You could call it a neat trick or just something so oddball insane that few programmers would dare breech the territory: A variable that holds the address of a function; a function pointer.
\n
\nPointers are scary enough in C, but a function pointer variable?<\/p>\n
In last week’s Lesson<\/a>, I showed how one function works as an argument to some other function. That operation takes place because a function has an address, a location in memory. That’s a pointer!<\/p>\n It’s possible in C to create a function pointer — a variable that holds the address of a function. This possibility begs the question: Why<\/em>?<\/p>\n Because this post, like the last two, is about oddball things in C that most programmers don’t think of. This is definitely an item some interloping Python programmer would consider as insane. And that’s fine by me.<\/p>\n The format to declare a function variable looks like this:<\/p>\n ret_type<\/em> is the function’s return value, such as int<\/em>, void<\/em>, or whatever.<\/p>\n name<\/em> is the function’s name as a variable. It’s a pointer, so the asterisk is prefixed to the name. Because of the order of precedence, the name and asterisk must be enclosed in parentheses.<\/p>\n arg_type<\/em> is the list of function arguments, just as you’d find in the function’s prototype. I recommend specifying only the types (int<\/em>, void<\/em>, and so on), not specific variable names.<\/p>\n As with all pointer variables, you must initialize the function pointer to a function’s address to use it. After that, you can use the function pointer’s name to reference the function.<\/p>\n Here’s your sample code:<\/p>\n The output:<\/p>\n The function pointer Now to seriously address the interloping Python programmer’s inquiry: Why bother<\/em>?<\/p>\n Honestly, there is absolutely no reason to use a function pointer as shown in the sample code.<\/p>\n The only time you need to use a function pointer construction is when your code contains a function that requires another function as an argument — as is done in the qsort()<\/em> function. In that case, the following format is implemented in the function’s declaration as well as inside the function. For example:<\/p>\n Inside the function, the variable That’s it!<\/p>\n Any other insanity you want to try using the function pointer technique is purely up to you.<\/p>\n","protected":false},"excerpt":{"rendered":" Creating a variable that holds a function. Continue reading ret_type<\/em> (*name<\/em>)(arg_type<\/em>);<\/code><\/p>\n\r\n#include <stdio.h>\r\n\r\nint funct1(void)\r\n{\r\n return(42);\r\n}\r\n\r\nint funct2(void)\r\n{\r\n return(24);\r\n}\r\n\r\nint main()\r\n{\r\n int (*fp)(void); \/* variable fp is a function pointer *\/<\/span>\r\n\r\n fp = funct1;\r\n printf(\"The solution is %d\\n\",fp());\r\n fp = funct2;\r\n printf(\"The solution is %d\\n\",fp());\r\n\r\n return(0);\r\n}<\/pre>\nThe solution is 42\r\nThe solution is 24<\/code><\/pre>\nfp<\/code> is declared in Line 15. It’s assigned the address of funct1()<\/em> at Line 17, then called in Line 18. It’s assigned to funct2()<\/em> at Line 19, and used again at Line 20.<\/p>\nvoid qsort(void *base, size_t nel, size_t width, int (*compar)(const void *a, const void *b));<\/code><\/p>\ncompar(a,b)<\/code> is used to call the user-define function that compares the two values for the sort, which is just how the variable fp()<\/code> is used in this Lesson’s sample code.<\/p>\n