APUE第7章中的hello world退出，代码为0

Question

APUE chapter 7 says this snippet would exit with exit code other than 0 (13 in example). But it exit with code 0 on my computer:

#include    <stdio.h>

main()
{
    printf("hello, world\n");
}

Compile & Running environment:

Darwin cuixiaochens-MacBook-Pro.local 17.4.0 Darwin Kernel Version 17.4.0: Sun Dec 17 09:19:54 PST 2017; root:xnu-4570.41.2~1/RELEASE_X86_64 x86_64

Gcc version:

♪ apue.3e git:(master) gcc -v Configured with: --prefix=/Library/Developer/CommandLineTools/usr --with-gxx-include-dir=/usr/include/c++/4.2.1 Apple LLVM version 9.0.0 (clang-900.0.39.2) Target: x86_64-apple-darwin17.4.0 Thread model: posix InstalledDir: /Library/Developer/CommandLineTools/usr/bin

Answer 1

APUE is trying to illustrate a real phenomenon, and it's still possible to observe that phenomenon with a modified program.

Normally, when a function declared to return a value (ie return type other than void ) returns by "falling off the end", its return value is "indeterminate," and a program that observes that value has "undefined behavior." A common way for that undefined behavior to manifest, is for the function's return value to be equal to the return value of the last call made within the function, because that value is still in the return-value register. APUE was trying to demonstrate this by falling off the end of main , but since C99 there is a special rule for falling off the end of main which says that this behaves as-if main had properly returned 0. Many compilers apply this rule even in their C89 mode, since in C89 it was undefined, so they're allowed to do anything they like.

So, to observe the phenomenon APUE wants you to observe, you need to use a function other than main , and you also need to take steps to prevent interprocedural analysis, such as splitting the program into two translation units and then not using link-time optimization.

/* file1.c */
#include <stdio.h>

int print_hello(void)
{
    printf("hello world\n");
}

/* file2.c */
#include <stdio.h>
extern int print_hello(void);

int main(void);
{
    int n = print_hello();
    printf("print_hello returned %d\n", n);
    return 0;
}

Compile each file separately with eg cc -c and then link them together as a third operation. On my computer, with my compiler, this prints

hello world
print_hello returned 12

Please do keep in mind that this program does have undefined behavior. On a computer with a different calling convention (eg a SPARC) it might print a garbage number; if I had allowed interprocedural optimization to happen, the compiler might have deduced that the only way the program doesn't have undefined behavior is if it's never run, and therefore emitted no code at all for it.

(The program would not have undefined behavior if the value returned by print_hello was never used. This is another special case in the language, for the sake of backward compatibility with code written before void return types were a possibility.)

Answer 2

Tour APUE chapter is wrong. The main function in the C language is guaranteed by the standard to return zero if there is no return statement( if we consider standard no older than 20 years)