One thing I take for granted is which C standard I’m using. The differences between the versions are subtle, and the compiler chooses its standard by default. But this choice can be altered for compatibility or historical reasons.
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\nEvery so often, the Committee That Does Such Things releases a new C standard, improving the language and offering new features while trying to maintain compatibility with the universe of older (and still operating) C programs. Here are the current standards:<\/p>\n
K&R.<\/strong> The original C written by Brian Kernighan and Dennis Ritchie and documented when they published The C Programming Language<\/em> in 1978.<\/p>\n ANSI C \/ C89.<\/strong> This version of C is backed by the ANSI standard body. It standardized C for use across multiple compilers.<\/p>\n C99.<\/strong> This version added the long long<\/em> data types, C++ style C11.<\/strong> New keywords were introduced, primarily the underscore keywords.<\/p>\n C17.<\/strong> Various minor improvements.<\/p>\n C2x.<\/strong> A major update that I’ll cover as soon as many of its features are implemented in popular compilers.<\/p>\n Even within these standards, compilers and libraries exist to extend the language.<\/p>\n To help sort out which version is which, and avoid compatibility issues in C code, two defined constants are available: The The The following code tests the standard constants and outputs the results.<\/p>\n The if<\/em> test at Line 5 checks for the The Here is a sample run with the program compiled by clang<\/em> in macOS, no other options:<\/p>\n Your compiler may output Here is a run of the code ( The final switch I use is If compatibility is an issue for any code, using these constants is one way to ensure that the program compiles. Otherwise, it’s a curious tidbit to know which C standard your compiler uses.<\/p>\n","protected":false},"excerpt":{"rendered":" C has many standards, including the upcoming exciting C23. Which is which? Continue reading \/\/<\/code> comments, and Unicode support.<\/p>\n__STDC__<\/code> and __STDC_VERSION__<\/code>.<\/p>\n__STDC__<\/code> constant is defined in ANSI C \/ C89 forward. It’s set to 1 or TRUE.<\/p>\n__STDC_VERSION__<\/code> constant was introduced with C99. It’s a long unsigned<\/em> value equal to the C version year and month represented as a six digit value.<\/p>\n2022_12_10-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n if( __STDC__ )\r\n {\r\n printf(\"The C version is at least C89\\n\");\r\n#ifdef __STDC_VERSION__\r\n printf(\"C standard version: %lu\\n\",__STDC_VERSION__);\r\n#endif\r\n }\r\n else\r\n {\r\n printf(\"Somehow you got ahold of K&R C. Neat!\\n\");\r\n }\r\n\r\n return(0);\r\n}<\/pre>\n__STDC__<\/code> constant, which should exist in all modern compilers. If not, the program won’t compile, so my else<\/em> statement (Line 14) is unnecessary. Presence of the __STDC__<\/code> constant indicates at least C89, so a printf()<\/em> statement confirms at Line 7.<\/p>\n#ifdef<\/code> preprocessor test at Line 8 checks for the __STDC_VERSION__<\/code> constant, which isn’t present in C89. If found, its value is output.<\/p>\nThe C version is at least C89
\nC standard version: 201710<\/code><\/p>\n201112<\/code> as the version; it all depend on the defaults. Even so, you can use the -std<\/code> switch at the command line to set a specific C version for compiling.<\/p>\na.out<\/code>) compiled with various switches to set the C language version:<\/p>\n\r\n$ clang -Wall -std=c89<\/span> 1210.c \r\n$ .\/a.out\r\nThe C version is at least C89<\/span>\r\n$ clang -Wall -std=c99<\/span> 1210.c \r\n$ .\/a.out\r\nThe C version is at least C89\r\nC standard version: 199901<\/span>\r\n$ clang -Wall -std=c11<\/span> 1210.c \r\n$ .\/a.out\r\nThe C version is at least C89\r\nC standard version: 201112<\/span>\r\n$ clang -Wall -std=c17<\/span> 1210.c \r\n$ .\/a.out\r\nThe C version is at least C89\r\nC standard version: 201710<\/span>\r\n$ clang -Wall -std=c2x<\/span> 1210.c \r\n$ .\/a.out\r\nThe C version is at least C89\r\nC standard version: 202000<\/span><\/pre>\n-std=c2x<\/code> for the next version of C. Some compilers have a tiny subset of this version ready, so using the switch may not generate an error. Still, the value output is 202000<\/code> – not 2023nn<\/em><\/code>, which would be present for compilers that implement the full version of C23.<\/p>\n