[中文版本](/posts/c教程/zh-CN/05-输入输出与文件/046-Error Checking with errno and perror)

Even the best-written C programs can encounter errors, missing files, permission issues, division by zero, or failed memory allocations. Unlike some languages that throw exceptions, C reports errors manually using return values and a global variable namederrno.

Understanding how to useerrno,perror(), andstrerror() is essential for writing robust, production-grade C programs that fail gracefully and informatively.

The Idea Behind errno

errno is a global integer (declared in<errno.h>) that stores an error code whenever a library function fails.

• On success, most functions leaveerrno unchanged.
• On failure, they typically seterrno and return an error value (oftenNULL,-1, or0 depending on the function).
• You must check the return value first, only then should you inspecterrno.

Example:

#include <stdio.h>
#include <errno.h>
int main(void) {
    FILE *fp = fopen("missing.txt", "r");
    if (!fp) {
        printf("Error code: %d\n", errno);
        perror("fopen failed");
    }
    return 0;
}

Output:

Error code: 2
fopen failed: No such file or directory

Common errno Codes

Code	Name	Meaning
2	ENOENT	No such file or directory
13	EACCES	Permission denied
12	ENOMEM	Not enough memory
22	EINVAL	Invalid argument
17	EEXIST	File already exists
5	EIO	Input/output error
111	ECONNREFUSED	Connection refused (network)

Include<errno.h> to use these symbolic names:

if (errno == ENOENT) printf("File missing.\n");

Using perror()

perror() prints a human-readable error message tostderr, based on the current value oferrno.

perror("File open error");

Example:

#include <stdio.h>
#include <errno.h>
int main(void) {
    FILE *f = fopen("ghost.txt", "r");
    if (!f)
        perror("Unable to open file");
    return 0;
}

Output:

Unable to open file: No such file or directory

Using strerror()

If you want to use the error message in your own formatted output, usestrerror() from<string.h>:

#include <stdio.h>
#include <string.h>
#include <errno.h>
int main(void) {
    FILE *f = fopen("/root/secret.txt", "r");
    if (!f)
        printf("Error (%d): %s\n", errno, strerror(errno));
    return 0;
}

Output:

Error (13): Permission denied

Tiny Code: Robust File Reader

Here’s a simple file reader that checks for errors at every step:

#include <stdio.h>
#include <string.h>
#include <errno.h>
int main(int argc, char *argv[]) {
    if (argc != 2) {
        fprintf(stderr, "Usage: %s <filename>\n", argv[0]);
        return 1;
    }
    FILE *fp = fopen(argv[1], "r");
    if (!fp) {
        fprintf(stderr, "Error opening %s: %s\n", argv[1], strerror(errno));
        return 1;
    }
    char buf[128];
    while (fgets(buf, sizeof(buf), fp))
        printf("%s", buf);
    if (ferror(fp)) {
        fprintf(stderr, "Error reading file: %s\n", strerror(errno));
    }
    fclose(fp);
    return 0;
}

Usage:

./readfile poem.txt

If the file is missing:

Error opening poem.txt: No such file or directory

Clearing and Resetting errno

Some functions may seterrno even if they succeed later. To be safe, you can clear it before a call:

#include <errno.h>
errno = 0;
FILE *f = fopen("file.txt", "r");
if (!f) perror("fopen");

This ensures you don’t read a leftover error from an earlier operation.

Checking Other System Errors

errno isn’t limited to file I/O, it applies to many system calls and library functions:

#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
int main(void) {
    FILE *f = fopen("/dev/full", "w"); // special Linux device that fails on write
    if (f) {
        if (fputc('A', f) == EOF)
            perror("Write failed");
        fclose(f);
    }
    return 0;
}

Output:

Write failed: No space left on device

Why It Matters

errno and its helpers (perror,strerror) make your programs explain themselves when things go wrong. This is vital for:

• System tools that must report specific error causes
• Debugging production code
• Writing portable, maintainable programs

Good C developers never just “fail silently.”

Try It Yourself

1. Open a file that doesn’t exist, then create it and try again.
2. Simulate a read error by usingferror() after reading a closed file.
3. Try writing to a directory (fopen("/tmp", "w")) and inspecterrno.
4. Print all known error codes and messages using a loop andstrerror().
5. Write a small “safe_open” function that wrapsfopen with error reporting.

With error handling mastered, you now know how to make C programs both informative and reliable. Next, you’ll explore command-line arguments (argc,argv), the gateway to building flexible, scriptable tools that process user input dynamically.