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close_range.2: Include a better example program 

The current example program can't really be used to demonstrate the
effect of close_range(). Replace it by a program that does show the
effect of this system call.

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>
This commit is contained in:
Michael Kerrisk 2021-02-14 23:04:45 +01:00
parent 3bb4fe47a5
commit 336bd62ba2
1 changed files with 79 additions and 33 deletions
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112
man2/close_range.2
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@ -1,4 +1,5 @@
.\" Copyright (c) 2020 Stephen Kitt <steve@sk2.org>
.\" and Copyright (c) 2021 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
@ -174,63 +175,108 @@ and the profile can control access to
.BR close_range ()
without affecting the calling process.
.SH EXAMPLES
The following program executes the command given on its command-line,
after opening the files listed after the command and then using
The program shown below opens the files named in its command-line arguments,
displays the list of files that it has opened
(by iterating through the entries in
.IR /proc/PID/fd ),
uses
.BR close_range ()
to close them:
to close all file descriptors greater than or equal to 3,
and then once more displays the process's list of open files.
The following example demonstrates the use of the program:
.PP
.in +4n
.EX
/* close_range.c */
$ \fBtouch /tmp/a /tmp/b /tmp/c\fP
$ \fB./a.out /tmp/a /tmp/b /tmp/c\fP
/tmp/a opened as FD 3
/tmp/b opened as FD 4
/tmp/c opened as FD 5
/proc/self/fd/0 ==> /dev/pts/1
/proc/self/fd/1 ==> /dev/pts/1
/proc/self/fd/2 ==> /dev/pts/1
/proc/self/fd/3 ==> /tmp/a
/proc/self/fd/4 ==> /tmp/b
/proc/self/fd/5 ==> /tmp/b
/proc/self/fd/6 ==> /proc/9005/fd
========= About to call close_range() =======
/proc/self/fd/0 ==> /dev/pts/1
/proc/self/fd/1 ==> /dev/pts/1
/proc/self/fd/2 ==> /dev/pts/1
/proc/self/fd/3 ==> /proc/9005/fd
.EE
.in
.PP
Note that the lines showing the pathname
.I /proc/9005/fd
result from the calls to
.BR opendir (3).
.SS Program source
\&
.EX
#define _GNU_SOURCE
#include <fcntl.h>
#include <linux/close_range.h>
#include <stdio.h>
#include <stdlib.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <sys/types.h>
#include <string.h>
#include <unistd.h>
#include <dirent.h>
/* Show the contents of the symbolic links in /proc/self/fd */
static void
show_fds(void)
{
DIR *dirp = opendir("/proc/self/fd");
if (dirp == NULL) {
perror("opendir");
exit(EXIT_FAILURE);
}
for (;;) {
struct dirent *dp = readdir(dirp);
if (dp == NULL)
break;
if (dp\->d_type == DT_LNK) {
char path[PATH_MAX], target[PATH_MAX];
snprintf(path, sizeof(path), "/proc/self/fd/%s",
dp\->d_name);
ssize_t len = readlink(path, target, sizeof(target));
printf("%s ==> %.*s\en", path, (int) len, target);
}
}
closedir(dirp);
}
int
main(int argc, char *argv[])
{
char *newargv[] = { NULL };
char *newenviron[] = { NULL };
if (argc < 3) {
fprintf(stderr, "Usage: %s <command> <file>...\en", argv[0]);
exit(EXIT_FAILURE);
}
for (int i = 2; i < argc; i++) {
if (open(argv[i], O_RDONLY) == \-1) {
perror(argv[i]);
for (int j = 1; j < argc; j++) {
int fd = open(argv[j], O_RDONLY);
if (fd == \-1) {
perror(argv[j]);
exit(EXIT_FAILURE);
}
printf("%s opened as FD %d\en", argv[j], fd);
}
if (syscall(__NR_close_range, 3, ~0U, 0) == \-1) {
show_fds();
printf("========= About to call close_range() =======\en");
if (syscall(__NR_close_range, 3, \(ti0U, 0) == \-1) {
perror("close_range");
exit(EXIT_FAILURE);
}
execve(argv[1], newargv, newenviron);
perror("execve");
show_fds();
exit(EXIT_FAILURE);
}
.EE
.in
.PP
Running any program with the above, with files to open:
.PP
.in +4n
.EX
.RB "$" " ./close_range " <program> " /dev/null /dev/zero"
.EE
.in
.PP
and inspecting the open files in the resulting process will show that
the files have indeed been closed.
.SH SEE ALSO
.BR close (2)