How to clear a char* string array in C

[rchan0021]

Given that

char *string = (char*)malloc(500);

how would I go about clearing *string?

In java for example if I wanted to make a string empty I would just say,

String var = "";

What would be the C equivalent?

Thanks in advance.

[lisati]

A couple of ways come to mind. Given that strings in C are usually terminated by an ASCII zero, the easiest would be to set the first byte to zero.
Another way might be to use memset() to set the whole string to zeros.

[MindSz]

if you want to "empty" a string 'name' just do
name[0] = '\0';
Now this doesn't erase all the characters in the string, but since the first character is now a termination character the string looks empty to your program.

If you want to erase all the characters in the string then you need to use a loop.

[Simian Man]

The ways above are valid, but I usually do:


strcpy(str, "");


Which does the same thing, but conveys the meaning a little better I think.

[rchan0021]

I tried both methods, but no go. Let me better explain what I am trying to do. I need to make a program that given a directory, i.e. /home/ , the program will list all files within that directory and also any sub directories. My problem is that when using my recursive function printDir(), my char* curFileFullPath doesn't clear between calls to printDir(). So the program works only for the first file, after that curFileFullPath should clear so that a new path can be put in it, but it doesn't clear so if i said /home/ the first time, the second time around it would be /home/home/..., instead of /home/...

Please help, lol.
__________________________________________________ __________________

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <dirent.h>

void printDir(char *dirPath);

int main(void)
{
char *directory = (char*)malloc(500);

printf("Enter a directory to list (must have tailend / , i.e. /home/): ");
scanf("%s", directory);

printDir(directory);

free(directory);

return 0;
}

void printDir(char *dirPath) /* dirPath MUST have a tailend / else code won't work, i.e. /home/ */
{
struct dirent *entry = NULL;
struct stat s;

char *curFileFullPath = (char*)malloc(500);

DIR *mydir = opendir(dirPath);

while ((entry = readdir(mydir))) /* If we get EOF, the expression is 0 and the loop stops. */
{
if (strcmp(entry->d_name, "..") == 0 || strcmp(entry->d_name, ".") == 0)
{
continue;
}
printf("content of curFileFullPath: %s\n\n",curFileFullPath );
strcat(curFileFullPath, dirPath);
strcat(curFileFullPath, entry->d_name);

if ( stat(curFileFullPath,&s) == 0 )
{
if ( s.st_mode & S_IFDIR )
{
printf("[Dir]\t%s\n", entry->d_name);
strcat(curFileFullPath, "/");
printDir(curFileFullPath);
}
else
{
printf("%s\n", entry->d_name);
}
}
else
{
printf("stat read error\n");
}
}
}

[MindSz]

you could maybe try to loop through your string (until strlen() or size-of-array) and set all characters to '\0' before storing the new name?

[lisati]

you could maybe try to loop through your string (until strlen() or size-of-array) and set all characters to '\0' before storing the new name?

This can be done with memset (http://www.elook.org/programming/c/memset.html)

[MadCow108]

the problem goes beyond simple clearing (string[0] = 0 is the best solution when one only handles normal c style strings)

to fix your program move the strcat(..., dirPath) before the while loop and save the length there
and then just always reset after this length in the loop:
curFileFullPath[len] = 0;
printf("content of curFileFullPath: %s\n\n",curFileFullPath );
strcat(curFileFullPath, entry->d_name);


but you still have memory and resource leaks
use free and closedir

if the original buffer is big enough you technically don't even need the copy of the path
replace curFileFullPath with dirPath, remove the malloc and first strcat in printDir and you only have a resource leak left

[cb951303]

or you could use "calloc" instead of "malloc" and get a zero initialized array?

[falconindy]

Rather than using strcat twice, just use snprintf:


snprintf(curFileFullPath, strlen(dirPath) + strlen(entry->d_name) + 1, "%s%s", dirPath, entry->d_name);

This will overwrite the contents of the string, and neatly append a \0 to the end of the format.

[rchan0021]

Thank Everyone! With your advice I finally got my program working.