Learning C Language in 2019
Recently, I finally had some time to learn some C language. C code needs to be compiled into a binary executable file before it can be executed, so I configured the relevant environment for C/C++ according to the official tutorial of VS Code. Programming, debugging, and outputting results in one step makes the learning process more efficient.
I plan to start from The C Programming Language, and I typed the first example code in chapter one, hello.c:
#include<stdio.h>
// include information about standard input/output library
main()
// define a function named main that receives no arguments values
{
// statements of main are enclosed in braces
printf("hello, world\n");
// main calls library function printf to print this sequence of characters; \n represents the newline character
}
After executing cc hello.c to compile it, I saw the error message:
hello.c:3:1: warning: return type defaults to ‘int’ [-Wimplicit-int]
main()
^~~~
The reason is that from the C99 standard, no type specifiers have been removed. The book uses the C89 standard, see The C89 Draft - 3.5.2 Type specifiers for details.
Well, now I will find out which standard cc is using by executing the following command to view the pre-compilation macros defined by cc:
cc -E -dM - </dev/null | grep "STDC_VERSION"
#define **STDC_VERSION** 201112L
The computer is Ubuntu 18.04, and the default standard used is C11.
There are approximately three commonly known C standards: c90 (or c89), c99, c11, which were released in 1990, 1999, and 2011, respectively. The original ANSI C standard (X3.159-1989) was approved in 1989 and released in 1990. It was later (in 1990) approved as an ISO standard (ISO/IEC 9899:1990). There is no technical difference between C89 and C90.
Well, let's start solving the problem. There are two options for the above code to work properly:
1. Modify the code according to the C11 standard
#include <stdio.h>
int main()
{
printf("hello, world\n");
return 0;
}
And it will output normally.
2. Use the C89 or C90 standard
cc -std=c89 hello.c
# or cc -std=c90 hello.c
And it will also output normally.
In the Linux operating system, cc is linked to gcc through a soft link, so using cc and gcc in Linux has the same effect.
Supplementary material: