K&R Exercise 5-15: sorting with extra options
I'd use a typedef for a pointer to your compare functions:

typedef int (*cmpfnc)(void *,void *);


This simplifies things and eliminates the ugly cast used in main's call to qsort1

I'd remove the &amp; from &amp;strcmp et. al. It is less idiomatic than strcmp. Both say "I want the address of ...". This allows the address of a function and a function pointer of the same type to be used interchangeably syntactically.

That is, you can say:

typedef int (*cmpfnc)(void *,void *);

cmpfnc myptr;

myptr = (cmpfnc) strcmp;

strcmp(x,y);
myptr(x,y);


And, they're all interchangeable. Also, note that (*myptr)(x,y) is simplified.

If you compile with [more] warning (e.g. -Wall), setting compareFunctionPointers needs a cast for strcmp. Using strcmp directly here is a slight blemish (because its signature uses const char * instead of void *, but it works in practice with the cast.

You could wrap strcmp with a wrapper function: int mystrcmp(void *left,void *right) { return strcmp(left,right); } as an alternative to using the cast. This would allow the compiler to do more type checking. But, using strcmp directly would probably run faster.

For most of your functions, you're breaking type safety of the arguments, so you have to be careful when doing that to be sure that things match up when the program runs.

Side note: You use sortDecreasing in main, but rename it in qsort1. Note that the the name in main is much more descriptive of its meaning. Also, you could replace the value with 1 or -1 and simplify qsort1 with a multiply (see my cleaned up code below).



Here is your code annotated with what I think could be cleaned up a bit [Note that it's based on the code in your original post, so I had to guess about some missing functions]:

#include &lt;string.h&gt;

#define MAX_NO_OF_LINES     100

// NOTE/BUG: this should be a typedef
int (*compareFunctionPointers[3]) (void *, void *);

void
swap(void **v,int left,int right)
{
    void *tmp;

    tmp = v[left];
    v[left] = v[right];
    v[right] = tmp;
}

int
numcmp(void *left,void *right)
{
    return *(int *) right - *(int *) left;
}

// NOTE/BUG: this should use a typedef
void
qsort1(void *v[], int left, int right, int (*comp) (void *, void *), int increesingOrDecresing)
{
    int last,
     compare;

    // Base case
    if (left &gt;= right) {
        return;
    }

    swap(v, left, (right + left) / 2);
    last = left;
    for (int i = left + 1; i &lt;= right; i++) {
        compare = (increesingOrDecresing) ? -(*comp) (v[i], v[left])
            : (*comp) (v[i], v[left]);
        if (compare &lt; 0)
            swap(v, ++last, i);
    }
    swap(v, last, left);
    qsort1(v, left, last - 1, comp, increesingOrDecresing);
    qsort1(v, last + 1, right, comp, increesingOrDecresing);
}

enum {
    COMPARE_DEFAULT,
    COMPARE_FOLDED,
    COMPARE_NUMERICLY
};

char *pointersToLines[MAX_NO_OF_LINES];

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

    // Store pointers to the diffrent types of
    // compare functions in a array of
    // function pointers.
    // NOTE/BUG: these require a cast
    // NOTE/BUG: leave off the "&amp;" as it is more idiomatic
    compareFunctionPointers[COMPARE_DEFAULT] = &amp;strcmp;
#if 0
    compareFunctionPointers[COMPARE_FOLDED] = &amp;strcmpFold;
#else
    compareFunctionPointers[COMPARE_FOLDED] = &amp;strcasecmp;
#endif
    compareFunctionPointers[COMPARE_NUMERICLY] = &amp;numcmp;

    int numberOfLines;
    int sortDecreasing = 0;             // 1 if the input is to be sorted in a decreasing order
    int compareFunction = COMPARE_DEFAULT;  // Index for the compare-function

    // Parse the command line arguments
    if (argc &gt; 1) {
        for (int i = 1; *(argv + i); i++) {
            char *c = *(argv + i);

            if (strcmp(c, "-n") == 0)
                compareFunction = COMPARE_NUMERICLY;
            if (strcmp(c, "-r") == 0)
                sortDecreasing = 1;
            if (strcmp(c, "-f"))
                compareFunction = COMPARE_FOLDED;

        }
    }

#if 0
    if ((numberOfLines = readLines(pointersToLines, MAX_NO_OF_LINES)) &gt;= 0) {
#endif

    // NOTE/BUG: too much casting, particularly on compareFunctionPointers
        qsort1((void **) pointersToLines, 0, numberOfLines - 1, (int (*)(void *, void *)) compareFunctionPointers[compareFunction], sortDecreasing);

#if 0
        writeLines(pointersToLines, numberOfLines);
    }
#endif

    return 0;
}




Here is a more cleaned up version incorporating the ideas I've expressed above:

#include &lt;string.h&gt;

#define MAX_NO_OF_LINES     100

typedef int (*cmpfnc)(void *,void *);

cmpfnc compareFunctionPointers[3];

void
swap(void **v,int left,int right)
{
    void *tmp;

    tmp = v[left];
    v[left] = v[right];
    v[right] = tmp;
}

int
numcmp(void *left,void *right)
{
    return *(int *) right - *(int *) left;
}

void
qsort1(void *v[], int left, int right, cmpfnc comp, int sortdir)
{
    int last,
     compare;

    // Base case
    if (left &gt;= right) {
        return;
    }

    swap(v, left, (right + left) / 2);
    last = left;
    for (int i = left + 1; i &lt;= right; i++) {
        compare = sortdir * comp(v[i], v[left]);
        if (compare &lt; 0)
            swap(v, ++last, i);
    }
    swap(v, last, left);
    qsort1(v, left, last - 1, comp, sortdir);
    qsort1(v, last + 1, right, comp, sortdir);
}

enum {
    COMPARE_DEFAULT,
    COMPARE_FOLDED,
    COMPARE_NUMERICLY
};

char *pointersToLines[MAX_NO_OF_LINES];

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

    // Store pointers to the diffrent types of
    // compare functions in a array of
    // function pointers.
    compareFunctionPointers[COMPARE_DEFAULT] = (cmpfnc) strcmp;
    compareFunctionPointers[COMPARE_FOLDED] = (cmpfnc) strcasecmp;
    compareFunctionPointers[COMPARE_NUMERICLY] = numcmp;

    int numberOfLines;
    int sortDecreasing = 1;             // 1=increasing, -1=decreasing
    int compareFunction = COMPARE_DEFAULT;  // Index for the compare-function

    // Parse the command line arguments
    if (argc &gt; 1) {
        for (int i = 1; *(argv + i); i++) {
            char *c = *(argv + i);

            if (strcmp(c, "-n") == 0)
                compareFunction = COMPARE_NUMERICLY;
            if (strcmp(c, "-r") == 0)
                sortDecreasing = -1;
            if (strcmp(c, "-f"))
                compareFunction = COMPARE_FOLDED;

        }
    }

#if 0
    if ((numberOfLines = readLines(pointersToLines, MAX_NO_OF_LINES)) &gt;= 0) {
#endif

    // NOTE/BUG: too much casting, particularly on compareFunctionPointers
        qsort1((void **) pointersToLines, 0, numberOfLines - 1,
        compareFunctionPointers[compareFunction], sortDecreasing);

#if 0
        writeLines(pointersToLines, numberOfLines);
    }
#endif

    return 0;
}