Tuesday, 18 October 2016

char c; int i; c[&i]; // index a simple var using pointer in c compiles (and runs often without segfault)

    char c;
    int i;
    //printf("%02x",c[i]); // doesn't compile, error, cannot access c as array
    // "error: subscripted value is neither array nor pointer nor vector"

c[&i] there is SOOOO BAD and WROOONG. Semantically wrong. But apparently not syntactically wrong in c.

Why does it compile?
Because 1. array[i] == *(array+i) == *(i+array) == i[array]
And 2. c casts char to int happily

In case of c[&i] c is the index added to pointer &i.
Now &i + value of c indexes an undefined area of memory so it is not good to use this!!

    char c;
    int i[10];
    // normal access into array (bad to use a char as array index though)
    for(c=0;c<10;c++) printf("%02x",i[c]); // normal-ish
    for(c=0;c<10;c++) printf("%02x",*(i+c)); // equivalent pointer arithmetic
    for(c=0;c<10;c++) printf("%02x",*(c+i)); // equivalent
    for(c=0;c<10;c++) printf("%02x",c[i]);    // allowed BUT NOT NORMAL

    char c;
    int i;
    int buffer[20]; // buffer helps prog not seg fault!
    // abnormal access into memory at offset of (int)c from &i
    //for(c=0;c<10;c++) printf("%02x",&i[c]); // COMPILE ERROR
    for(c=0;c<10;c++) printf("%02x",*(&i+c)); // a bit weird
    for(c=0;c<10;c++) printf("%02x",*(c+&i)); // not normal
    for(c=0;c<10;c++) printf("%02x",c[&i]);    // but it compiles and runs(mostly)

    for(c=0;c<10;c++) printf("%02x,",i[&c]); // this is all
    printf(" ");
    for(c=0;c<10;c++) printf("%02x,",*(i+&c)); // a bit weird
    printf(" ");
    for(c=0;c<10;c++) printf("%02x,",*(&c+i)); // not normal
    //for(c=0;c<10;c++) printf("%02x,",&c[i]);    // COMPILE ERROR

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