qsort turns all values of array to null

Question

I have a program which reads in the lines of a text file and stores them in linesArr, I have also defined a function compFunc which takes two strings as inputs, makes copies of them and converts the copies to lowercase before returning the value of strcmp(copy1, copy2). I am trying to use this function to sort linesArr into alphabetical order with qsort(linesArr, size, 255, compFunc. But the values in the array turn from {"Bob", "James", "Alice"} to {(null), (null), (null)}.

This is how linesArr is initialised.

char **linesArr = (char**)malloc(size*sizeof(char));
for (int i = 0; i < size; i++) {
    linesArr[i] = (char*)malloc(255*sizeof(char));
}

And it it filled with values from the text using the file pointer fp

for (int i = 0; i < size; i++) {    
    fgets(line, 255, fp);
    strcpy(linesArr[i], line);
}

Why is qsort deleting the values in the array?

Answer 1

For starters this memory allocation

char **linesArr = (char**)malloc(size*sizeof(char));
                                             ^^^^^

is incorrect, You need to write

char **linesArr = (char**)malloc(size*sizeof(char *));
                                             ^^^^^^

Secondly the call of qsort must look like

qsort(linesArr, size, sizeof( char * ), compFunc);

because the array pointed to by the pointer linesArr is an array of pointers.

Here is a demonstrative program.

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

int compFunc( const void *a, const void *b )
{
    const char *s1 = *( const char ** )a;
    const char *s2 = *( const char ** )b;
    
    while ( *s1 && tolower( ( unsigned char )*s1 ) == tolower( ( unsigned char )*s2 ) )
    {
        ++s1;
        ++s2;
    }
    
    return tolower( ( unsigned char )*s1 ) - tolower( ( unsigned char )*s2 );
}

int main(void) 
{
    enum { size = 3, len = 255 };
    
    char **linesArr = malloc( size * sizeof( char * ) );
    
    for ( size_t i = 0; i < size; i++ )
    {
        linesArr[i] = malloc( len * sizeof( char ) );
    }
    
    char line[len];
    
    for ( size_t i = 0; i < size; i++ )
    {
        fgets( line, sizeof( line ), stdin );
        line[ strcspn( line, "\n" ) ] = '\0';
        strcpy( linesArr[i], line );
    }
    
    qsort( linesArr, size, sizeof( *linesArr ), compFunc );
    
    for ( size_t i = 0; i < size; i++ )
    {
        puts( linesArr[i] );
    }
    
    for ( size_t i = 0; i < size; i++ )
    {
        free( linesArr[i] );
    }
    free( linesArr );
    
    return 0;
}

The program output might look like

Bob
James
Alice
Alice
Bob
James