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CALLING CHILD PROGRAMS FROM A TURBO C PARENT PROGRAM.
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It is sometimes desirable to call and run another program from the
current Turbo C program, as for example a DOS program like DEBUG.COM
There are basically three functions available for this purpose:
exec... which leaves the parent program to run a child program.
spawn... which runs the child program and then returns to the parent.
system which runs the DOS COMMAND.COM file from inside the C program.
The ... after the first two function names indicates that the suffices
l,v,p and e can be added to the family name of the function.
The suffix p indicates that the function will search the DOS PATH for the
child program, otherwise only the current directory is searched.
The suffix l is used when the number of arguments, described below, is
known in advance, whilst v indicates a variable number of arguments.
The suffix e indicates that the argument 'env' may be passed to the child
process, to permit a different environment for the child process.
exec... has a number of parameters or arguments starting with the 'path',
which is the file name of the child process, followed by arg0 to
argn and concluding with a mandatory NULL parameter.
spawn... has the same parameters but preceded by the parameter 'mode',
which determines whether the parent process is put on hold using
P_WAIT as the parameter or overlayed by the child program by
using the parameter P_OVERLAY
system has only one parameter, the DOS command to be executed.
There are example programs for exec... and spawn... in the Reference Guide.
Specific examples of the use of the function execlp can be found in the
programs FILEWRIT.C, LINKLIST.C and TWOLINKS.C, whilst STRUCTUR.C uses
the function spawnlp and the program DOSOPS.C uses the function system.
On error, the exec... function returns -1, so it is wise to check for this
as shown in LINKLIST.C below:
stat = execlp("DEBUG.COM","DEBUG.COM",NULL);
if (stat == -1) printf("Execlp error = %d\n",stat);
On error, the system function returns -1, so in the program DOSOPS.C a
function status() is created which receives the value returned by the
function system() and reports the failure, as shown below:
status(int val)
{
if (val == -1)
printf("Error in system function call to COMMAND.COM\n");
}
main()
{
....
....
status(system("DIR *.SYS"));
....
}