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cgccturbo-c

Different behavior of printf() in Turbo C and gcc when trying to print a pointer

The code below gives the output without error in Turbo C compiler, and gives the address of variable and its value both: 

int a=5,*fp;
fp=&a;
printf("%d %d\n",fp,*fp);

But when I compile the same code in Linux with GCC compiler, it gives an error: 

`warning: format ‘%d’ expects argument of type ‘int’, but argument 2 has type ‘int *’ [-Wformat=]
printf("%d %d\n",fp,*fp);`

But the same code with the %p format specifier works in GCC Compiler ,to which I agree. The question is: how come it's working on Turbo C platform?

P.S. The issue is not that (in Turbo C) the error is not reported but it's that on Turbo C it gives a signed integer value that is unchanged on repeated execution of the program; can it be garbage?

P.P.S Turbo C is running on MSDOS platform and GCC on 64-bit Linux, if that helps.

Solution

  • printf() assumes you will pass a signed int for the %d specifier.

    On the platform and compiler you are using, ints and pointers are probably the same size and printf() is able to display it correctly. Your pointer is reinterpreted as an int.

    Compiling and running this on a different platform where, for example, ints are 32-bit and pointers are 64-bit (such as gcc on x64) would be undefined behaviour. If you are lucky enough, it would crash. If not, you'd get garbage.