The main Function Return Value in C: Exit Status, Conventions, and Shell Integration

main returns an int. That int is the program's exit status, delivered to the parent process. By convention, 0 means success and nonzero means failure. The C standard defines EXIT_SUCCESS and EXIT_FAILURE in stdlib.h as portable values. In C99 and later, reaching the end of main without a return statement is equivalent to return 0. In C89, it was undefined behavior. The shell captures the exit status in $? on Unix and %ERRORLEVEL% on Windows. Scripts branch on it.

Three invocations, three outcomes. The shell sees 0 for success and 1 for failure. A script can chain: if ./main_return data.txt; then echo "processed"; else echo "failed"; fi. The exit status is the program's final word to the outside world. Use it deliberately. Return distinct nonzero values for distinct error categories if your callers need to distinguish them.

#The Implicit return 0 in C99#link

In C99 and later, if control reaches the end of main without a return statement, the compiler inserts return 0 implicitly. This is specific to main. Any other function that reaches its closing brace without returning a value has undefined behavior if the caller uses the result. The implicit return 0 is a convenience for simple programs. In production code, write the return explicitly. It documents intent and satisfies static analysis tools.

The exit status is a contract. Scripts depend on it. CI systems alert on it. Monitoring dashboards aggregate it. Return 0 when the work is done. Return nonzero when it is not. Be specific about the nonzero value if your callers need to distinguish error types. Next: comments, and why the best comment is the one you did not need to write.