Thread: stdin "incompatible pointer type" as FILE*

Lesson 9 in the C tutorial says the declaration of fgets calls for a FILE* but will take stdin. But when I try this with my own function (which I'm trying to generalize) I get a warning:

Code:

warning: passing argument 1 of ‘readLine’ from incompatible pointer type

Why? Here's the code:

Code:

#include <stdio.h>
#include <string.h>

void readLine(char* string, FILE* stream){
    int i;
    fgets(string, strlen(string), stream);
    for(i = 0; i < strlen(string); i++){
        if(string[i] == '\n'){
            string[i] = '\0';
            return;
        }
    }
    return;
}

int main(){
    char* input[256];
    printf("Please enter a number: ");
    readLine(input, stdin);
    printf("You entered &#37;s.\n", input);
    return 0;
}

It's actually 'input' that it is complaining about.

Originally Posted by Jesdisciple

Code:

warning: passing argument 1 of ‘readLine’ from incompatible pointer type

For future reference, the warnings from your compiler are not in C-array-index-style (so when it says argument 1, it really means the first argument, and not the second).

Originally Posted by Jesdisciple

Lesson 9 in the C tutorial says the declaration of fgets calls for a FILE* but will take stdin. But when I try this with my own function (which I'm trying to generalize) I get a warning:

Code:

warning: passing argument 1 of ‘readLine’ from incompatible pointer type

Why? Here's the code:

Code:

#include <stdio.h>
#include <string.h>

void readLine(char* string, FILE* stream){
    int i;
    fgets(string, strlen(string), stream);
    for(i = 0; i < strlen(string); i++){
        if(string[i] == '\n'){
            string[i] = '\0';
            return;
        }
    }
    return;
}

int main(){
    char* input[256];
    printf("Please enter a number: ");
    readLine(input, stdin);
    printf("You entered %s.\n", input);
    return 0;
}

as already pointed out, the problem is in line 1 of your main() function. What you have there is actually a multidimensional array, which conflicts with your declaration of the readLine() function.
Also be aware that your readline function should actually be

Code:

void readLine(const char *string, FILE *stream)
{
    /* ... */
}

Well, it's actually not necessary, but the first argument is a pointer, which will be read-only. So any attempt to modify it should cause an Segmentation Fault. the const makes that variable constant, so that you can't modify it. Your implementation is fine too, but it might become a hard-to-find problem in your code, if you acidentially write to that char *string and your not aware of the read-only condition.

Originally Posted by tabstop

For future reference, the warnings from your compiler are not in C-array-index-style (so when it says argument 1, it really means the first argument, and not the second).

Oh, thanks. That threw me for a loop (no pun intended).

Umm, I'll just leave the write permission as it is until I've had more experience.

Originally Posted by matsp

However, there is a small bug here:

Code:

fgets(string, strlen(string), stream);

strlen for an uninitialized string on the stack will be undefined. It may be zero, it may be hundreds of thousands, and it may even crash the system - if there isn't a zero before the end (bottom) of the stack, you'd have a crash. This may be unlikely. However, I can almost guarantee that the length here is incorrect in relation to the actual size of the string. The best option would be to pass the actual size (from sizeof(input)) along the other arguments to the readLine function.

I've changed my declaration for input to

Code:

char input[256];

And I wonder why I can't retrieve the length I applied to it? Since C (as I understand it) is supposed to be a bare language with the mechanics showing and ready to be used, both beautiful and ugly, I would think this would be an externally accessible datum...

Originally Posted by Jesdisciple

I've changed my declaration for input to

Code:

char input[256];

And I wonder why I can't retrieve the length I applied to it? Since C (as I understand it) is supposed to be a bare language with the mechanics showing and ready to be used, both beautiful and ugly, I would think this would be an externally accessible datum...

When you've declared a character array as you have, there's no real guarantee as to what's contained within the array. A character array is an array or bytes, and can contain pretty much anything in C. You make it special, i.e. a "string", through the use of a null terminator. Therefore, strlen requires the presence of a null terminator to determine the length of the string. With an array, you can, however, use sizeof(input) to get the available size.

Yes, that works much better.

Thanks a bunch!