将读取的数字转换为C中的无符号短型

Question

I'm trying to convert a read number into unsigned short .

My program saves a specific number in a file. Afterwards I'm trying to read it and to cast it to another type. Actually the read value is correct but when I cast it, it's a different value.

Here is how I read my file:

fseek(fp, 0, SEEK_END);

int length = ftell(fp);
fseek(fp, 0, SEEK_SET);

char* buffer = malloc(length);
if (buffer) {
   fread(buffer, 1, length, fp);
}

fclose(fp);

Let's assume that the value is "1337"; Buffer would have the correct value of "1337";

Now here is how i try to convert the value into an unsigned short :

unsigned short num = (unsigned short)buffer;

The var num is now not anymore that same value as buffer .

What am I doing wrong?

Answer 1

This has nothing to do with reading from files. You can't just cast a string (actually a pointer to char ) to a number and expect it to do anything sensible.

What you should do instead is to use a conversion function such as strtoul (declared in <stdlib.h> ) or sscanf (declared in <stdio.h> ). But make sure to give it a string, ie ensure buffer is nul-terminated.

Answer 2

You can use atoi function to convert buffer to integer. Please see below working code.

int main()
{
    FILE *fp = fopen("data.txt", "r");

    if(fp == NULL)
        exit(0);

    fseek(fp, 0, SEEK_END);

    int length = ftell(fp);
    fseek(fp, 0, SEEK_SET);

    char* buffer = malloc(length + 1);
    if (buffer) 
    {
       fread(buffer, 1, length, fp);
    }

    //unsigned long num = (unsigned long)atol(buffer);
    unsigned short num = (unsigned short)atoi(buffer);

    printf("Number = %u\n", num);

    fclose(fp);
    return 0;
}

If you see theoretical aspect of it, you have two bytes of data, each byte contribute to value depending on byte position.

Say number is 1337, in hex it is 0x05 0x39.

So corresponding integer value = (0x05 * 2 ^ 8) + 0x39 = (5 * 256) + 57 = 1337.

Hope it helps you.