-fno-builtin

When compiling without -fno-builtin, the compiler can replace calls to certain standard C library functions with inline code or with calls to other library functions. The Run-time ABI for the Arm^® Architecture lists library functions that the compiler can use. This means that your re-implementations of the standard C library functions might not be used, and might be removed by the linker.

Default

-fno-builtin is disabled by default.

Example

This example shows the result of compiling the following program with and without -fno-builtin:

#include "stdio.h"

void foo( void )
{
    printf("Hello\n");
}

1. Compile without -fno-builtin:

   armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -nostdlib foo.c -o foo.o

2. Run the following fromelf command to show the disassembled output:

   fromelf --disassemble foo.o -o foo.s
   ...
   ||foo|| PROC
           ADR      r0,|L3.8|
           B        puts
   |L3.8|
           DCD      0x6c6c6548
           DCD      0x0000006f
           ENDP
   ...

   Note:

   The compiler has replaced the printf() function with the puts() function.

3. Compile with -fno-builtin:

   armclang -c -O2 -g --target=arm-arm-none-eabi -mcpu=cortex-a9 -mfpu=none -nostdlib -fno-builtin foo.c -o foo.o

4. Run the following fromelf command to show the disassembled output:

   fromelf --disassemble foo.o -o foo.s
   ...
   ||foo|| PROC
           ADR      r0,|L3.8|
           B        printf
   |L3.8|
           DCD      0x6c6c6548
           DCD      0x00000a6f
           ENDP
   ...

   Note:

   The compiler has not replaced the printf() function with the puts() function when using the -fno-builtin option.