CODEWITHSUNDEEP
macos
Rose Perrone
Rose Perrone

Reputation: 63576

Why is sysctlbyname not defined if I include it in a block as a dispatch_once arg?

I import:

#import <sys/types.h>
#import <sys/sysctl.h>

When I use the function, sysctlbyname("hw.machine", NULL, &size, NULL, 0); outside dispatch_once, the compiler finds its definition in . When I use it inside the block inside the second argument to dispatch_once, I get this error:

No matching function for call to 'sysctlbyname'

Here's the full code:

#import <sys/types.h>
#import <sys/sysctl.h>

NSString *GetMachineName(void) {
  static NSString *machine = nil;

  if (nil == machine) {
    size_t size;

    sysctlbyname("hw.machine", NULL, &size, NULL, 0); // DEFINED IN SYSCTL.H


    static dispatch_once_t once;
    dispatch_once(&once, ^ {
      // Set 'oldp' parameter to NULL to get the size of the data
      // returned so we can allocate appropriate amount of space
      sysctlbyname("hw.machine", NULL, &size, NULL, 0); // DEFINED IN SYSCTL.H

      // Allocate the space to store name
      char *name = (char *)malloc(size);

      // Get the platform name
      sysctlbyname("hw.machine", name, &size, NULL, 0);

      // Place name into a string
      machine = [NSString stringWithCString:name encoding:NSASCIIStringEncoding];

      // Done with this
      free(name);
    });

    if ([machine isEqualToString:@"iPhone1,1"]) return IPHONE_1G_PLATFORM;
    if ([machine isEqualToString:@"iPhone1,2"]) return IPHONE_3G_PLATFORM;
    if ([machine isEqualToString:@"iPhone2,1"]) return IPHONE_3GS_PLATFORM;
    if ([machine isEqualToString:@"iPhone3,1"]) return IPHONE_4G_PLATFORM;
    if ([machine isEqualToString:@"iPod1,1"]) return IPOD_1G_PLATFORM;
    if ([machine isEqualToString:@"iPod2,1"]) return IPOD_2G_PLATFORM;
    if ([machine isEqualToString:@"iPod3,1"]) return IPOD_3G_PLATFORM;
    if ([machine isEqualToString:@"iPad1,1"]) return IPAD_1G_PLATFORM;
    if ([machine isEqualToString:@"iPad2,1"]) return IPAD_2G_PLATFORM;
    if ([machine isEqualToString:@"i386"] || [machine isEqualToString:@"x86_64"]) return SIMULATOR_PLATFORM;
    return NULL;
  }

  return machine;
}

Upvotes: 3

Views: 1215

Answers (1)

Rose Perrone
Rose Perrone

Reputation: 63576

To fix, I needed to put the declaration of size inside the block, because as it was, the argument wasn't decorated with __block, so it compiles to a const, mismatching the argument list of sysctlbyname. Because of function overloading, the compiler found that the function didn't exist.

TLDR: The compiler thought the function I was using was 

int sysctlbyname(const char *, void *, const size_t*, void *, size_t);

rather than

int sysctlbyname(const char *, void *, *size_t, void *, size_t);

Upvotes: 1

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