Nothing paralyzes a C programmer like double-asterisk notation. What does it mean? Can you use it? How is it passed to a function and then referenced? I, too, fall victim to this confusion. So a good explanation is in order.
\n
\nDouble-pointer notation is best demonstrated with an array of strings:<\/p>\n
This declaration represents an array of pointers, the memory locations of the various strings where they’re stored in memory. (I added NULL as the last item to identify the end of the list.)<\/p>\n You can use array notation to reference each string: When passing the array to a function, you can use Function one()<\/em> is passed Within the one()<\/em> function, variable The two()<\/em> function is passed All this asterisk nonsense can be so confusing! It’s easy to understand how any C programmer, not just a beginner, can be driven nuts with this stuff.<\/p>\n","protected":false},"excerpt":{"rendered":" Here’s the difference between char *words[] = {
\n \"alpha\", \"beta\", \"gamma\", \"delta\", NULL
\n};<\/code><\/p>\nwords[1]<\/code> is beta<\/code>, for example. You can also use pointer notation: *(words+1)<\/code> is beta<\/code>.<\/p>\nwords<\/code> or *words<\/code>. What’s passed and how the function deals with it is demonstrated in this code:<\/p>\n2024_07_20-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nvoid one(char **words)\r\n{\r\n \/*\r\n **words is the base of the array\r\n **words is also the first string's first character\r\n *words is the first string\r\n *\/<\/span>\r\n puts(\"You have passed the entire array:\");\r\n while(*words)\r\n puts(*words++);\r\n}\r\n\r\nvoid two(char *word)\r\n{\r\n \/*\r\n word is a string\r\n *word is the first character of the string\r\n *\/<\/span>\r\n puts(\"You have passed a single string:\");\r\n puts(word);\r\n}\r\n\r\nint main()\r\n{\r\n char *words[] = {\r\n \"alpha\", \"beta\", \"gamma\", \"delta\", NULL\r\n };\r\n\r\n one(words);\r\n two(*words);\r\n\r\n return 0;\r\n}<\/pre>\nwords<\/code>, which is the base of the pointer array. In the function’s declaration, the argument is written **words<\/code>. The reason is that words<\/code> in the main()<\/em> function is the base of the array. In the one()<\/em> function, words<\/code> represents an array of pointers, which is why it’s referenced as **words<\/code>.<\/p>\n*words<\/code> represents a string in the array. The while<\/em> loop outputs each word. If you use **words<\/code> in the function, it represents the first character in the first string. Remember, **words<\/code> in the function’s declaration describes the type<\/em> of data passed. In the function, the double-asterisk represents a pointer-pointer: the address of the first string and the address of its first character.<\/p>\n*words<\/code>, which in the main()<\/em> function represents a single string. For the two()<\/em> function declaration it represents a single string, *word<\/code>. Within the function, word<\/code> represents the string; *word<\/code> is the first character of the string.<\/p>\n*data<\/code> and **data<\/code> as arguments in a function. Continue reading