Phases of translation

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C
 
C language
 
Basic Concepts
 

The C source file is processed by the compiler as if the following phases take place, in this exact order. Actual implementation may combine these actions or process them differently as long as the behavior is the same.

Phase 1

1) The individual bytes of the source code file (which is generally a text file in some multibyte encoding such as UTF-8) are mapped, in implementation defined manner, to the characters of the source character set. In particular, OS-dependent end-of-line indicators are replaced by newline characters.
The source character set is a multibyte character set which includes the basic source character set as a single-byte subset, consisting of the following 96 characters:
a) 5 whitespace characters (space, horizontal tab, vertical tab, form feed, new-line)
b) 10 digit characters from '0' to '9'
c) 52 letters from 'a' to 'z' and from 'A' to 'Z'
d) 29 punctuation characters: _ { } [ ] # ( ) < > % : ; . ? * + - / ^ & | ~ ! = , \ " '
2) Trigraph sequences are replaced by corresponding single-character representations.(until C23)

Phase 2

1) Whenever backslash appears at the end of a line (immediately followed by the newline character), both backslash and newline are deleted, combining two physical source lines into one logical source line. This is a single-pass operation: a line ending in two backslashes followed by an empty line does not combine three lines into one.
Run this code
#include <stdio.h>
 
#define PUTS p\
u\
t\
s
/* Line splicing is in phase 2 while macros
 * are tokenized in phase 3 and expanded in phase 4,
 * so the above is equivalent to #define PUTS puts
 */
 
int main(void)
{
 /* Use line splicing to call puts */ PUT\
S\
("Output ends here\\
0Not printed" /* After line splicing, the remaining backslash
               * escapes the 0, ending the string early.
               */
);
}
2) If a non-empty source file does not end with a newline character after this step (whether it had no newline originally, or it ended with a backslash), the behavior is undefined.

Phase 3

1) The source file is decomposed into comment, sequences of whitespace characters (space, horizontal tab, new-line, vertical tab, and form-feed), and preprocessing tokens, which are the following
a) header names: <stdio.h> or "myfile.h"
b) identifiers
c) preprocessing numbers, which cover integer constants and floating constants, but also cover some invalid tokens such as 1..E+3.foo or 0JBK
d) character constants and string literals
e) operators and punctuators, such as +, <<=, <%, or ##.
f) individual non-whitespace characters that do not fit in any other category
2) Each comment is replaced by one space character
3) Newlines are kept, and it's implementation-defined whether non-newline whitespace sequences may be collapsed into single space characters.

If the input has been parsed into preprocessing tokens up to a given character, the next preprocessing token is generally taken to be the longest sequence of characters that could constitute a preprocessing token, even if that would cause subsequent analysis to fail. This is commonly known as maximal munch. 

int foo = 1;
// int bar = 0xE+foo; // error: invalid preprocessing number 0xE+foo
int bar = 0xE/*Comment expands to a space*/+foo; // OK: 0xE + foo
int baz = 0xE + foo; // OK: 0xE + foo
int pub = bar+++baz; // OK: bar++ + baz
int ham = bar++-++baz; // OK: bar++ - ++baz
// int qux = bar+++++baz; // error: bar++ ++ +baz, not bar++ + ++baz
int qux = bar+++/*Saving comment*/++baz; // OK: bar++ + ++baz

The sole exception to the maximal munch rule is: 

#define MACRO_1 1
#define MACRO_2 2
#define MACRO_3 3
#define MACRO_EXPR (MACRO_1 <MACRO_2> MACRO_3) // OK: <MACRO_2> is not a header-name

Phase 4

1) Preprocessor is executed.
2) Each file introduced with the #include directive goes through phases 1 through 4, recursively.
3) At the end of this phase, all preprocessor directives are removed from the source.

Phase 5

1) All characters and escape sequences in character constants and string literals are converted from source character set to execution character set (which may be a multibyte character set such as UTF-8, as long as all 96 characters from the basic source character set listed in phase 1 have single-byte representations). If the character specified by an escape sequence isn't a member of the execution character set, the result is implementation-defined, but is guaranteed to not be a null (wide) character.

Note: the conversion performed at this stage can be controlled by command line options in some implementations: gcc and clang use -finput-charset to specify the encoding of the source character set, -fexec-charset and -fwide-exec-charset to specify the encodings of the execution character set in the string literals and character constants that don't have an encoding prefix(since C11).

Phase 6

Adjacent string literals are concatenated.

Phase 7

Compilation takes place: the tokens are syntactically and semantically analyzed and translated as a translation unit.

Phase 8

Linking takes place: Translation units and library components needed to satisfy external references are collected into a program image which contains information needed for execution in its execution environment (the OS).

References

  • C23 standard (ISO/IEC 9899:2024):
  • 5.1.1.2 Translation phases (p: TBD)
  • 5.2.1 Character sets (p: TBD)
  • 6.4 Lexical elements (p: TBD)
  • C17 standard (ISO/IEC 9899:2018):
  • 5.1.1.2 Translation phases (p: 9-10)
  • 5.2.1 Character sets (p: 17)
  • 6.4 Lexical elements (p: 41-54)
  • C11 standard (ISO/IEC 9899:2011):
  • 5.1.1.2 Translation phases (p: 10-11)
  • 5.2.1 Character sets (p: 22-24)
  • 6.4 Lexical elements (p: 57-75)
  • C99 standard (ISO/IEC 9899:1999):
  • 5.1.1.2 Translation phases (p: 9-10)
  • 5.2.1 Character sets (p: 17-19)
  • 6.4 Lexical elements (p: 49-66)
  • C89/C90 standard (ISO/IEC 9899:1990):
  • 2.1.1.2 Translation phases
  • 2.2.1 Character sets
  • 3.1 Lexical elements

See also

C++ documentation for Phases of translation
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