How to append a NULL into the char array?

Question

I'm trying to run a shell script using posix_spawn, by trying to convert from NSArray to char array with a NULL in the last element, I end up crashing my system. Below is the original code that has no error.

NSArray *arg_array = nil;
arg_array = @[@"/bin/bash", @"/var/somefile.sh"];
char **argv = NULL;
NSInteger numargv = arg_array.count;

if (numargv) {
    argv = (char **)calloc(numargv, sizeof(char*))  ;

    if (argv) {

        for (NSInteger i=0;i<numargv;i++) {
            NSString *nsString = arg_array[i];

            if (i==0){
                NSArray* spliteArray = [nsString componentsSeparatedByString: @"/"];
                NSString* cStringFirstArgv = [spliteArray lastObject];
                nsString = cStringFirstArgv;
            }

            char *cString = (char *)malloc([nsString lengthOfBytesUsingEncoding:NSUTF8StringEncoding] + 1); // + 1 for \0

            if (cString) {  
                strcpy(cString, nsString.UTF8String);
                argv[i] = cString;
            } else {
                // error
            }
        }
    } 
}


pid_t pid;
int status;
//const char *argv[] = {"bash", "/var/test.sh", NULL}; This is what I expected argv to be, a NULL in the last element
posix_spawnp(&pid, "bash", NULL, NULL, (char* const*)argv, NULL);
waitpid(pid, &status, WEXITED);

argv currently is {"bash", "/var/test.sh"}, however I trying to make it to an array like this:{"bash", "/var/test.sh", NULL} so that I could run posix_spawn successfully. Any idea how to fix this? Thanks in advance!

Well, you need calloc(numargv + 1 in the first place.

Sulthan
– Sulthan

2019-08-12 14:03:55 +00:00
Commented Aug 12, 2019 at 14:03 — Sulthan
– Sulthan, Commented Aug 12, 2019 at 14:03
That's what I did too, but it end up crashing the system.

Gregor Isack
– Gregor Isack

2019-08-12 14:07:23 +00:00
Commented Aug 12, 2019 at 14:07 — Gregor Isack
– Gregor Isack, Commented Aug 12, 2019 at 14:07
You just need cStringWithEncoding:

Cy-4AH
– Cy-4AH

2019-08-12 14:50:53 +00:00
Commented Aug 12, 2019 at 14:50 — Cy-4AH
– Cy-4AH, Commented Aug 12, 2019 at 14:50

Sulthan · Accepted Answer · 2019-08-12 14:19:00Z

1

The following will work:

NSArray *arg_array = nil;
arg_array = @[@"/bin/bash", @"/var/somefile.sh"];
char **argv = NULL;
NSInteger numargv = arg_array.count;

argv = (char **)calloc(numargv + 1, sizeof(char*));

if (argv) {
    for (NSInteger i = 0; i < numargv; i++) {
        NSString *nsString = arg_array[i];

        if (i == 0){
            NSString* application = [[nsString componentsSeparatedByString: @"/"] lastObject];
            nsString = application;
        }

        NSData* stringData = [nsString dataUsingEncoding:[NSString defaultCStringEncoding]];

        char *cString = (char *)malloc(stringData.length + 1);
        [stringData getBytes:cString length:stringData.length];
        cString[stringData.length] = 0;

        argv[i] = cString;
    }
}

pid_t pid;
int status;
//const char *argv[] = {"bash", "/var/test.sh", NULL}; This is what I expected argv to be, a NULL in the last element
posix_spawnp(&pid, "bash", NULL, NULL, (char* const*)argv, NULL);
waitpid(pid, &status, WEXITED);

I think there were two problems in your code:

You were missing the +1 in your calloc for the NULL terminator.
You don't actually set the ending 0 for your string arguments.

answered Aug 12, 2019 at 14:19

Sulthan

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1 Comment

Gregor Isack Over a year ago

Your code is so much more cleaner, and I didn't realize using NSData is more easier! Thank you!

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