C using malloc and realloc to dynamically increase string length - c

Question

Currently learning memory management in C, and I am currently running into issues increasing string length as a loop iterates.
The method I am trying to figure out logically works like this:
// return string with "X" removed
char * notX(char * string){
result = "";
if(for int = 0; i < strlen(string); i++){
if (string[i] != 'X') {
result += string[i];
}
}
return result;
}
Simple enough to do in other languages, but managing the memory in C makes it a bit challenging. Difficulties I run into is when I use malloc and realloc to initialize and change size of my string. In my code I currently tried:
char * notX(char * string){
char* res = malloc(sizeof(char*)); // allocate memory for string of size 1;
res = ""; // attempted to initialize the string. Fairly certain this is incorrect
char tmp[2]; // temporary string to hold value to be concatenated
if(for int = 0; i < strlen(string); i++){
if (string[i] != 'X') {
res = realloc(res, sizeof(res) + sizeof(char*)); // reallocate res and increasing its size by 1 character
tmp[0] = string[i];
tmp[1] = '\0';
strcat(res, tmp);
}
}
return result;
}
Note, I have found success in initializing result to be some large array like:
char res[100];
However, I would like to learn how to address this issue with out initializing an array with a fixed size since that might potentially be wasted memory space, or not enough memory.

Answer 1

realloc needs the number of bytes to allocate. size is incremented for each character added to res. size + 2 is used to provide for the current character being added and the terminating zero.
Check the return of realloc. NULL means a failure. Using tmp allows the return of res if realloc fails.
char * notX(char * string){
char* res = NULL;//so realloc will work on first call
char* tmp = NULL;//temp pointer during realloc
size_t size = 0;
size_t index = 0;
while ( string[index]) {//not the terminating zero
if ( string[index] != 'X') {
if ( NULL == ( tmp = realloc(res, size + 2))) {//+ 2 for character and zero
fprintf ( stderr, "realloc problem\n");
if ( res) {//not NULL
res[size] = 0;//terminate
}
return res;
}
res = tmp;//assign realloc pointer back to res
res[size] = string[index];
++size;
}
++index;//next character
}
if ( res) {//not NULL
res[size] = 0;//terminate
}
return res;
}

Answer 2

2 main errors in this code:
the malloc and the realloc function with parameter that call sizeof(char*). In this case the result of sizeof(char*) is the size of a pointer, not of a char, so you have to substitute the char* with char in the sizeof function.
res = ""; is incorrect. You primarly have a memory leak because you lose the pointer to the just allocated memory in malloc function, secondary but not less important, you have an undefined behavior when call realloc function over res initialized as an empty string ( or better a constant string), after the above initialization the memory is no longer dinamically managed. To substitute this initialization i think a memset to 0 is the best solution.