Programming in C

General Utility Functions
491
arguments.” qsort calls this function whenever it needs to compare two elements in
the array, passing it pointers to the elements to compare.The function, which is usersupplied,
is expected to compare the two elements and return a value less than zero,
equal to zero, or greater than zero if the first element is less than, equal to, or greater
than the second element, respectively.
Here is an example of how to use qsort to sort an array of 1,000 integers called
data:
#include
...
int main (void)
{
int data[1000], comp_ints (void *, void *);
...
qsort (data, 1000, sizeof(int), comp_ints);
...
}
int comp_ints (void *p1, void *p2)
{
int i1 = * (int *) p1;
int i2 = * (int *) p2;
return i1 - i2;
}
Another routine called bsearch, which is not described here, takes similar arguments
to qsort and performs a binary search of an ordered array of data.
int rand (void)
Returns a random number in the range [0, RAND_MAX], where RAND_MAX is defined in
and has a minimum value of 32767. See also srand.
void srand (seed)
Seeds the random number generator to the unsigned int value seed.
int system (s)
Gives the command contained in the character array pointed to by s to the system
for execution, returning a system-defined value. If s is the null pointer, system
returns a nonzero value if a command processor is available to execute your commands.
As an example, under Unix, the call
system ("mkdir /usr/tmp/data");
causes the system to create a directory called /usr/tmp/data (assuming you have the
proper permissions to do so).