Can't solve memory management (valgrind) in this c program - c

Question

My problem is as follows. I have to create a C program that concatenates all the parameters in input in one buffer and then this return this buffer. My solution works, but there are memory management issues.
How can I fix the problem?
#define RIALLOCA(buf, newsize) buf = realloc(buf, newsize);
char *mystrcat(char *buf, size_t sz, char *first, ...) {
va_list l;
va_start(l, first);
buf = malloc(strlen(buf) + 1);
if (sz < strlen(first) + 1) {
sz += (strlen(first) + 1);
}
RIALLOCA(buf, sz + 1 + 16);
strncat(buf, first, strlen(first));
char *nextString = va_arg(l, char *);
while (nextString != NULL) {
// sz += strlen(nextString);
RIALLOCA(buf, strlen(buf) + strlen(nextString) + 1 + 16);
strncat(buf, nextString, strlen(nextString));
nextString = va_arg(l, char *);
}
va_end(l);
return buf;
}
int main(int argc, char *argv[]) {
if (argc != 7) {
printf("troppi pochi argomenti\n");
return -1;
}
char *buffer = NULL;
RIALLOCA(buffer, 16); // macro che effettua l'allocazione
buffer[0] = '\0';
buffer = mystrcat(buffer, 16, argv[1], argv[2], argv[3], argv[4], argv[5],
argv[6], NULL);
printf("%s\n", buffer);
free(buffer);
return 0;
}
This is problem with Valgrind

Answer 1

The problem is that - when you call buf = malloc() inside mystrcat() - you are causing a memory leak. Memory had already been dynamically allocated for buf within your main().
Change
buf = malloc(strlen(buf)+1);
to
buf = realloc(buf, strlen(buf) + 1);
Also ... I agree with user3629249: providing macros for standard library functions in the way you have done seems redundant - at least in this context.

Answer 2

in function: mystrcat()
This statement:
buf = malloc(strlen(buf) + 1);
is not correct. Because buf is already a pointer to allocated memory AND this is not modifying the pointer buf back in the main() function but rather just the parameter on the stack.
To correct:
in main() (notice the additional '&' on parameter: buffer)
buffer = mystrcat(&buffer, 16, argv[1], argv[2], argv[3], argv[4],
argv[5], argv[6], NULL);
in mystrcat() remove this line
buf = malloc(strlen(buf) + 1);
regarding:
if(sz < strlen(first) + 1)
{
sz += (strlen(first) + 1);
}
since sz contains 16 and the first character in buf is '\0' This if() statement will never be entered.
Since the passed parameter buf is now a pointer to a pointer, due to the change in main(), the signature should be:
char* mystrcat(char **buf, size_t sz, char *first, ...)
and all references to buf should be de-referencing that parameter to access the pointer value in main()
There is plenty more, but the above are the underlying problems