sscanf converting from octal: How does it know?-CodePudding

I have this code which converts a string to an int

unsigned int formatInt(char *ptr) {
    int res;
    if (sscanf(ptr, "%i", &res) == -1) exit(-1);
    return res;
}

I fed it a char * pointing to the first char of "00000000041".

Conversion to int returns me 33 (Implicit Octal to Decimal conversion)

"00000000041" is actually a string (char[12]), but it's the size of a file in octal.

How did the compiler know it was in octal ? 00000000041 could perfectly be a decimal (41)

CodePudding user response：

Recognizing the string as octal is a function of the %i format specifier to scanf. From the man page:

   i      Matches an optionally signed integer; the next pointer must be a
          pointer to int.  The integer is read in base  16  if  it  begins
          with  0x  or  0X,  in base 8 if it begins with 0, and in base 10
          otherwise.  Only characters that  correspond  to  the  base  are
          used.

So because the string begins with a 0 and %i was used, it is interpreted as an octal string.