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let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,fileno(stdin))
$2 = 1
(gdb) call close($1)
...
(gdb) detach
...

for those not familiar with gdb , "attach" "call" "detach" are gdb commands . get information with "help attach" inside a gdb prompt . and "open" "dup2" "close" are library functions . get information with "man 2 open" .

here O_WRONLY equals 1 and fileno(stdin) equals 1 . use literal values or gdb may complain lack of symbols .

and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done

let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,fileno(stdout))
$2 = 1
(gdb) call close($1)
...
(gdb) detach
...

for those not familiar with gdb , "attach" "call" "detach" are gdb commands . get information with "help attach" inside a gdb prompt . and "open" "dup2" "close" are library functions . get information with "man 2 open" .

here O_WRONLY equals 1 and fileno(stdout) equals 1 . use literal values or gdb may complain lack of symbols .

and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done

let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,1)
$2 = 0
(gdb) call close($1)
$3 = 0
(gdb) detach

for those not familiar with gdb , "attach" "call" "detach" are gdb commands . get information with "help attach" inside a gdb prompt . and "open" "dup2" "close" are library functions . get information with "man 2 open" .

here O_WRONLY is referenced through fcntl.h , and has value 1 . and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done

let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,fileno(stdin))
$2 = 1
(gdb) call close($1)
...
(gdb) detach
...

for those not familiar with gdb , "attach" "call" "detach" are gdb commands . get information with "help attach" inside a gdb prompt . and "open" "dup2" "close" are library functions . get information with "man 2 open" .

here O_WRONLY equals 1 and fileno(stdin) equals 1 . use literal values or gdb may complain lack of symbols . 

and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done

let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,1)
$2 = 0
(gdb) call close($1)
$3 = 0
(gdb) detach

here according to header fcntl.h O_WRONLY has value 1 . and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done

let me yank here the solution @jimmij pointed to , with minor modification .

(gdb) attach <pid>    
... 
(gdb) call open("/dev/null",O_WRONLY)
$1 = 3
(gdb) call dup2($1,1)
$2 = 0
(gdb) call close($1)
$3 = 0
(gdb) detach

for those not familiar with gdb , "attach" "call" "detach" are gdb commands . get information with "help attach" inside a gdb prompt . and "open" "dup2" "close" are library functions . get information with "man 2 open" .

here O_WRONLY is referenced through fcntl.h , and has value 1 . and if we want to find a file descriptor already opened , we can .

$ cd /proc/<pid>/fd
$ for i in * ; do
          if [[ `readlink $i` == "/dev/null" ]]
                  fd=$i ; break
          fi
  done