Trying to use/include/compile 3rd party library, libmagic. C/C++ filetype detection

Question

After looking for a way to detect the filetype of a file stream, I found that the Unix file command uses libmagic and I\'m trying to make use of the library myself, but I can\'t

Answer 1

__FILE__ is a reserved pre-processing symbol macro used for debugging/logging purposes. Consider this as an example:

// This file is called test.c
char *p = NULL;
if (!(p = malloc((1 * sizeof(char) + 1)))){
   printf("Error in file: %s @ line %d\n\tMalloc failed\n", __FILE__, __LINE__);
   exit(-1);
}

If the call to malloc failed you will see the output in the above example like this:

Error in file: test.c @ line 23
       Malloc failed

Notice how the code picks up the original source code. The above example illustrates the usage of this.

I think your code should be something like this:

// fileTypeTest.cpp, placed in file-5.03/src/ (source from link above)
#include <stdio.h>
#include "magic.h"
int main(int argc, char **argv) {
  if (argc > 1){
     magic_t myt = magic_open(MAGIC_CONTINUE|MAGIC_ERROR/*|MAGIC_DEBUG*/|MAGIC_MIME);
     magic_load(myt,NULL);
     printf("magic output: '%s'\n",magic_file(myt,argv[1]));
     magic_close(myt);
  }
  return 0;
}

The code above checks if there is a parameter that is passed into this program and the parameter would be a filename, i.e. argv[0] points to the executable name (the compiled binary), argv[1] points to the array of chars (a string) indicating the filename in question.

To compile it:

g++ -I/usr/include -L/usr/lib/libmagic.so  fileTestType.cpp -o fileTestType
g++ -L/usr/lib -lmagic fileTestType.cpp -o fileTestType

Edit: Thanks Alok for pointing out the error here...

If you are not sure where the libmagic reside, look for it in the /usr/local/lib, and /usr/local/include - this depends on your installation.

See this to find the predefined macros here.

Hope this helps, Best regards, Tom.

Answer 2

Where is magic.h in the filesystem? Is the preprocessor finding it? If not, use -I<path>.

Answer 3

I don't know why you think the above "obviously" doesn't work. See How to mix C and C++ in the C++ FAQ for details.

Looks like magic.h has proper extern "C" { } enclosures. So, compiling your code with g++ should work nicely. You can #include <magic.h> in your .cpp file, and use all the libmagic functions.

• Is there a particular error you're getting?
• I have not checked your use of the libmagic functions.
• You need to link with libmagic. Your g++ command needs -lmagic.
• Since magic.h is most likely in a standard place, you should use #include <magic.h>.

Tell us what your error is for more specific help.