Returning string from C system() function [duplicate] - c

Question

This question already has an answer here:
Getting output of a system command from stdout in C
(1 answer)
Closed 4 years ago.
I just learned C and fascinates with pointers. Recently I discovered C function system(). I am able to get return values from a program I executed via system ("program.exe"). Eg, program.c:
#include <stdio.h>
int main(){
printf("hello world\n");
return 123;
}
and this code calls program.exe, called call.c
#include <stdio.h>
#include <stdlib.h>
int main(){
int a;
printf("calling program.exe:\n");
a=system("program.exe");
printf("program.exe returned %d at exit\n",a);
return 0;
}
When I execute call.exe, I get this
calling program.exe:
hello world
pm returned 123 at exit
I was like, wow! This return value and system() function thing is like a new way to interprocess communication for me. But my question is, can I get a string return from the system() function?
Ii tried changing program.c "int main()" to "char * main()", and return 123 to return "bohemian rhapsody" and change "int a;" to "char *a;", printf format %d to %s in call.c, but I only get funny characters when I execute call.exe. I wonder what's wrong?

Answer 1

No you can't return a string from system(), and indeed a program under most modern desktop operating systems can only "return" an integer exit code when it terminates.
If you do want to get a string back from the executed program, one way would be to use popen() to invoke the program, and then have that program write the string to stdout.

Answer 2

The program that you call 'call.c' calls system(3), which does the following,
suspends the current process,
starts a child process,
and waits for the child process to complete
The return value from the called process, 'program.c/exe' is an integer (size is system dependent usually 16-bit), and what is happening behind the scenes is that the system(3) call uses the wait(2) call to (blocking) suspend execution until the child process returns.
Note that the string is not returned, but the child process prints to stdout. See the popen(3) call if you want to obtain the string (or binary) output from the child process.
See the manual page for wait(3) to see how to process the results returned by the called program, e.g. 'man -s2 wait',
WAIT(2) System Calls Manual WAIT(2)
NAME
wait, wait3, wait4, waitpid -- wait for process termination
SYNOPSIS
#include <sys/wait.h>
pid_t
wait(int *stat_loc);
pid_t
wait3(int *stat_loc, int options, struct rusage *rusage);
pid_t
wait4(pid_t pid, int *stat_loc, int options, struct rusage *rusage);
pid_t
waitpid(pid_t pid, int *stat_loc, int options);