Dealing with non-option arguments in getopt before options are checked - c

Question

I am writing a program and I want to get the non-option arguments before checking the flags.
For example, if the arguments are ./a.out -a -b 50 filename
I want to use filename and then do task_a() and/or task_b()depending on the flags. filename can be any argument in argv[]
int opt;
float value;
while ((opt = getopt(argc, argv, "ab:")) != -1)
case 'a':
task_a(filename);
break;
case 'b':
value = atof(optarg);
task_b(filename, value);
break;
So basically I need to extract filename before using it in my tasks. How can I implement this?

Answer 1

getopt will pass you the command line arguments in the order that it finds them on the command line. This is generally NOT the order that you want to process them.
Instead, have your program set its options first before performing the operations - so something like ( given #include <stdbool.h> ) :
// Set the defaults
bool doTaskA = false;
book doTaskB = false;
float brightness = -1;
int opt;
float value;
while ((opt = getopt(argc, argv, "ab:")) != -1) {
case 'a':
doTaskA = true;
break;
case 'b':
doTaskB = true;
brightness = atof(optarg);
break;
}
filename = ....,,
if (doTaskA) {
task_a(filename);
}
if (doTaskB) {
task_b(filename, brightness);
}
I chose the name “brightness” as a meaningful attribute name - naturally you should choose your own. As good practice, do NOT name your variables “optionA” or some such - variable names should describe what they ARE, not the specific implementation details.
Always imagine that your options can change letters over time (what is now option “a” might become option “R” in six months time) - such changes should not affect anything except the “getopt” arguments and “case” statements.
EDITED TO CHANGE :
I originally wrote this with “if (brightness >= 0.0)”, but decided that in general a separate doTaskB variable is cleaner, more explicit approach that does not limit the possible values of the argument.