Can't run make using C++ stdlib system call - c++

Question

I've got the following code in C++
if (should_run_make) {
std::string make = "make -C ";
make.append(outdir);
std::cout << "Make cmd is " << make << std::endl;
system(make.c_str());
}
This reports the following:
Make cmd is make -C /home/hamiltont/temp/ make: Entering directory
/home/hamiltont/temp' make: *** No targets. Stop.
make: Leaving directory/home/hamiltont/temp'
However, doing it manually works fine in multiple ways e.g.
[hamiltont#4 generator]$ make -C /home/hamiltont/temp/
make: Entering directory `/home/hamiltont/temp'
g++ -O3 -I/usr/include/openmpi-x86_64 -L/usr/local/lib -L/usr/lib64/openmpi/lib -lmpi -lmpi_cxx -lboost_serialization -lboost_mpi stg_impl.cpp -o impl
make: Leaving directory `/home/hamiltont/temp'
[hamiltont#4 generator]$ cd /home/hamiltont/temp/
[hamiltont#4 temp]$ make
g++ -O3 -I/usr/include/openmpi-x86_64 -L/usr/local/lib -L/usr/lib64/openmpi/lib -lmpi -lmpi_cxx -lboost_serialization -lboost_mpi stg_impl.cpp -o impl

Answer 1

Are you generating the makefile from within your C program? That's the only reason I could imagine would cause that specific error message.
make: *** No targets. Stop.
Reproducing the error
Here's how I could generate that message:
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp = fopen("Makefile", "w");
fputs("all:\n\techo Done.\n", fp);
system("make");
fclose(fp);
return 0;
}
This, predictably, prints:
make: *** No targets. Stop.
I say predictably because Makefile will be empty! This is because IO is buffered...
Fixed version
So, I close the file before calling system(), which flushes the buffer (fflush() would also do the trick):
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *fp = fopen("Makefile", "w");
fputs("all:\n\techo Done.\n", fp);
fclose(fp);
system("make");
return 0;
}
Output:
echo Done.
Done.
I used C's IO functions for clarity, but the same rules apply to <iostream>.