (so we don't accumulate mappings). -- Paul Falstad pfalstad@phoenix.princeton.edu And on the roads, too, vicious gangs of KEEP LEFT signs! If Princeton knew my opinions, they'd have expelled me long ago.