In this recipe, you will get to know how the Clang frontend can be used for different purposes.
Clang can be used as the high-level compiler driver. Let us show it using an example:
hello world
C code, test.c
:$ cat test.c #include<stdio.h> int main() { printf("hello world "); return 0; }
a.out
file, which on execution gives the output as expected:$ clang test.c $ ./a.out hello world
Here the test.c
file containing C code is created. Using Clang we compile it and produce an executable that on execution gives the desired result.
–E
flag. In the following example, create a C code having a #define directive defining the value of MAX and use this MAX as the size of the array you are going to create:$ cat test.c #define MAX 100 void func() { int a[MAX]; }
$ clang test.c -E # 1 "test.c" # 1 "<built-in>" 1 # 1 "<built-in>" 3 # 308 "<built-in>" 3 # 1 "<command line>" 1 # 1 "<built-in>" 2 # 1 "test.c" 2 void func() { int a[100]; }
In the test.c
file, which will be used in all the subsequent sections of this recipe, MAX is defined to be 100
, which on preprocessing is substituted to MAX in a[MAX]
, which becomes a[100]
.
test.c
file from the preceding example using the following command, which displays the output on standard output:| $ clang -cc1 test.c -ast-dump TranslationUnitDecl 0x3f72c50 <<invalid sloc>> <invalid sloc>|-TypedefDecl 0x3f73148 <<invalid sloc>> <invalid sloc> implicit __int128_t '__int128'|-TypedefDecl 0x3f731a8 <<invalid sloc>> <invalid sloc> implicit __uint128_t 'unsigned __int128'|-TypedefDecl 0x3f73518 <<invalid sloc>> <invalid sloc> implicit __builtin_va_list '__va_list_tag [1]'`-FunctionDecl 0x3f735b8 <test.c:3:1, line:5:1> line:3:6 func 'void ()'`-CompoundStmt 0x3f73790 <col:13, line:5:1>`-DeclStmt 0x3f73778 <line:4:1, col:11>`-VarDecl 0x3f73718 <col:1, col:10> col:5 a 'int [100]'
Here, the –cc1
option ensures that only the compiler front-end should be run, not the driver, and it prints the AST corresponding to the test.c
file code.
test.c
file in previous examples, using the following command:|$ clang test.c -S -emit-llvm -o - |; ModuleID = 'test.c' |target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128" |target triple = "x86_64-unknown-linux-gnu" | |; Function Attrs: nounwind uwtable |define void @func() #0 { |%a = alloca [100 x i32], align 16 |ret void |}
The –S
and –emit-llvm
flag ensure the LLVM assembly is generated for the test.c
code.
test.c
testcode, pass the –S
flag to Clang. It generates the output on standard output because of the option –o –
:|$ clang -S test.c -o - | .text | .file "test.c" | .globl func | .align 16, 0x90 | .type func,@function |func: # @func | .cfi_startproc |# BB#0: | pushq %rbp |.Ltmp0: | .cfi_def_cfa_offset 16 |.Ltmp1: | .cfi_offset %rbp, -16 | movq %rsp, %rbp |.Ltmp2: | .cfi_def_cfa_register %rbp | popq %rbp | retq |.Ltmp3: | .size func, .Ltmp3-func | .cfi_endproc
When the –S
flag is used alone, machine code is generated by the code generation process of the compiler. Here, on running the command, machine code is output on the standard output as we use –o –
options.
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