Thread: strange negative numbers ?!!

Code:

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

int main()
{

	FILE *out;
	FILE *in;
	char *ce;


	if((ce=(char *)malloc(sizeof(char))) == NULL)
    {
        printf("Error: No enough memory\n");
        getchar();
        return 1;
    }

	in = fopen("CP1256.txt","rb");
	out = fopen("output.txt","wb");

	if(in == NULL)
	{
		printf("Can`t open file:%s\n",argv[1]);
		exit(0);
	}


	while(fread(ce,sizeof(char),1,in) > 0)
	{
		printf("%d\n",*ce);   //it`s print negative numbers 
	}

	fclose(in);
	fclose(out);

	return 0;
}

can i know why the *ce contain negative numbers ??

main.c
../main.c: In function `main':
../main.c:25: error: `argv' undeclared (first use in this function)
../main.c:25: error: (Each undeclared identifier is reported only once
../main.c:25: error: for each function it appears in.)
*** Errors occurred during this build ***

To what does ce point?

point to the first char in a the input file ??

Originally Posted by Meshal

point to the first char in a the input file ??

Sorry, I'm going blind.

Code:

if((ce=(char *)malloc(sizeof(char))) == NULL)

I have to wonder why, though.

Originally Posted by Doodle77

because char is a signed data type, or at least, you're printing it as signed.
use printf("%u\n",ce) instead of printf("%d\n",ce)

That won't fix the problem, but it is true that that should be changed. The conversion of char to int happens before the printf function is called. If this change is made by itself, it will simply reinterpret the negative numbers as very large integers.

You also need to change the the type of ce from char * to unsigned char *.

Explanation on why changing printf is not enough (ignore if not interested):
Basically, there are two steps in interpreting the type of additional argument passed to printf and similar functions. The first is the natural conversion from to the basic types such as void *, int, unsigned, and double. Actual conversion is done based on the type of the argument passed, including sign extension. Then inside the printf function, each argument is interpreted based on the type indicated in the format string. If the two types are not the same you get undefined behavior, but if the difference is only the the signedness, the results are easy to predict -- they would be the same as the cast from the signed type to the unsigned type (or vice versa).

thanks all guy`s , the problem fixed when i change the char to unsigned char , i couldn`t notice that ..