return(sizeof(array)) gives unexpected result [duplicate]

Question

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What should I do to get the whole return value of c-program from command line? (2 answers)

Closed 8 years ago.

I wrote the following program to determine the size of a static array. When I ran it, I got a result I can't explain. I've done some searching on stackexchange and google, but nothing I've read has given me a hint.

#include <stdio.h>
int main()
{
  int arrSize, intSize, elemSize;
  int input[9][9];
  arrSize = sizeof(input);
  intSize = sizeof(int);
  elemSize = sizeof(input[0]);
  printf("Array: %d, Element: %d, Int: %d\n", arrSize, elemSize, intSize);
  return sizeof(input);
}

When I compile and run this program, I get the following result (using linux):

./a.out ; echo $?
Array: 324, Element: 36, Int: 4
68

I see from http://c-faq.com/malloc/sizeof.html that sizeof is computed at compile time, and if I change the return to return sizeof(input[0]) I get 36 which is expected, and I get 4 if I change it to return sizeof(input[0][0]) as expected. So why does sizeof(input) give 68 in the return, but when stored it gives the expected 324?

Answer 1

The exit code for your system must be max 255 we can see that 324 % 256 = 68.

Answer 2

After a child process terminated, its parent process could get the status information of this child process by

waitpid(-1, &status, 0);

And the return status could be extracted from status by WEXITSTATUS(status), according to waitpid(2) this macro

returns the exit status of the child. This consists of the least significant 8 bits of the status argument that the child specified in a call to exit(3) or _exit(2) or as the argument for a return statement in main().

Therefore, if your main() returns 324, the return code you get from shell would be 324 % 256, i.e. 68.

Answer 3

From the bash shell and scripting language reference documentation's "Exit status" section:[2] The exit status of an executed command is the value returned by the waitpid system call or equivalent function. Exit statuses fall between 0 and 255, though, as explained below, the shell may use values above 125 specially. Exit statuses from shell builtins and compound commands are also limited to this range. Under certain circumstances, the shell will use special values to indicate specific failure modes. For more information go to:Exit status in Unix

That's why your program is giving this output because 324 is the size of array but shell only can show up to 256 so

324%256=68;

If you want to output more, you could write it to stdout or to a file instead.