Modifying a char * in a struct

Question

I am trying to modify a char * that i have in a struct by i am having some issues.

#define MAXLINELENGTH 1000
#define MAXINSTRUCTIONS 65536

struct labelMem {
    char *name;
    int pc;
};

struct labelStr {
    struct labelMem labels[MAXINSTRUCTIONS];
};




while (func(string s) == 0) {
    strncpy(programLabels.labels[labelCounter].name, label, MAXLINELENGTH);
        labelCounter++;
}

I tried a few different ways of arranging my structs in an array but each time i have issues modifying my char * var.

Any ideas on how to fix this would be appreciated.

You need to allocate memory for `name` inside `labelMem` struct — P0W, May 07 '18 at 05:09
Either allocate memory for `name` or make it an array. You can't just store data to a pointer like that, it must be pointing some memory space where the data can fit. — Havenard, May 07 '18 at 05:11
Ok yea that makes sense. Ive done a lot of coding in c++ so i sometimes forget the basics. — Chase, May 07 '18 at 05:24
If you have `label` and have not yet allocated, then allocate for `strlen (label) + 1` and then just `strcpy` - no need to `strncpy` at that point. (and you know there is NO `string` type in C, so you must already have a `typedef` somewhere? if so, see [Is it a good idea to **typedef** pointers?](http://stackoverflow.com/questions/750178/is-it-a-good-idea-to-typedef-pointers)) — David C. Rankin, May 07 '18 at 05:35
Or as you know the length, you might as well just use `memcpy` for the `length + 1`. Btw, using `strncpy` is almost always *wrong*. `strncpy` is not "safe strcpy". — Antti Haapala -- Слава Україні, May 07 '18 at 06:28

Increasingly Idiotic · Answer 1 · 2018-05-07T05:39:53.437

2

Without a call to malloc pointers do not actually point to anything.

You need to allocate memory for a pointer before you use it. You could change your program to

while (func(string s) == 0) {
    // Allocate memory and check for errors
    programLabels.labels[labelCounter].name = malloc (strlen (label) + 1);
    if (!programLabels.labels[labelCounter].name) { /* handle error */ }

    strncpy(programLabels.labels[labelCounter].name, label, MAXLINELENGTH);
    labelCounter++;
}

edited May 07 '18 at 05:39

answered May 07 '18 at 05:24

Increasingly Idiotic

5,700
5
35
73

1

How about `programLabels.labels[labelCounter].name = malloc (strlen (label) + 1); if (!programLabels.labels[labelCounter].name) { /* handle error */ }` then `strncpy`. It would be rather wasteful to allocate `MAXLINELENGTH` every time... (but yes, you are correct that it needs to be allocated) – David C. Rankin May 07 '18 at 05:32
oops. I should have paid closer attention! – Increasingly Idiotic May 07 '18 at 05:40
That's OK, it takes a while to snap to those things. You just kinda have to run through a loop of the code in your head to make sure all the pieces fit. – David C. Rankin May 07 '18 at 05:41

Modifying a char * in a struct

1 Answers1