/in/foo.c: In function 'main':
/in/foo.c:14:28: warning: passing argument 4 of 'qsort' from incompatible pointer type [-Wincompatible-pointer-types]
     qsort(a,n,sizeof(a[0]),cmp);//(数组，需要排序的数字个数，单个数字所占内存大小，比较函数）
                            ^~~
In file included from /in/foo.c:2:0:
/usr/include/stdlib.h:725:13: note: expected '__compar_fn_t {aka int (*)(const void *, const void *)}' but argument is of type 'long int (*)(const void *, const void *)'
 extern void qsort (void *__base, size_t __nmemb, size_t __size,
             ^~~~~