g++/clang override linker for specific functions - c++

Question

Is there a simple way to specify alternative functions (to the linker) to be used rather than the standard ones?
I have a wrapper around open/close/read/write system functions. I can test the functionality of the good path for these functions relatively easily.
But testing the potential errors is harder. To do this I need to get open/close/read/write to generate specific error codes for each test. Is there a way to link in alternative versions of these functions that I can then program to set the appropriate errno value before returning?

Answer 1

The linker option --wrap is for this purpose.
Some code that calls open:-
main.c
#include <fcntl.h>
#include <unistd.h>
#include <stdio.h>
#define N 32
int main(int argc, char *argv[])
{
char buf[N] = {0};
int fd = open(argv[1],O_RDONLY);
read(fd,buf,N - 1);
puts(buf);
close(fd);
return 0;
}
For simplicity it's a program, but it needn't be.
Using the true open:
$ gcc -Wall -c main.c
$ gcc -o prog main.o
$ echo "Hello world" > hw.txt
$ ./prog hw.txt
Hello world
Your alternative open must be called __wrap_open, and
must refer to the real open, if it needs to, as __real_open:
dbg_open.c
#include <stdio.h>
extern int __real_open(const char *path, int oflag);
int __wrap_open(const char *path, int oflag)
{
printf("In tester %s\n",__FUNCTION__);
return __real_open(path,oflag);
}
There's no need to recompile main.c to replace open with __wrap_open in prog; just
reuse main.o in a different linkage of prog
$ gcc -Wall -c dbg_open.c
$ gcc -o prog main.o dbg_open.o -Wl,--wrap=open
$ ./prog hw.txt
In tester __wrap_open
Hello world
If your __wrap_foo alternatives needed to knock out C++ functions
foo then you would need to obtain the mangling of foo to specify in the linkage
option --wrap=mangled-foo. But since you want to knock out system calls
you're spared that complication.