learn CPP, the code is an exercise of string substitution, in which the expression
p = strstr (pdjm)! = NULL in the while loop will report an error at this point.
if you change this sentence to ((p = strstr)! = NULL), there will be no problem if you execute the left first.
feels like it was executed first on the right. What I don"t understand is why the sentence strstr! = NULL is wrong.

the Visual Studio 2017 compiler used is the default configuration has not been changed,

-sharpdefine _CRT_SECURE_NO_WARNINGS
-sharpinclude <stdio.h>
-sharpinclude <stdlib.h>
-sharpinclude <string.h>
-sharpinclude <ctype.h>

                            // 
int replaceSubstr(char *src,char **dst,char *sub, char *new_sub)
{
    char *out = NULL;
    char *p = NULL;  
    char *q = NULL; 

    int sub_len = 0;
    int new_sub_len = 0;

    if (src == NULL || dst == NULL || new_sub == NULL) {
        fprintf(stderr, "src == NULL || dst == NULL || new_sub == NULL");
        return -1;
    }

    out = (char *)malloc(sizeof(char *) * 4096); //4096 * char*
    
    if (out == NULL)
    {
        fprintf(stderr, "malloc out error\n");
        return -1;
    }
    
    memset(out, 0, 4096); //4096

    p = src; // p
    q = p;   // q

    sub_len = strlen(sub); // 
    new_sub_len = strlen(new_sub); //   

    while (  p = strstr(p, sub) != NULL) {  // strstr
        // 
                    
        //q p-qsub
        strncat(out, q, p - q); //out
        strncat(out, new_sub, new_sub_len); //sub

        p += sub_len; // p

        q = p;  //q

        if (*p == "\0") { // *p0 
            break;
        }
    }

    if (*q != "\0") {
        strncat(out, q,( src + strlen(src) - q ));
    }

    *dst = out;
    //free();
    return 0;
}

int main(void) 
{
    char *str = "213abcd123231abcd213321asdabcd123214abcd";
    char *dst = NULL;

    replaceSubstr(str,&dst,"abcd","XX");
     
    printf("%s\n", dst);
    return 0;
}