How do I sort the elements of argv in C?

问题

I am trying to alphabetically sort the elements of argv.

The following line of code is giving me problems:

qsort(argv[optind], argc - optind, sizeof(argv[optind]), sort);

Specifically, the last argument is giving me trouble, the comparison function, which is given below:

int
sort(const void *a, const void * b)
{    
    return(strcmp( (char*)a, (char*)b ));
}

At the moment, it compiles fine, but I end up getting a segmentation fault when I run it.

回答1:

The first argument should be argv+optind, as this is the address of the first element in the sequence to be sorted.

回答2:

The man page for qsort(3) contains one example, which does exactly what you want. It also explains why:

http://linux.die.net/man/3/qsort

Summary: You are missing one level of referencing on the qsort() first argument, and missing one level of dereferencing inside the sort() function.

回答3:

Here's my attempt at sorting argv

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

int mycomp(const void *a, const void *b) {
  /* function code removed to prevent homework copy/paste */
}

int main(int argc, char **argv) {
  int i;
  qsort(argv + 1, argc - 1, sizeof *argv, mycomp);
  for (i = 1; i < argc; i++) printf("i: %d ==> '%s'\n", i, argv[i]);
  return 0;
}

And a sample run of the program

$ ./a.out one two three four five six seven
i: 1 ==> 'five'
i: 2 ==> 'four'
i: 3 ==> 'one'
i: 4 ==> 'seven'
i: 5 ==> 'six'
i: 6 ==> 'three'
i: 7 ==> 'two'

回答4:

The problem is in the structure of the argv array.

It is structured like this

program\0arg1\0argument2\0a3\0\0

The qsort function assumes that all elements are the same size, but in this case they are not. You specify the size of argv[optind], but not all elements are that size.

Edit: I was mistaken, you do not pass the string length to qsort, but the length of the pointers. So argv contains an array of pointers. The goal is to sort the pointers.

That means that you pass the array of pointers to qsort, and that the sort function should expect a pointer. Like this:

int
sort(const void *a, const void * b)
{
    return(strcmp( *(char**)a, *(char**)b ));
}

qsort(argv+optind, argc - optind, sizeof(argv[optind]), sort);

来源：https://stackoverflow.com/questions/3707051/how-do-i-sort-the-elements-of-argv-in-c

标签

sorting

argv