为什么不在 stderr 上打印浮点错误消息？

Question

I am running c language code using a java application using java.lang.ProcessBuilder . If any output and error generated by this c language code, I am able to read those via process.getInputStream() and process.getErrorStream() . But in some cases (when exit code = 136 or 139) like if the code execution failed due to floating point error , I am not getting any error in process's error stream. So, I tried to run my c language code directly on shell and redirected stderr and stdout to seperate files. Code.c

#include<stdio.h>

int main() {
    int x = 1/0;
    return 0;
}

Command I am running:

# gcc Code.c -o Code.out -std=c99
# ./Code.out 2>err.log 1>out.log
Floating point exception (core dumped)

As you can see above error is printing on shell only but not redirecting to err.log . Also, nothing is there in err.log. My question is why this is not printing in stderr stream? As per my understanding if it is not printing in stderr I will not be able to read it via process.getErrorStream. Whatever the error message generated by code execution, I wanted to show it to end user.

Answer 1

As dave_thompson_085 comments, the Floating point exception (core dumped) message is from the Shell rather than the program.

This is a test program:

#include<stdio.h>

int main() {
    fprintf(stdout, "normal output, %d\n", __LINE__);
    fprintf(stderr, "error output, %d\n", __LINE__);
    fflush(stdout);
    fflush(stderr);

    int x = 1/0;

    fprintf(stdout, "normal output, %d\n", __LINE__);
    fprintf(stderr, "error output, %d\n", __LINE__);
    fflush(stdout);
    fflush(stderr);

    return 0;
}

# gcc code.c -o Code -std=c99
# ./Code  2>err.log 1>out.log
normal output, 4
error output, 5
Floating point exception (core dumped)

# cat out.log
normal output, 4

$cat err.log
error output, 5

When the float exception occurs, it is captured by the OS and forced to exit.