Problem - Kevin Tafuro

The two easiest and safest functions to use are execv( ) and execve( ); the only difference
between the two is that execv( ) calls execve( ), passing environ for the environment
pointer. If you have already sanitized the environment (see Recipe 1.1), it’s
reasonable to call execv( ) without explicitly specifying an environment to use. Otherwise,
a new environment can be built and passed to execve( ).
The argument lists for the functions are built just as they will be received by main( ).
The first element of the array is the name of the program that is running, and the last
element of the array must be a NULL. The environment is built in the same manner as
described in Recipe 1.1. The first argument to the two functions is the full path and
filename of the executable file to load and execute.
As a courtesy to the new program, before executing it you should close any file
descriptors that are open unless there are descriptors that you intentionally want to
pass along to it. Be sure to leave stdin, stdout, and stderr open. (See Recipe 1.5 for a
discussion of file descriptors.)
Finally, if your program was executed setuid or setgid and the extra privileges have
not yet been dropped, or they have been dropped only temporarily, you should drop
them permanently before executing the new program. Otherwise, the new program
will inherit the extra privileges when it should not. If you use the spc_fork( ) function
from Recipe 1.6, the file descriptors and privileges will be handled for you.
Another function provided by the standard C runtime library for executing programs
is system( ). This function hides the details of calling fork( ) and the appropriate
exec*( ) function to execute the program. There are two reasons why you should
never use the system( ) function:
• It uses the shell to launch the program.
• It passes the command to execute to the shell, leaving the task of breaking up the
command’s arguments to the shell.
The system( ) function works differently from the exec*( ) functions; instead of
replacing the currently executing program, it creates a new process with fork( ). The
new process executes the shell with execve( ) while the original process waits for the
new process to terminate. The system( ) function therefore does not return control to
the caller until the specified program has completed.
Yet another function, popen( ), works somewhat similarly to system( ). It also uses
the shell to launch the program, passing the command to execute to the shell and
leaving the task of breaking up the command’s arguments to the shell. What it does
differently is create an anonymous pipe that is attached to either the new program’s
stdin or its stdout file descriptor. The new program’s stderr file descriptor is always
inherited from the parent. In addition, it returns control to the caller immediately
with a FILE object connected to the created pipe so that the caller can communicate
with the new program. When communication with the new program is finished, you
30 | Chapter 1: Safe Initialization
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