C string problem

Question

I made this function:

char** parse_cmd(const char* cmdline) {
    int i;
    int j = 0 ,k = 0;
    char ch[100][100];
    for(i=0; i<strlen(cmdline); i++) {
        if(cmdline[i] != ' ') {
            ch[j][k] = cmdline[i];
            k++;
        } else {
            j++;
            k = 0;
        }
    }

    return ch;
}

But when I compile the program I have this warning:

shell.c: In function ‘parse_cmd’:
shell.c:25:2: warning: return from incompatible pointer type
shell.c:25:2: warning: function returns address of local variable

Why?

Apart from anything else you can't return local arrays from functions like that, or indeed at all. — user2100815
– user2100815, Commented May 15, 2011 at 17:06

Constantinius · Accepted Answer · 2011-05-15 17:16:25Z

3

You try to return a pointer to a memory location that will not be associated with the array after the function returned. If you want to permanently allocate the memory, then you have to copy it with malloc (or any similar function) before returning it.

e.g:

char** parse_cmd(const char* cmdline) {
int i;
int j = 0 ,k = 0;
char **ch = (char**)malloc(100*100);
for(i=0; i<strlen(cmdline); i++) {
    if(cmdline[i] != ' ') {
        ch[j][k] = cmdline[i];
        k++;
    } else {
        j++;
        k = 0;
    }
}
return ch;
}

EDIT: fixed typo. Thanx

edited May 15, 2011 at 17:16

answered May 15, 2011 at 17:08

Constantinius

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4 Comments

Chris Lutz Over a year ago

char **ch not char ch**. And you can't allocate a char ** with one malloc.

Constantinius Over a year ago

You can, if you cast, but that seems like a hack, doesn' it? :/

Chris Lutz Over a year ago

No you can't. Dereferencing a char ** gives a value of type char *. It expects to point somewhere. You need to a) allocate a char (*)[100], b) do a loop to allocate each row, or c) change it to char * and manually index via multiplication, i.e. ch[i * 100 + j] instead of ch[i][j].

Paul R Over a year ago

Also you shouldn't cast the result of malloc in C.

David Heffernan · Accepted Answer · 2011-05-15 17:06:27Z

3

You're returning a pointer to memory that lives on the stack. Use heap allocated memory instead.

answered May 15, 2011 at 17:06

David Heffernan

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Comments

Himadri Choudhury · Accepted Answer · 2011-05-15 17:07:02Z

2

The ch array is deallocated at the end of the function, that memory is stored on the stack which is invalid after the function returns. You should instead create a array in the calling function and pass the pointer into the parse_cmd() function.

answered May 15, 2011 at 17:07

Himadri Choudhury

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1 Comment

Chris Lutz Over a year ago

+1 but it should also be noted that char [N][O] does not decay to char **, but to char (*)[O].

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