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--- exit-status [February 23, 2026 at 13:29] – yanevskiv
+++ exit-status [May 14, 2026 at 11:38] (current) – external edit 127.0.0.1
@@ Line 1: / Line 1: @@
+# Exit status
+**Exit status** lets the user of your program know if the program ended in success or failure.
+Commonly, 0 is success, any other value is failure (most commonly, failure is 1 -- a bit counter intuitive at first!)
+Your users can inspect it with `echo $?` after your program ends. Thanks to the exit status, your user can chain program with other programs using `&&` and put your program in `if [ ... ]; then ... fi` statements when they write shell scripts.
+## Echo
+When you run a program, the shell remembers the exit status in the `$?` variable.
+You can inspect that variable with `echo $?`. The `echo` command is going to replace the value of `$?` with its own exit status (likely a success)
+Here's a simple prog
+```c
+/*
+Compile: gcc main.c -o exit_status
+Shell:
+    > ./exit_status 17
+    > echo $?
+    > echo $?
+*/
+#include <stdlib.h>
+int main(int argc, char *argv[])
+{
+    return (argc == 2 ? atoi(argv[1]) : EXIT_FAILURE);
+}
+```
+The program itself prints nothing -- but the shell remembers the exit status of the last program in `$?` variable. The shell expands the variable, gives the value "17" to the `echo` commands, the echo commands prints "17", and then returns the value 0 to the shell. The shell then remembers "0" in `$?` variable.
+## Constants
+The standard C library header [[stdlib.h|<stdlib.h>]] provides the following the following constants two constants:
+ - `EXIT_SUCCESS`
+ - `EXIT_FAILURE`
+You can find them in `/usr/include/stdlib.h` with grep command (-A means "lines after", -B means "lines before", -e means "expression")
+```c
+// $ grep -A 1 -B 3 -e EXIT_FAILURE /usr/include/stdlib.h
+/* We define these the same for all machines.
+   Changes from this to the outside world should be done in `_exit'.  */
+#define EXIT_FAILURE    1       /* Failing exit status.  */
+#define EXIT_SUCCESS    0       /* Successful exit status.  */
+```
+R 0 or 1
+```c
+// Compile: gcc main.c -o my_cat
+// Run:
+//   $ echo "Hello world!" > file.txt
+//   $ ./my_cat file.txt
+//   Hello world!
+int main(int argc, char *argv[])
+{
+    // Check arguments
+    if (argc != 2) {
+        fprintf(stderr, "Error: Expected one. argument");
+        return EXIT_FAILURE;
+    }
+    // Open file
+    FILE *file = NULL;
+    if ((file = fopen(argv[1], "r")) == NULL) {
+        fprintf(stderr, "Error: Can't open file '%s' for reading", file);
+        return EXIT_FAILURE;
+    }
+    // Read and print chars
+    int ch;
+    while ((ch = fgetc(file)) != EOF) {
+        putchar(ch);
+    }
+    // Close file
+    fclose(file);
+    // Return success
+    return EXIT_SUCCESS;
+}
+```
+## Why is 0 success?
+In C programming language, zero is considered false. What's funny is that if you use `EXIT_SUCCESS` in an if-statement directly the if-block is skipped -- because `EXIT_SUCCESS` is defined as 0!
+```c
+if (EXIT_SUCCESS) {
+    printf("This never prints...");
+}
+```
+So why is `EXIT_SUCCESS` defined as 0?
+That's because the original intent of exit status was
+```c
+#define SUCCESS 0
+#define FAILURE_ONE 1
+#define FAILURE_TWO 2
+...
+#define FAILURE_LAST 255
+int main()
+{
+    if (/*... something goes wrong 1 ...*/)
+        return FAILURE_ONE;
+    if (/*... something goes wrong 2 ...*/)
+        return FAILURE_TWO;
+    return SUCCESS;
+}
+```
+But there are way more than 255 ways a program can fail -- and it's much better if you get a descriptive error in **stderr** instead of a single number.
+Think about it! If you forget a semicolon -- would you rather the compiler tell where exactly? Or would you rather it tell you something cryptic "Error code: 233"?
+That's why, when most programs fail, they print a detailed report in **stderr** but return `EXIT_FAILURE` which is defined as 1.
+```bash
+ $ gcc main.c -o program
+main.c: In function ‘main’:
+main.c:5:26: error: expected ‘;’ before ‘}’ token
+|     printf("Hello world")
+      |                          ^
+      |                          ;
+| }
+      | ~
+ $ echo $?
+```
+## Exit status: 130
+When a program is interrupted by a signal the exit status
+```
+exit_status = 128 + signal_number
+Ctrl + A = 128
+Ctrl + B = 129
+Ctrl + C = 130
+... and so on ...
+```
+When you cancel a program with Ctrl + C, you're sending `SIGINT` signal ("signal interrupt") to the program which interrupts its execution and the value of `$?` is going to say 130.