how do I compare a char pointer and a car in C

Question

I have a char pointer that holds a single character. I want to create a while loop that will continue to run as long as that character is not '~'. But I cannot seem to get the characters to compare correctly.

char *currChar;
while(currChar != '~') {
    //read();
    printf("Current Char:%c\n", currChar);
}

read puts the last value entered by the user into currChar. The printF will always output the correct information (i.e. a ~ if the user pressed ~) but for some reason he while loop will never be exited.

If i try to do something like while(*currChar != '~') I get a segmentation fault error.

What can I do to get this to work?

Thanks in advance.

Your `char*` pointer does not *contain* a single `char`. It may *point to* a single `char`. That `char` object is `*currChar`. — Keith Thompson, Nov 18 '14 at 00:16
possible duplicate of [What does "dereferencing" a pointer mean?](http://stackoverflow.com/questions/4955198/what-does-dereferencing-a-pointer-mean) — Deduplicator, Nov 18 '14 at 00:20
BTW, I would not use `read()` as the function name, since it is a Posix function. try `getchar()` — technosaurus, Nov 18 '14 at 03:21

score 1 · Answer 1 · answered Nov 18 '14 at 00:18

Your char pointer does not hold a single character. It points to a single character in memory. You need to declare an actual char somewhere.

E.g.,

char currChar = ' ';  /* Initialize to something not '~' */
while( currChar != '~' ) {
    /* read().  If your read function requires a pointer, then provide &currChar. */
    printf("Current character: %c\n", currChar);
}

So what's wrong with your code is that char *currChar; creates an uninitialized pointer. That pointer is not pointing anywhere. You could do this:

char currChar = ' ';
char *currChar_ptr = &currChar;

Now you have a character. And your pointer is pointing to that character.

@self: It's pseudo code copied directly from the OP's pseudo code. The idea is that in the commented code area, there is a function that reads a character. — indiv, Nov 18 '14 at 00:22

score 0 · Answer 2 · edited Nov 18 '14 at 01:09

0

You need to compare *currChar. But the reason you get a segfault is because the first time you compare *currChar to '~', you are comparing an undefined pointer to '~'.

So call read(); before you enter the loop, and make sure you allocate space to store your char in the first place.

Or:

char *currChar = '_';

do {
    // read();
    printf("Current Char:%c\n", currChar);
} while (*currChar != '~');

edited Nov 18 '14 at 01:09

Alex Weinstein

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answered Nov 18 '14 at 00:21

stromdotcom

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The read() method call is from the code he posted. Neither you nor I know what this read() is or whether it should or should not have arguments. – stromdotcom Nov 18 '14 at 00:40
2

Its irrelevant to the question. He already said his `read()` method is working correctly. – stromdotcom Nov 18 '14 at 00:42
`read()` was commented out of the code OP posted, but you think it's relevant, so explain it. – Weather Vane Nov 18 '14 at 00:42
And? Why is this confusing to you? His issue was how to compare a char* to a char literal, not how to read a char from input. – stromdotcom Nov 18 '14 at 00:46
1

Stop bickering. This is not helpful to either you or the OP. I just commented out read() to make it match the original question. – Alex Weinstein Nov 18 '14 at 01:09

score 0 · Answer 3 · answered Nov 18 '14 at 00:24

0

You have to read the user input before you enter the loop.

This code works:

char *currChar;
do{
    read();
    printf("Current Char:%c\n", *currChar);
    currChar++;
}while(*currChar != '~')

answered Nov 18 '14 at 00:24

Ludovic Feltz

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`read()` without arguments? `currChar++` ? Is that "hit and hope"? – Weather Vane Nov 18 '14 at 00:39
I though `read()` was an OP function. And what is the problem with `currChar++` i just increment the pointer! Is that a problem? – Ludovic Feltz Nov 18 '14 at 00:42

how do I compare a char pointer and a car in C

3 Answers3