[中文版本](/posts/c教程/zh-CN/07-贴近系统/069-Error Handling and Return Codes)

Every C program, from the tiniest script to the Linux kernel itself, relies on error codes and return values to communicate success or failure. Unlike higher-level languages, C gives you no exceptions, only clear, explicit status codes anderrno.

Mastering these patterns will make your programs robust, predictable, and professional.

Step 1. Exit Codes and main()

Every process returns an integer exit code to the operating system. Conventionally:

• 0→ success
• nonzero → failure or specific error

#include <stdio.h>
int main(void) {
    printf("Everything OK!\n");
    return 0; // exit success
}

Check it in your shell:

./program
echo $?

Output:

Everything OK!
0

If you return a nonzero value:

return 1;

then$? will be1, meaning failure.

Step 2. Using EXIT_SUCCESS and EXIT_FAILURE

Instead of hardcoded numbers, use standard macros from<stdlib.h>:

#include <stdio.h>
#include <stdlib.h>
int main(void) {
    printf("Failed to open file.\n");
    return EXIT_FAILURE;
}

These improve portability and readability.

Step 3. The Global errno

When a library or system call fails, it usually sets a global variable namederrno. It’s declared in<errno.h>.

#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
int main(void) {
    int fd = open("nonexistent.txt", O_RDONLY);
    if (fd == -1) {
        printf("Error opening file: %s\n", strerror(errno));
    }
}

Output:

Error opening file: No such file or directory

errno stores an integer code, butstrerror() converts it into a readable message.

Step 4. Common errno Values

Code	Macro	Meaning
2	ENOENT	No such file or directory
13	EACCES	Permission denied
12	ENOMEM	Out of memory
22	EINVAL	Invalid argument
9	EBADF	Bad file descriptor
11	EAGAIN	Resource temporarily unavailable

You can check them directly:

if (errno == EACCES) { ... }

Step 5. The perror() Function

A simpler way to print error messages isperror(), it automatically uses the currenterrno.

#include <stdio.h>
#include <fcntl.h>
#include <unistd.h>
int main(void) {
    int fd = open("nothing.txt", O_RDONLY);
    if (fd == -1) {
        perror("open");
    }
}

Output:

open: No such file or directory

Step 6. Returning Meaningful Codes

Good C programs translate internal errors into meaningful exit codes.

Example: file copy program

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
    if (argc != 3) {
        fprintf(stderr, "Usage: %s src dest\n", argv[0]);
        return 1;
    }
    FILE *src = fopen(argv[1], "r");
    if (!src) {
        perror("fopen src");
        return 2;
    }
    FILE *dst = fopen(argv[2], "w");
    if (!dst) {
        perror("fopen dst");
        fclose(src);
        return 3;
    }
    fclose(src);
    fclose(dst);
    return 0;
}

Now each exit code represents a specific type of failure.

Step 7. Resetting and Checking errno

Some system calls seterrno only when they fail. So always reset it before use if you plan to inspect it later:

#include <errno.h>
errno = 0;
if (some_syscall() == -1) {
    perror("syscall failed");
}

Step 8. Custom Error Handling Helpers

You can create your own function to simplify error handling.

#include <stdio.h>
#include <stdlib.h>
void die(const char *msg) {
    perror(msg);
    exit(EXIT_FAILURE);
}
int main(void) {
    FILE *f = fopen("no.txt", "r");
    if (!f) die("fopen");
}

Output:

fopen: No such file or directory

This pattern appears throughout Unix utilities.

Step 9. Handling Partial Failures

Not all errors should abort your program. Sometimes you should log, retry, or ignore.

FILE *f = fopen("optional.conf", "r");
if (!f) {
    fprintf(stderr, "Warning: config file missing, using defaults.\n");
}

This kind of graceful degradation is good design.

Step 10. Tiny Code: Safe File Reader

#include <stdio.h>
#include <errno.h>
#include <string.h>
int main(void) {
    FILE *f = fopen("data.txt", "r");
    if (!f) {
        fprintf(stderr, "Error: %s\n", strerror(errno));
        return EXIT_FAILURE;
    }
    char buf[64];
    while (fgets(buf, sizeof(buf), f))
        printf("%s", buf);
    fclose(f);
    return EXIT_SUCCESS;
}

Output (if missing file):

Error: No such file or directory

Why It Matters

Error handling separates toy programs from real software:

• Every syscall can fail, be prepared.
• Always check return values.
• Always report why it failed.

By convention:

• Return0 on success.
• Return nonzero for recoverable or fatal errors.
• Print messages tostderr, notstdout.

Try It Yourself

1. Open a nonexistent file and print the fullerrno value.
2. Build a small utility that returns specific codes for specific problems.
3. Useperror() vsstrerror() and compare their outputs.
4. Add adie() helper to your previous exercises.
5. Write a safe wrapper that retries system calls whenerrno == EAGAIN.

Next, you’ll put all of this together in Practice 70: Building a Mini Shell in C, where you’ll handle processes, pipes, and signals to create your own working Unix shell prototype.