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cprintfwarningsgetchar

What is wrong with this C function? (printf() with getchar())

printf( "%3o\t%2x\t%3d\t%c\n", c = getchar(), c, c, c );

I'm getting a warning that says "unsequenced modification and access to 'c' [-Wunsequenced]". The error is fairly easy to fix; all I have to do is separate getchar() from printf(), but I just want to have a better understanding of why this instruction is producing a warning, and what would go wrong if I actually ran it.

Does this have anything to do with the implementation of printf() using CPP macros for the variable-length argument list?

Solution

  • The order in which arguments passed to a function call are evaluated is unspecified. There is no guarantee that the result of getchar() will be assigned to c before the last three c arguments are read.

    Calling c = getchar() outside of the function call fixes the issue.