更改传递给void函数的String的值

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!