Changing value of String passed to void function - c

Question

I am writing a method to remove whitespaces from a String passed as a pointer to a void function as follows -
char * mystrcpy(char * dest, char * str) {
dest = malloc(sizeof(str));
char * tep = dest;
while(*dest++ = *str++) {
}
return tep;
}
void trimWhiteSpace(char * str){
int counter = 0;
int i = 0;
char * res = malloc(sizeof(str));
for(i = 0; str[i]; i++) {
if(str[i] != ' ')
res[counter++] = str[i];
}
res[counter] = '\0';
mystrcpy(str, res);
printf("I got %s\n", str);
}
However, when I try to test my method using the main -
int main() {
char * myStr = (char*)malloc(sizeof(char) * 50);
myStr = "Hello World ! ";
trimWhiteSpace(myStr);
printf("%s", myStr);
return 0;
}
the program prints
"HelloWorld!"
inside of the void trimWhiteSpace(char * str) method but that is not the case with the main which prints the unmodified string.
How do I make sure that the main method prints the same string as printed by the trimWhiteSpace method?

Answer 1

Your program starts with a memory leak.:)
int main() {
char * myStr = (char*)malloc(sizeof(char) * 50);
myStr = "Hello World ! ";
//...
At first you allocated memory and assigned its address to pointer myStr
char * myStr = (char*)malloc(sizeof(char) * 50);
and in the next statement you reassigned the pointer with the address of a string literal
myStr = "Hello World ! ";
Take into account that string literals are immutable in C and C++. Any attempt to modify a string literal results in undefined behaviour.
You should use standard C function strcpy that to copy the string literal in the allocated memory.
strcpy( myStr, "Hello World ! " );
Or another error. You are using operator sizeof applied to a pointer
char * mystrcpy(char * dest, char * str) {
dest = malloc(sizeof(str));
^^^^^^^^^^^
Usually sizeof( char * ) is equal to 4 or 8 depending on the environment where the program was compiled.
You should use another standard C function strlen. For example
char * mystrcpy(char * dest, char * str) {
dest = malloc( strlen( str ) + 1);
There are other errors in the program.
Nevertheless the function that removes blanks could be written much simpler.
Here is a demonstrative program
#include <stdio.h>
char * trimWhiteSpace( char * str )
{
char *q = str;
char c = ' ';
while ( *q && *q != c ) ++q;
char *p = q;
while ( *q )
{
if ( *++q != c ) *p++ = *q;
}
return str;
}
int main( void )
{
char myStr[] = "Hello World ! ";
puts( myStr );
puts( trimWhiteSpace( myStr ) );
}
Its output is
Hello World !
HelloWorld!