CODEWITHSUNDEEP
iosobjective-ccnsstring
Luke Smith
Luke Smith

Reputation: 1320

Converting NSStrings to C chars and calling a C function from Objective-C

I'm in an Objective-C method with various NSStrings that I want to pass to a C function. The C function requires a struct object be malloc'd so that it can be passed in - this struct contains char fields. So the struct is defined like this:

struct libannotate_baseManual {
    char *la_bm_code;  // The base code for this manual (pointer to malloc'd memory)
    char *la_bm_effectiveRevisionId; // The currently effective revision ID (pointer to malloc'd memory or null if none effective)
    char **la_bm_revisionId; // The null-terminated list of revision IDs in the library for this manual (pointer to malloc'd array of pointers to malloc'd memory)
};

This struct is then used in the following C function definition:

void libannotate_setManualLibrary(struct libannotate_baseManual **library) { ..

So that's the function I need to call from Objective-C.

So I have various NSStrings that I basically want to pass in there, to represent the chars - la_bm_code, la_bm_effectiveRevisionId, la_bm_revision. I could convert those to const chars by using [NSString UTF8String], but I need chars, not const chars.

Also I need to do suitable malloc's for these fields, though apparently I don't need to worry about freeing the memory afterwards. C is not my strong point, though I know Objective-C well.

Upvotes: 1

Views: 99

Answers (1)

trojanfoe
trojanfoe

Reputation: 122391

strdup() is your friend here as that both malloc()s and strcpy()s for you in one simple step. It's memory is also released using free() and it does your const char * to char * conversion for you!

NSString *code = ..., *effectiveRevId = ..., *revId = ...;
struct libannotate_baseManual *abm = malloc(sizeof(struct libannotate_baseManual));
abm->la_bm_code = strdup([code UTF8String]);
abm->la_bm_effectiveRevisionId = strdup([effectiveRevId UTF8String]);
const unsigned numRevIds = 1;
abm->la_bm_effectiveRevisionId = malloc(sizeof(char *) * (numRevIds + 1));
abm->la_bm_effectiveRevisionId[0] = strdup([revId UTF8String]);
abm->la_bm_effectiveRevisionId[1] = NULL;

const unsigned numAbms = 1;    
struct libannotate_baseManual **abms = malloc(sizeof(struct libannotate_baseManual *) * (numAbms + 1));
abms[0] = abm;
abms[1] = NULL;
libannotate_setManualLibrary(abms);

Good luck, you'll need it. It's one of the worst interfaces I've ever seen.

Upvotes: 3

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