PK07789: FOPEN FIFO READ WRITE PIPE DEADLOCK SEARIALIZATION RESTRICTION

APAR status

• Closed as documentation error.

Error description

• Using fopen() with a mutex results in a deadlock state
  during the blocking kernel call.If one thread opens a
  fifo for writing in blocking mode and a second thread in
  the same process opens a regular file, then the second
  thread is blocked until another process opens the fifo
  for reading.
  The fopen() function serializes all accesses to the function
  when in multi-threaded environment. fopen() behavior is such
  that when one thread is opening a file using fopen(), will
  cause all other threads that are doing fopen() to be blocked
  until the first thread finishes.
  .
  The projected closing code for this APAR is DOC
  

Local fix

• Use open() with O_NONBLOCK on the read side of the FIFO.
  

Problem summary

• ****************************************************************
  * USERS AFFECTED: z/OS Language Environment users running      *
  *                 multi-threaded C/C++ applications that are   *
  *                 using fopen() to open named pipes, FIFO      *
  *                 files.                                       *
  ****************************************************************
  * RECOMMENDATION:                                              *
  ****************************************************************
  If the fopen() function is used to open named pipes in a multi-
  threaded environment, a deadlock will occur.  The deadlock is
  caused by a named pipe waiting for the other end of the pipe to
  be opened, while still holding the fopen() multi-thread mutex.
  

Problem conclusion

• keywords:
  fopen open multithread multithreaded
  
  documentation change:
  
  z/OS C/C++ Programming Guide
  
  Add the following to the end of "Using named pipes" section
  
  | Restrictions:
  | If fopen() is used to open named pipes in a multi-threaded
  | environment, a deadlock will occur.  This deadlock is caused
  | by a named pipe waiting for the other end of the pipe to be
  | opened, while still holding the fopen() multi-thread mutex.
  | To prevent this deadlock, use open() to open the named pipe,
  | instead of fopen().
  
  z/OS C/C++ Run-Time Library Reference
  
  Add the following to section "fopen() - Open a File", after the
  "Large Files for HFS" section.
  
  | Named Pipes in Multi-Threaded Environment
  |
  | fopen() may not be used to open named pipes in multi-
  | threaded environment.  If used, a deadlock will occur.  See
  | the C/C++ Programming Guide for a detailed explanation.
  

Temporary fix

Comments

APAR Information

• APAR is sysrouted FROM one or more of the following:

• APAR is sysrouted TO one or more of the following:

Fix information

Applicable component levels