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File:,  Node: Running a Command,  Next: Process Creation Concepts,  Up: Processes

26.1 Running a Command

The easy way to run another program is to use the `system' function.
This function does all the work of running a subprogram, but it doesn't
give you much control over the details: you have to wait until the
subprogram terminates before you can do anything else.

 -- Function: int system (const char *COMMAND)
     This function executes COMMAND as a shell command.  In the GNU C
     library, it always uses the default shell `sh' to run the command.
     In particular, it searches the directories in `PATH' to find
     programs to execute.  The return value is `-1' if it wasn't
     possible to create the shell process, and otherwise is the status
     of the shell process.  *Note Process Completion::, for details on
     how this status code can be interpreted.

     If the COMMAND argument is a null pointer, a return value of zero
     indicates that no command processor is available.

     This function is a cancellation point in multi-threaded programs.
     This is a problem if the thread allocates some resources (like
     memory, file descriptors, semaphores or whatever) at the time
     `system' is called.  If the thread gets canceled these resources
     stay allocated until the program ends.  To avoid this calls to
     `system' should be protected using cancellation handlers.

     The `system' function is declared in the header file `stdlib.h'.

   *Portability Note:* Some C implementations may not have any notion
of a command processor that can execute other programs.  You can
determine whether a command processor exists by executing
`system (NULL)'; if the return value is nonzero, a command processor is

   The `popen' and `pclose' functions (*note Pipe to a Subprocess::)
are closely related to the `system' function.  They allow the parent
process to communicate with the standard input and output channels of
the command being executed.