realloc fails when printf is in code / weird chars - c

Question

I try to program a simple implementation of ls for school
#include <dirent.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/stat.h>
int normalsort( const void *string1, const void *string2) {
char *const *char1 = string1;
char *const *char2 = string2;
return strcasecmp(*char1, *char2);
}
int listDir (char* path, int listToggeled, int classifyToggeled){
DIR *dir = opendir(path);
struct dirent *entry;
struct stat s;
char** listofentries;
char symbol = '-';
int counter = 0;
while ((entry = readdir(dir)) != NULL){
if (entry->d_name[0] != '.'){
listofentries = realloc(listofentries, (counter + 1) * sizeof(char*));
listofentries[counter] = entry->d_name;
counter++;
}
}
qsort(listofentries, counter, sizeof(char*), normalsort);
for (int i = 0; i < counter; i++){
char* entryname = listofentries[i];
if (entryname[0] != '.'){
printf("%s", entryname);
if (classifyToggeled == 1){
lstat(entryname, &s);
if (S_ISDIR(s.st_mode)) {
symbol = '/';
} else if (S_ISLNK(s.st_mode)) {
symbol = '#';
} else if ((S_ISREG(s.st_mode)) && (s.st_mode & S_IXUSR)) {
symbol = '*';
} else {
symbol = ' ';
}
printf("%c", symbol);
}
if (listToggeled == 1){
printf("\n");
}
else {
printf(" ");
}
}
}
closedir(dir);
return 0;
}
int main(int argc, char **argv) {
int classifyToggeled = 0;
int listToggeled = 0;
char* dirToList = ".";
if (argc == 1){
listDir(dirToList, listToggeled, classifyToggeled);
return 0;
}
for (int i = 1; i < argc; i++){
char* currentArg = argv[i];
//Check if -F is set
if (strcmp(currentArg, "-F") == 0 || strcmp(currentArg, "-1F") == 0 || strcmp(currentArg, "-F1") == 0){
classifyToggeled = 1;
}
//Check if -1 is set
if (strcmp(currentArg, "-1") == 0 || strcmp(currentArg, "-1F") == 0 || strcmp(currentArg, "-F1") == 0){
listToggeled = 1;
}
//List out all folders
if (currentArg[0] != '-'){
dirToList = currentArg;
}
}
//If no folders were listed yet, list current folder
//printf("dirtolist: %s", dirToList); <-- This line
listDir(dirToList, listToggeled, classifyToggeled);
if (listToggeled == 0){
printf("\n");
}
return 0;
}
It has a few bugs:
When the printf line marked above is not commented out, the program tells realloc(): invalid old size
When this line is commented out this only happens if the program is executed without any parameters
/bin/ and /sbin/ print out weird characters
I think this is some kind of memmory issue but I have hardly any C knowledge to just see it

Answer 1

Your code exhibits undefined behaviour because you passed a non-static, uninitialised pointer to realloc whose contents were indeterminate.
From C11, Memory Management Functions:
The realloc function deallocates the old object pointed to by ptr and
returns a pointer to a new object that has the size specified by size.
....if ptr does not match a pointer earlier returned by a memory
management function, or if the space has been deallocated by a call to
the free or realloc function, the behavior is undefined.
Possible fixes:
Initialize it with 0 / NULL.
Declare it as static.¹
Replace it with a call to malloc.
The realloc function returns a pointer to the new object (which may
have the same value as a pointer to the old object), or a null pointer
if the new object could not be allocated.
realloc returns NULL on failure, your code should check for it.
#include <errno.h>
char *line = 0;
errno = 0;
char *new = realloc (line, size);
if (!new) {
perror ("realloc");
/* realloc() failed to allocate memory, handle error accordingly. */
}
The above code snippet can be rewritten as:
#include <errno.h>
errno = 0;
char *new = malloc (size);
if (!new) {
perror ("malloc");
/* malloc() failed to allocate memory, handle error accordingly. */
}
Similarly, opendir returns a NULL pointer while lstat returns -1 to indicate an error. Check for them.
[1] — Objects declared with static storage duration and are always initialised. In the absence of a definition, they are implicitly initialised to 0.
From the C standard C11 6.7.9/10:
"... If an object that has static or thread storage duration is not
initialized explicitly, then:
— if it has pointer type, it is initialized to a null pointer;
— if it has arithmetic type, it is initialized to (positive or
unsigned) zero;"
Though, it's good practice to explicitly set it to 0.

Answer 2

Bugs:
listofentries isn't initialized, it must be set to NULL or you can't use realloc.
You don't check if realloc succeeded or not.