getopt（）中的optarg始终为null

Question

I'm trying to process an argument parameter using the optarg parameter of getopt(), but it remains null. Could this be something to do with the c99 standard? I know I will need to actually copy the string from optarg but it never even gets set.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <getopt.h>
#include <string.h>

int main(int argc, char *argv[]) {
    char *optarg;
    int ch; 

    char *indir = NULL;

    while ((ch = getopt(argc, argv, "d:")) != -1) {
        switch(ch) {
            case 'd':
                indir = optarg;
                fprintf(stderr, "Optarg: %s\n", optarg);
                fprintf(stderr, "Dir name: %s\n", indir);
                break;
            default : 
                fprintf(stderr, "Usage:  test -d <input directory>\n");
                exit(1);
        }
    }
    if(indir == NULL){
        fprintf(stderr, "Input directory required.\n");
        exit(1);
    }
    else{
        printf("Input dir: %s\n", indir);
    }


    return 0;
}

Answer 1

optarg is something that's initialized by a call to getopt(), you're overwriting it by initializing it yourself. Remove the line "char* optarg;" and you should be good to go.