Opening multiple directories in C - c

Question

I am trying to make a program that creates a directory in which multiple directories are created, then in each directory I am creating a file. I cannot seem to open those "multiple directories" so that I can put my file there. I tried using name3 as a parameter, and I also tried creating a const char* with name3's value and nothing worked.
error: malloc.c:2379: sysmalloc: Assertion `(old_top == initial_top (av) && old_size == 0) || ((unsigned long) (old_size) >= MINSIZE && prev_inuse (old_top) && ((unsigned long) old_end & (pagesize - 1)) == 0)' failed. Aborted (core dumped)
here is my code
#include <dirent.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>
int make_directory(char * name) {
int checker=mkdir(name, S_IRWXU | S_IRWXG | S_IRWXO);
return checker;
}
char** getNames() {
char** names=malloc(10*sizeof(char*));
for(int i=0;i<10;i++) {
if(i==9) {
names[i]=malloc(3*sizeof(char));
names[i][0]='1';
names[i][1]='0';
names[i][2]='\0';
} else {
names[i]=malloc(2*sizeof(char));
names[i][0]=49+i;
names[i][1]='\0';
}
}
return names;
}
int makeTenDirs() {
char **names=getNames();
char *name2;
for(int i=0;i<10;i++) {
name2=NULL;
name2=getcwd(NULL,0);
strncat(name2,"/input/dir",11);
strncat(name2,names[i],1);
int s=make_directory(name2);
}
name2=NULL;
name2=getcwd(NULL,0);
strncat(name2,"/input/dir",11);
strncat(name2,names[0],1);
strncat(name2,"0",2);
int s=make_directory(name2);
}
int main() {
char **names=getNames();
FILE *file;
DIR *dir;
DIR *dir2;
struct dirent *dent;
char * name1="./input";
char *name3;
int proceed=make_directory("./input");
if(proceed==-1) {
printf("Error making the directory\n");
}
makeTenDirs();
dir=opendir("./input");
if(dir!=NULL) {
name3=getcwd(NULL,0);
while((dent=readdir(dir))!=NULL){
if(strcmp(dent->d_name,"..")!=0 && strcmp(dent->d_name,".")!=0) {
name3=getcwd(NULL,0);
strncat(name3,"/input/",8);
strncat(name3,dent->d_name,10);
printf("%s\n",name3);
dir2=opendir(name3);
if(dir2!=NULL) {
printf("alo");
}
}
}
}
closedir(dir);
free(names);
return 0;
}
Any tips on how to open the directories (and maybe put the files in them)?

Answer 1

Below is a working implementation. I cleaned up all the warnings (you had some unused variables, makeTenDirs was not returning a value). Always look at and fix the warnings, use -Wall -Wextra flags to enable them. As I thought before, you were invoking undefined behavior by overwriting the buffers of name2 and name3. The way you were using getcwd it was allocating exactly enough space for name2 and name3. As soon as you strcat to that, you overwrite the buffer, invoking UB. At that point, the program can behave in completely unpredictable ways, including appearing to work. You hope your program crashes when there's UB so you're alerted to the problem. Below, I've used a 2nd operating mode of getcwd that doesn't internally malloc memory and instead keeps everything in automatic memory (on the stack). This relieves you of having to manage memory manually. I've included comments that hopefully explain everything, let me know if you have questions.
#include <dirent.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <stdlib.h>
#include <dirent.h>
#include <linux/limits.h> // for PATH_MAX
int make_directory(char * name) {
int checker=mkdir(name, S_IRWXU | S_IRWXG | S_IRWXO);
return checker;
}
char** getNames() {
char** names=malloc(10*sizeof(char*));
for(int i=0;i<10;i++) {
if(i==9) {
names[i]=malloc(3*sizeof(char));
names[i][0]='1';
names[i][1]='0';
names[i][2]='\0';
} else {
names[i]=malloc(2*sizeof(char));
names[i][0]='1'+i;
names[i][1]='\0';
}
}
return names;
}
int makeTenDirs(char** names) {
// This is where your problems began. As I suggested before and confirmed
// when I actually ran the code, `getcwd(NULL, 0)` returns a pointer to
// dynamically allocated memory that's just big enough to hold the path.
// As soon as you strcat to that, you overflow the buffer causing
// undefined behavior. It crashed for me 2nd time through the loop, not
// immediately when the UB occurred. Your results could be entirely different,
// that is the essence of UB. To fix it, I will declare an automatic array
// of PATH_MAX large (4096 I believe). That should make this fixed size array
// able to handle any path on your _linux_ box (this code will not be portable
// to windows). Alternatively, you could do what you did before, just be sure
// to realloc the name2 memory _before_ strcat'ing so there's enough room for
// "/input/dir"
char name2[PATH_MAX];
// only acceptable place for one character variable names are loop index
// variables, and you'll even get some argument on that. Give your variables
// descriptive names (although, I'd probably just make this function void, not
// much utility in the return value here, which you didn't even return!)
int directoryMade = 0;
for(int i=0;i<10;i++) {
// This returns NULL if the `sizeof name` (==PATH_MAX == 4096) is too
// small to hold the path. That shouldn't be the case since no paths
// on the system should exceed PATH_MAX, but it's always a good idea
// to check for errors, that's what a lot of your C code should be
// doing, so get in the habit. Also, realize your old way created
// a memory leak each time since each call to getcwd would malloc more
// memory, and you overwrite the pointer to the previous block with the
// pointer to the new block. Now nothing is pointing at the previous
// block so you can't free it --> memory leak.
if (getcwd(name2, sizeof name2) == NULL)
{
perror("Path exceeded buffer length");
exit(-1);
}
// should be plenty of space to strcat now
strncat(name2,"/input/dir",11);
strncat(name2,names[i],1);
// not much utility b/c it keeps getting overwritten. You could check it each time
// and return if there's an error
directoryMade=make_directory(name2);
}
if (getcwd(name2, sizeof name2) == NULL)
{
perror("Path exceeded buffer length");
exit(-1);
}
strncat(name2,"/input/dir",11);
strncat(name2,names[0],1);
strncat(name2,"0",2);
directoryMade=make_directory(name2);
return directoryMade;
}
int main() {
char **names=getNames();
DIR *dir;
DIR *dir2;
struct dirent *dent;
int proceed=make_directory("./input");
if(proceed==-1) {
// you print an error here, but continue on anyway
// as if there was no error
printf("Error making the directory\n");
}
makeTenDirs(names); // you already fetched names, use them!
dir=opendir("./input");
if(dir!=NULL) {
// same problem here. name3 holds _exactly_ how much space it
// needs when you allow getcwd to malloc memory for it. As before,
// you can make name3 an array in automatic storage, or realloc it
// before strcat'ing
char name3[PATH_MAX];
// check for NULL return here too. Not showing it because I'm getting lazy
getcwd(name3, sizeof name3);
while((dent=readdir(dir))!=NULL){
if(strcmp(dent->d_name,"..")!=0 && strcmp(dent->d_name,".")!=0) {
// check for NULL return
getcwd(name3, sizeof name3);
strncat(name3,"/input/",8);
strncat(name3,dent->d_name,10);
printf("%s\n",name3);
dir2=opendir(name3);
if(dir2!=NULL) {
// printf is line-buffered, so this won't print right away unless
// you put a newline on it (or fflush(stdout);)
printf("alo\n");
// close this dir too?
closedir(dir2);
}
}
}
}
closedir(dir);
// this is NOT a complete free. You have a double pointer. You need to loop and
// free each of names[0], names[1], .. names[9], _then_ free(names). I'll leave
// that as an exercise. In general, you should have a matching number of malloc's
// and free's.
free(names);
return 0;
}