C deleting char in string

March 28, 2022

Now I’m using this code to delete some char in a string.

void eliminate(char *str, char ch){
    for(; *str != '\0';str++){
        if(*str == ch){
            strcpy(str, str+1);
            str--;
        }
    }
}

In the char *str there are some strings like

"sll $6, $5, 16"

After deleting "$", the string looks like this.

"sll 6, 5, 16"

But after deleting ",", the string became very strange.

"sll 6 5 6"

Is there any problem with the code above? And also, it only happens in Linux and online GDB. VS code in my window laptop eliminates the targeted char very well.

>Solution :

As pointed out in comments strcpy() is not safe when coping data with overlapping memory blocks. memmove(dst, src, len) is the alternative which uses an auxiliary buffer in case of src & dst memory overlaps.

You can simply skip the character to eliminate in a loop:

#include <stdio.h>

void drop_char (char *str, char ch) {
    if (!str) return;

    for(char* cp = str; 1 ; ) {
        if(*cp != ch)
            *str++ = *cp;
        if ('\0' == *cp++)
            break;
    }
}

int main () {
    char str [] = "sll     $6, $5, 16";
    printf ("Original   : [%s]", str);

    drop_char(str, '$');
    printf ("\nDropping $ : [%s]", str);

    drop_char(str, ',');
    printf ("\nDropping , : [%s]", str);

    printf ("\n");

    return 0;
}