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loop.4: Add an EXAMPLE program 

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2015-05-01 15:11:33 +02:00
parent 44fdc52128
commit 1a7f33706f
1 changed files with 69 additions and 0 deletions
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69
man4/loop.4
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@ -202,6 +202,75 @@ argument.
On success, the device number is returned as the result of the call.
If the device is in use, the call fails with the error
.BR EBUSY .
.SH EXAMPLE
The program below uses the
.I /dev/loop-control
device to find a free loop device, opens the loop device,
opens a file to be used as the underlying storage for the device,
and then associates the loop device with the backing store.
The following shell session demonstrates the use of the program:
.nf
.in +4n
$ \fBdd if=/dev/zero of=file.img bs=1MiB count=10\fP
10+0 records in
10+0 records out
10485760 bytes (10 MB) copied, 0.00609385 s, 1.7 GB/s
$ \fBsudo ./mnt_loop file.img\fP
loopname = /dev/loop5
.in
.fi
.SS Program source
\&
.nf
#include <fcntl.h>
#include <linux/loop.h>
#include <sys/ioctl.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#define errExit(msg) do { perror(msg); exit(EXIT_FAILURE); \\
} while (0)
int
main(int argc, char *argv[])
{
int loopctlfd, loopfd, backingfile;
long devnr;
char loopname[4096];
if (argc != 2) {
fprintf(stderr, "Usage: %s backing\-file\\n", argv[0]);
exit(EXIT_FAILURE);
}
loopctlfd = open("/dev/loop\-control", O_RDWR);
if (loopctlfd == \-1)
errExit("open: /dev/loop\-control");
devnr = ioctl(loopctlfd, LOOP_CTL_GET_FREE);
if (devnr == \-1)
errExit("ioctl\-LOOP_CTL_GET_FREE");
sprintf(loopname, "/dev/loop%ld", devnr);
printf("loopname = %s\\n", loopname);
loopfd = open(loopname, O_RDWR);
if (loopfd == \-1)
errExit("open: loopname");
backingfile = open(argv[1], O_RDWR);
if (backingfile == \-1)
errExit("open: backing\-file");
if (ioctl(loopfd, LOOP_SET_FD, backingfile) == \-1)
errExit("ioctl\-LOOP_SET_FD");
exit(EXIT_SUCCESS);
}
.fi
.SH FILES
.IR /dev/loop* :
the loop block special device files