Memory Leak with Copying a Character Pointer - c

Question

Since strsep is a destructive operation, I want to preserve the contents of the line array, so I make a copy of line and assign it to the variable newline. I then do the destructive operation on newline. I then try to free newline but I am getting a memory leak. I'm not 100% sure, but the memory leak is likely due to newline not being entirely freed. Before, when I performed the strsep operation destructively on line, I did not get the memory leak. I am compiling my C code with fsanitize=address to uncover the memory leak.
struct Item {
char* line;
struct Item *next;
};
struct Item* getArguments(char *line) {
struct Item *current = NULL;
struct Item *next = NULL;
struct Item *first = NULL;
char *found = NULL;
char *newline = strdup(line);
char blank[] = "";
while ((found = strsep(&newline," \n")) != NULL) {
if (strcmp(found, blank) != 0) {
next = malloc(sizeof(struct Item));
next->next = NULL;
next->line = malloc((strlen(found)+1) * sizeof(char));
strcpy(next->line, found);
if (current) {
current->next = next;
}
current = next;
if (first == NULL) {
first = current;
}
}
}
free(found);
free(newline);
newline = NULL;
return first;
}

Answer 1

Changed argument to const char *line as you explicitly don't modify it.
Retain a copy of address returned by strdup(line) for use with free() as strsep() modifies that pointer. Made it a const pointer to be really explicit about it.
Don't free(found).
Minimized scope of variables.
Assignments in a condition is a little hard to read so I changed the loop early return loop construct.
if(!*found) is the same as char blank[] = "": strcmp(found, blank) to check for empty string.
Check the return value of malloc() and strdup() and handle clean-up on failure. This change adds a lot of noisy error handling.
if(current) and if(first == NULL is really just a special case around the first iteration so collapsed those into if-else.
(not fixed) You could replace the malloc() with a calloc() to eliminate the line next->next = NULL.
(not fixed) Consider tweaking the interface to allow caller to differentiate between memory allocation errors and no strings found.
#define _POSIX_C_SOURCE 200809L
#define _DEFAULT_SOURCE
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
struct Item {
char *line;
struct Item *next;
};
void freeArguments(struct Item *first) {
while (first) {
struct Item *tmp = first->next;
free(first->line);
free(first);
first = tmp;
}
}
struct Item *getArguments(const char *line) {
struct Item *first = NULL;
char *const newline = strdup(line);
if(!newline)
goto err;
char *newline2 = newline;
for (struct Item *current = NULL;;) {
char *found = strsep(&newline2," \n");
if(!found)
break;
if (!*found)
continue;
struct Item *next = malloc(sizeof *next);
if(!next)
goto err;
next->line = strdup(found);
if(!next->line)
goto err;
next->next = NULL;
if (first)
current->next = next;
else
first = next;
current = next;
}
free(newline);
return first;
err:
freeArguments(first);
free(newline);
return NULL;
}
int main() {
const char *s = "hello\nworld again";
struct Item *first = getArguments(s);
for (struct Item *current = first; current; current = current->next)
printf("%s\n", current->line);
freeArguments(first);
}
and valgrind seems happy:
==1734409== HEAP SUMMARY:
==1734409== in use at exit: 0 bytes in 0 blocks
==1734409== total heap usage: 8 allocs, 8 frees, 1,108 bytes allocated
==1734409==
==1734409== All heap blocks were freed -- no leaks are possible