cast from pointer to integer of different size. compile error

Question

When I try to use toupper, the compiler gives me this error: "warning: cast from pointer to integer of different size." and "warning: assignment makes pointer from integer without cast." Why?

for(i=0;i<argc;i++){
  argv[i]=(char)toupper((char)argv[i]);
}

Which part of the error message is unclear? Or are you asking *why* it is an error to cast a pointer to an integer of a different size? — juanchopanza, Jul 12 '15 at 22:56
Remove the explicit casts. If you still get an error/warning, you are doing something wrong. — too honest for this site, Jul 12 '15 at 23:07
Are you trying to make the first letter into uppercase or the whole string? — Mikkel Christiansen, Jul 12 '15 at 23:18
Argc is how many strings, not the length. The rest have Iskar Jarak explained. The first dimension of argv is the strings of arguments, the second is each char of the string. — Mikkel Christiansen, Jul 12 '15 at 23:26

score 5 · Answer 1 · edited May 23 '17 at 11:51

5

argv is a char**, a pointer to a pointer. Think of it as an array of strings, not a string.

Indexing with [i] removes one level of indirection, so argv[i] is a pointer to a character, a char*.

toupper() is int toupper (int c); - it expects a single character as an integer, not a pointer to a character, but you are passing it a char* when you do argv[i], which you are then trying to cast to a char... a regular old character, which typically has a different size to a pointer.

argc says how many command line arguments there are, and argv[0] is usually the exectuable name. Use strlen to get the length of each argument in argv.

To convert the whole of each argument to upper case:

for (int i = 0; i < argc; i++) {
    size_t len = strlen(argv[i]);
    for (size_t j = 0; j < len; j++) {
        argv[i][j] = (char)toupper((unsigned char)argv[i][j]);
    }
}

You can read about why the argument to toupper() should be cast to unsigned char in Keith Thompson's excellent answer elsewhere on SO

edited May 23 '17 at 11:51

Community

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answered Jul 12 '15 at 22:56

Iskar Jarak

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The argument to `toupper` should be cast to `unsigned char`, not to `int`. – Keith Thompson Jul 12 '15 at 23:05
bear with me.. I am new to C, but how is argv[i][j] possible, when it was initialized with only one dimension? Further, this compiled, however when I ran the program, and gave it "abcdefg", the program returned "?? ?" – Ricky Yang Jul 12 '15 at 23:19
@RickyYang What makes you think `argv` was initialised with only one dimension? `argc` is the length of one dimension, essentially, but each `char*` in `argv` has a different length depending on the command line arguments you pass in. – Iskar Jarak Jul 12 '15 at 23:24
@RickyYang Take a look at http://ideone.com/oBFGcM `argv[0]` is typically the name of the program invoked, in this case `./prog` - see how the output is capitalised? Unfortunately ideone doesn't let you set command line arguments so you'll only see the default one in the output there. – Iskar Jarak Jul 12 '15 at 23:29
might this be different in C? Because the output I am getting is a bunch of spaces and question marks. – Ricky Yang Jul 12 '15 at 23:34
@rickyyang Your program is wrong, despite it compiles. Iskars only have the fault of making argv[0] uppercase, otherwise it is what you want. – Mikkel Christiansen Jul 12 '15 at 23:44
@RickyYang My code is C. Plain standards compliant C99. What compiler and OS are you using? – Iskar Jarak Jul 12 '15 at 23:52
@IskarJarak Except for the int and size_t in the for headers, your code would work in all c versions except some from the 70's. And if argv[0] isn't the name of the program, it is the empty string. – Mikkel Christiansen Jul 13 '15 at 00:00

cast from pointer to integer of different size. compile error

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