Analysis of open function o_cloexec mode and fcntl function fd_cloexec option

For the Open function O_cloexec mode, the FCNTL function fd_cloexec option is summarized as follows:

1. Calling the Open function O_cloexec mode opens a file descriptor that is closed in a new program that executes the exec call, and is an atomic operation.

2. Calling the Open function does not use the O_cloexec mode to open the file descriptor, and then calls the FCNTL function to set the FD_CLOEXEC option, the effect is the same as using the O_CLOEXEC option of the Open function, but calls the open, fcnt two L functions respectively, not atomic operations, There are race conditions in multi-threaded environment, so the Open function o_cloexec option is used instead.

3. Call the Open function o_cloexec the file descriptor opened by the mode, or use Fcntl to set the Fd_cloexec option, both of which are not closed in the child process resulting from the fork call.

4. The new file descriptor that is obtained by invoking the DUP race function clears the o_cloexec mode.

The test procedure is as follows:

#include <unistd.h>#include <sys/types.h>#include <fcntl.h>#include <stdio.h>#include <stdlib.h>#include <string.h>#include <errno.h>#define ERR_SYS (FMT, arg ...) \ Do{printf (FMT, # #arg); printf"\nerrno:%d%s\n", errno, Strerror (errno)); Exit (exit_failure);} while(0)intMain () {intFD, Fd2, Val; pid_t pid;#ifdef _o_cloexecif (FD = open ("My.txt",O_rdwr|o_creat| O_cloexec,0600)) <0)#elseif (FD = open ("My.txt",O_rdwr| O_creat,0600)) <0)#endifErr_sys ("Open Error");#ifdef _dupif ((fd2 = DUP (FD)) <0) Err_sys ("DUP error"); if (Write (FD2,"123",3) <0) Err_sys ("Write error");#endif#ifdef _fcntl_cloexecif (val = fcntl (fd, F_GETFD)) <0) Err_sys ("Fcntl (F_GETFD) Error");    Val |= fd_cloexec; if (Fcntl (FD, F_SETFD, Val) <0) Err_sys ("Fcntl (F_SETFD) Error");#endif#ifndef _forkif (Execl ("/bin/sleep","Sleep","10000", (void*)0) <0) Err_sys ("Execl Error");#else Switch(PID = fork ())) { Case-1: Err_sys ("Fork Error"); Case 0: Sleep (10000); Break;default: Sleep (10000); Break; }#endif    return 0;}

Compile into o_cloexec options with macro _o_cloexec

#gcc-D_O_CLOEXEC-o cloexec cloexec.c

Execute the program

#./cloexec 

View process and process resources, note that the process name changes to sleep after executing execl

#ps Aux|grep SleepRoot7900  0.0  0.0   4200   280pts/0S+ One: $   0:xx Sleep 10000Root7915  0.0  0.1   4384   836pts/1S+ One: the   0:xxgrep--color=autoSleep#lsof-P 7900COMMAND PID USER FD TYPE DEVICE size/off NODE NAMESleep   7900Root CWD DIR8,1     4096 163741/home/zozy/aupSleep   7900Root RTD DIR8,1     4096      2/Sleep   7900Root txt REG8,1    26156 131621/bin/SleepSleep   7900Root Mem REG8,1  2919792      A/usr/lib/locale/locale-archiveSleep   7900Root Mem REG8,1  1730024  15701/lib/i386-linux-gnu/libc-2.15. soSleep   7900Root Mem REG8,1   134344  15713/lib/i386-linux-gnu/LD-2.15. soSleep   7900Root0U CHR136,0      0T03/dev/pts/0Sleep   7900Root1U CHR136,0      0T03/dev/pts/0Sleep   7900Root2U CHR136,0      0T03/dev/pts/0

You can see that there are no files in the process resource My.txt

Compile with no macro _o_cloexec

#gcc  -o nocloexec cloexec.c

Execute the program

#./nocloexec 

View process and Process resources

#ps Aux|grep SleepRoot7925  0.0  0.0   4200   280pts/0S+ One:Wuyi   0:xx Sleep 10000Root7928  0.0  0.1   4384   836pts/1S+ One: the   0:xxgrep--color=autoSleep#lsof-P 7925COMMAND PID USER FD TYPE DEVICE size/off NODE NAMESleep   7925Root CWD DIR8,1     4096 163741/home/zozy/aupSleep   7925Root RTD DIR8,1     4096      2/Sleep   7925Root txt REG8,1    26156 131621/bin/SleepSleep   7925Root Mem REG8,1  2919792      A/usr/lib/locale/locale-archiveSleep   7925Root Mem REG8,1  1730024  15701/lib/i386-linux-gnu/libc-2.15. soSleep   7925Root Mem REG8,1   134344  15713/lib/i386-linux-gnu/LD-2.15. soSleep   7925Root0U CHR136,0      0T03/dev/pts/0Sleep   7925Root1U CHR136,0      0T03/dev/pts/0Sleep   7925Root2U CHR136,0      0T03/dev/pts/0Sleep   7925Root3U REG8,1        0 163759/home/zozy/aup/my. txt

You can see that there are files in the process resources my.txt

The test fcntl function can be set to compile the macro-d_fcntl_cloexec test, compile the test process as above, similarly, by turning on the-d_o_cloexec-d_fork compilation option to test the status of the descriptor in the child process using the O_cloexec mode, through the macro-d_ The DUP compilation option tests the effect of the DUP function on o_cloexec and compiles the test process slightly.

Analysis of open function o_cloexec mode and fcntl function fd_cloexec option