C errno.h Library Reference

?Why Use errno.h?

C doesn't have exceptions like C++ or Java. When something fails, functions return -1 or NULL, but why did it fail? errno tells you!

•

File Errors

ENOENT, EACCES

•

Memory Errors

ENOMEM

Math Errors

ERANGE, EDOM

01Introduction to errno.h

What is errno.h?

<errno.h> provides the error reporting mechanismfor C programs. When library functions encounter errors, they set the errno variable to indicate what went wrong.

Key Components

Name	Type	Description
errno	int (macro)	Error number set by system calls
perror()	Function	Print error message to stderr
strerror()	Function	Get error message string

02The errno Variable

errno is a global variable (actually a macro that expands to a modifiable lvalue) that holds the error code from the last system call or library function that failed.

Important Rules:

Always set errno = 0 before calling a function
Check errno only if the function indicates failure
errno is never set to 0 by any standard library function
errno may be changed by successful function calls

errno Usage

1#include <stdio.h>
2#include <errno.h>
3#include <math.h>
4#include <string.h>
5
6int main() {
7    // Example 1: File open error
8    errno = 0;  // Clear errno before call
9    FILE *fp = fopen("nonexistent_file.txt", "r");
10    
11    if (fp == NULL) {
12        printf("Error opening file!\n");
13        printf("errno value: %d\n", errno);
14        printf("Error message: %s\n", strerror(errno));
15    }
16    
17    // Example 2: Math error
18    errno = 0;
19    double result = sqrt(-1.0);  // Invalid: sqrt of negative
20    
21    if (errno != 0) {
22        printf("\nMath error occurred!\n");
23        printf("errno value: %d\n", errno);
24        printf("Error message: %s\n", strerror(errno));
25        printf("Result: %f\n", result);  // NaN
26    }
27    
28    // Example 3: Number too large
29    errno = 0;
30    result = exp(1000);  // Overflow
31    
32    if (errno == ERANGE) {
33        printf("\nRange error: number too large!\n");
34        printf("Error message: %s\n", strerror(errno));
35    }
36    
37    return 0;
38}
39
40/* Output:
41Error opening file!
42errno value: 2
43Error message: No such file or directory
44
45Math error occurred!
46errno value: 33
47Error message: Numerical argument out of domain
48Result: nan
49
50Range error: number too large!
51Error message: Numerical result out of range
52*/

03Common Error Codes

Macro	Value	Description
EDOM	33	Math argument out of domain (e.g., sqrt(-1))
ERANGE	34	Result out of range (overflow/underflow)
EILSEQ	84	Illegal byte sequence
ENOENT	2	No such file or directory
EACCES	13	Permission denied
ENOMEM	12	Out of memory
EEXIST	17	File already exists
EINVAL	22	Invalid argument
ENOSPC	28	No space left on device
EBADF	9	Bad file descriptor

Note:

Error codes and their numeric values may vary between platforms. Always use the macro names (ENOENT, EACCES, etc.) instead of numeric values for portability.

04perror() - Print Error Message

void perror(const char *s);

perror() prints a custom message followed by a colon and the error message corresponding to the current value of errno. Output goes to stderr.

perror() Usage

1#include <stdio.h>
2#include <errno.h>
3
4int main() {
5    // Try to open a non-existent file
6    FILE *fp = fopen("/nonexistent/path/file.txt", "r");
7    
8    if (fp == NULL) {
9        // perror prints to stderr
10        perror("Error opening file");
11        
12        // Equivalent manual approach:
13        // fprintf(stderr, "Error opening file: %s\n", strerror(errno));
14    }
15    
16    // Try to open a protected file (if on Unix-like system)
17    fp = fopen("/etc/shadow", "r");
18    
19    if (fp == NULL) {
20        perror("Cannot read shadow file");
21    }
22    
23    // Empty string still prints error
24    errno = ENOMEM;
25    perror("");  // Just prints the error message
26    
27    // NULL prints just the error
28    errno = EINVAL;
29    perror(NULL);
30    
31    return 0;
32}
33
34/* Output:
35Error opening file: No such file or directory
36Cannot read shadow file: Permission denied
37: Cannot allocate memory
38Invalid argument
39*/

05strerror() - Get Error String

char *strerror(int errnum);

strerror() returns a pointer to a string describing the error code passed as argument. More flexible than perror() as you can use the string however you want.

strerror() Usage

1#include <stdio.h>
2#include <string.h>
3#include <errno.h>
4
5int main() {
6    // Get error message for specific error codes
7    printf("ENOENT (%d): %s\n", ENOENT, strerror(ENOENT));
8    printf("EACCES (%d): %s\n", EACCES, strerror(EACCES));
9    printf("ENOMEM (%d): %s\n", ENOMEM, strerror(ENOMEM));
10    printf("EINVAL (%d): %s\n", EINVAL, strerror(EINVAL));
11    printf("ERANGE (%d): %s\n", ERANGE, strerror(ERANGE));
12    printf("EDOM   (%d): %s\n", EDOM, strerror(EDOM));
13    
14    // Use with errno after an error
15    FILE *fp = fopen("nonexistent.txt", "r");
16    if (fp == NULL) {
17        // Store errno immediately - it might change
18        int saved_errno = errno;
19        
20        // Now use it
21        printf("\nFile open failed: %s (errno=%d)\n", 
22               strerror(saved_errno), saved_errno);
23    }
24    
25    // Custom error logging function
26    printf("\n--- Custom Error Log ---\n");
27    errno = 0;
28    fp = fopen("/root/secret.txt", "r");
29    if (fp == NULL) {
30        fprintf(stderr, "[ERROR] %s:%d: Failed to open file: %s\n",
31                __FILE__, __LINE__, strerror(errno));
32    }
33    
34    return 0;
35}
36
37/* Output:
38ENOENT (2): No such file or directory
39EACCES (13): Permission denied
40ENOMEM (12): Cannot allocate memory
41EINVAL (22): Invalid argument
42ERANGE (34): Numerical result out of range
43EDOM   (33): Numerical argument out of domain
44
45File open failed: No such file or directory (errno=2)
46
47--- Custom Error Log ---
48[ERROR] example.c:35: Failed to open file: Permission denied
49*/

Thread Safety:

strerror() may return a pointer to a static buffer, making it not thread-safe. Use strerror_r() (POSIX) or strerror_s() (C11) in multi-threaded programs.

06Error Handling Best Practices

Error Handling Best Practices

1#include <stdio.h>
2#include <stdlib.h>
3#include <string.h>
4#include <errno.h>
5
6// Good error handling pattern
7FILE *safe_fopen(const char *path, const char *mode) {
8    errno = 0;  // Clear errno before call
9    
10    FILE *fp = fopen(path, mode);
11    
12    if (fp == NULL) {
13        // Save errno immediately
14        int err = errno;
15        
16        // Log detailed error
17        fprintf(stderr, "Error: Cannot open '%s' (mode '%s'): %s\n",
18                path, mode, strerror(err));
19        
20        return NULL;
21    }
22    
23    return fp;
24}
25
26// Safe string to number conversion
27long safe_strtol(const char *str, int *error) {
28    char *endptr;
29    errno = 0;  // Clear errno
30    
31    long val = strtol(str, &endptr, 10);
32    
33    // Check for errors
34    if (errno == ERANGE) {
35        *error = 1;
36        fprintf(stderr, "Error: Number out of range: %s\n", str);
37        return 0;
38    }
39    
40    if (endptr == str) {
41        *error = 2;
42        fprintf(stderr, "Error: No digits found in: %s\n", str);
43        return 0;
44    }
45    
46    if (*endptr != '\0') {
47        *error = 3;
48        fprintf(stderr, "Warning: Extra characters after number: %s\n", endptr);
49    }
50    
51    *error = 0;
52    return val;
53}
54
55int main() {
56    // Test safe_fopen
57    FILE *fp = safe_fopen("test.txt", "r");
58    if (fp) {
59        printf("File opened successfully\n");
60        fclose(fp);
61    }
62    
63    // Test safe_strtol
64    int error;
65    const char *tests[] = {"42", "9999999999999999999", "abc", "123abc"};
66    
67    printf("\n--- String to Long Conversion ---\n");
68    for (int i = 0; i < 4; i++) {
69        long val = safe_strtol(tests[i], &error);
70        if (error == 0) {
71            printf("'%s' -> %ld\n", tests[i], val);
72        }
73    }
74    
75    return 0;
76}
77
78/* Output:
79Error: Cannot open 'test.txt' (mode 'r'): No such file or directory
80
81--- String to Long Conversion ---
82'42' -> 42
83Error: Number out of range: 9999999999999999999
84Error: No digits found in: abc
85Warning: Extra characters after number: abc
86'123abc' -> 123
87*/

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code rarely checks errno properly, leading to silent failures that are hard to debug.

If you search online to "read a file and parse numbers," it will likely use fopen(), strtol(), and other functions without checking errno. When these fail (file not found, invalid number, overflow), the copied code continues with garbage values instead of handling the error.

The Trap: Online sources produce "happy path" code that assumes everything succeeds. They often forget to: (1) set errno to 0 before the call, (2) check the return value, (3) check errno immediately after, and (4) use perror() or strerror() for meaningful error messages.

The Reality: Robust C code must check for errors after every operation that can fail. Always follow the pattern: set errno = 0, call the function, check return value and errno, handle errors appropriately. This is tedious but essential — beginners skip it, you shouldn't.

08Frequently Asked Questions

Q:Why should I check errno immediately after a function call?

A: Because errno can be overwritten by the next function call, even if it succeeds! Save it to a local variable immediately: int err = errno;

Q:What's the difference between perror() and strerror()?

A: perror() prints directly to stderr with your prefix. strerror() returns a string you can format yourself, store, or log. Use perror for quick debugging, strerror for custom error handling.

Q:Why should I set errno = 0 before calling a function?

A: Some functions don't modify errno on success. If errno already had a non-zero value from an earlier error, you might mistake it for a new error. Clear it first to be safe.

Q:Is errno thread-safe?

A: Yes, in modern C (C11+) and POSIX systems! Each thread has its own errno. However,strerror() may not be thread-safe — usestrerror_r() in multi-threaded code.

08Quick Reference

Error Checking Pattern

main.c

errno = 0;           // Clear
result = function(); // Call
if (result == ERROR) {
    int err = errno;  // Save
    handle_error(err);
}

Functions

perror(msg) - Print to stderr
strerror(errno) - Get error string

← Previous

stddef.h Library

?Why Use errno.h?

C doesn't have exceptions like C++ or Java. When something fails, functions return -1 or NULL, but why did it fail? errno tells you!

•

File Errors

ENOENT, EACCES

•

Memory Errors

ENOMEM

Math Errors

ERANGE, EDOM

01Introduction to errno.h

What is errno.h?

<errno.h> provides the error reporting mechanismfor C programs. When library functions encounter errors, they set the errno variable to indicate what went wrong.

Key Components

Name	Type	Description
errno	int (macro)	Error number set by system calls
perror()	Function	Print error message to stderr
strerror()	Function	Get error message string

02The errno Variable

errno is a global variable (actually a macro that expands to a modifiable lvalue) that holds the error code from the last system call or library function that failed.

Important Rules:

Always set errno = 0 before calling a function
Check errno only if the function indicates failure
errno is never set to 0 by any standard library function
errno may be changed by successful function calls

errno Usage

1#include <stdio.h>
2#include <errno.h>
3#include <math.h>
4#include <string.h>
5
6int main() {
7    // Example 1: File open error
8    errno = 0;  // Clear errno before call
9    FILE *fp = fopen("nonexistent_file.txt", "r");
10    
11    if (fp == NULL) {
12        printf("Error opening file!\n");
13        printf("errno value: %d\n", errno);
14        printf("Error message: %s\n", strerror(errno));
15    }
16    
17    // Example 2: Math error
18    errno = 0;
19    double result = sqrt(-1.0);  // Invalid: sqrt of negative
20    
21    if (errno != 0) {
22        printf("\nMath error occurred!\n");
23        printf("errno value: %d\n", errno);
24        printf("Error message: %s\n", strerror(errno));
25        printf("Result: %f\n", result);  // NaN
26    }
27    
28    // Example 3: Number too large
29    errno = 0;
30    result = exp(1000);  // Overflow
31    
32    if (errno == ERANGE) {
33        printf("\nRange error: number too large!\n");
34        printf("Error message: %s\n", strerror(errno));
35    }
36    
37    return 0;
38}
39
40/* Output:
41Error opening file!
42errno value: 2
43Error message: No such file or directory
44
45Math error occurred!
46errno value: 33
47Error message: Numerical argument out of domain
48Result: nan
49
50Range error: number too large!
51Error message: Numerical result out of range
52*/

03Common Error Codes

Macro	Value	Description
EDOM	33	Math argument out of domain (e.g., sqrt(-1))
ERANGE	34	Result out of range (overflow/underflow)
EILSEQ	84	Illegal byte sequence
ENOENT	2	No such file or directory
EACCES	13	Permission denied
ENOMEM	12	Out of memory
EEXIST	17	File already exists
EINVAL	22	Invalid argument
ENOSPC	28	No space left on device
EBADF	9	Bad file descriptor

Note:

Error codes and their numeric values may vary between platforms. Always use the macro names (ENOENT, EACCES, etc.) instead of numeric values for portability.

04perror() - Print Error Message

void perror(const char *s);

perror() prints a custom message followed by a colon and the error message corresponding to the current value of errno. Output goes to stderr.

perror() Usage

1#include <stdio.h>
2#include <errno.h>
3
4int main() {
5    // Try to open a non-existent file
6    FILE *fp = fopen("/nonexistent/path/file.txt", "r");
7    
8    if (fp == NULL) {
9        // perror prints to stderr
10        perror("Error opening file");
11        
12        // Equivalent manual approach:
13        // fprintf(stderr, "Error opening file: %s\n", strerror(errno));
14    }
15    
16    // Try to open a protected file (if on Unix-like system)
17    fp = fopen("/etc/shadow", "r");
18    
19    if (fp == NULL) {
20        perror("Cannot read shadow file");
21    }
22    
23    // Empty string still prints error
24    errno = ENOMEM;
25    perror("");  // Just prints the error message
26    
27    // NULL prints just the error
28    errno = EINVAL;
29    perror(NULL);
30    
31    return 0;
32}
33
34/* Output:
35Error opening file: No such file or directory
36Cannot read shadow file: Permission denied
37: Cannot allocate memory
38Invalid argument
39*/

05strerror() - Get Error String

char *strerror(int errnum);

strerror() returns a pointer to a string describing the error code passed as argument. More flexible than perror() as you can use the string however you want.

strerror() Usage

1#include <stdio.h>
2#include <string.h>
3#include <errno.h>
4
5int main() {
6    // Get error message for specific error codes
7    printf("ENOENT (%d): %s\n", ENOENT, strerror(ENOENT));
8    printf("EACCES (%d): %s\n", EACCES, strerror(EACCES));
9    printf("ENOMEM (%d): %s\n", ENOMEM, strerror(ENOMEM));
10    printf("EINVAL (%d): %s\n", EINVAL, strerror(EINVAL));
11    printf("ERANGE (%d): %s\n", ERANGE, strerror(ERANGE));
12    printf("EDOM   (%d): %s\n", EDOM, strerror(EDOM));
13    
14    // Use with errno after an error
15    FILE *fp = fopen("nonexistent.txt", "r");
16    if (fp == NULL) {
17        // Store errno immediately - it might change
18        int saved_errno = errno;
19        
20        // Now use it
21        printf("\nFile open failed: %s (errno=%d)\n", 
22               strerror(saved_errno), saved_errno);
23    }
24    
25    // Custom error logging function
26    printf("\n--- Custom Error Log ---\n");
27    errno = 0;
28    fp = fopen("/root/secret.txt", "r");
29    if (fp == NULL) {
30        fprintf(stderr, "[ERROR] %s:%d: Failed to open file: %s\n",
31                __FILE__, __LINE__, strerror(errno));
32    }
33    
34    return 0;
35}
36
37/* Output:
38ENOENT (2): No such file or directory
39EACCES (13): Permission denied
40ENOMEM (12): Cannot allocate memory
41EINVAL (22): Invalid argument
42ERANGE (34): Numerical result out of range
43EDOM   (33): Numerical argument out of domain
44
45File open failed: No such file or directory (errno=2)
46
47--- Custom Error Log ---
48[ERROR] example.c:35: Failed to open file: Permission denied
49*/

Thread Safety:

strerror() may return a pointer to a static buffer, making it not thread-safe. Use strerror_r() (POSIX) or strerror_s() (C11) in multi-threaded programs.

06Error Handling Best Practices

Error Handling Best Practices

1#include <stdio.h>
2#include <stdlib.h>
3#include <string.h>
4#include <errno.h>
5
6// Good error handling pattern
7FILE *safe_fopen(const char *path, const char *mode) {
8    errno = 0;  // Clear errno before call
9    
10    FILE *fp = fopen(path, mode);
11    
12    if (fp == NULL) {
13        // Save errno immediately
14        int err = errno;
15        
16        // Log detailed error
17        fprintf(stderr, "Error: Cannot open '%s' (mode '%s'): %s\n",
18                path, mode, strerror(err));
19        
20        return NULL;
21    }
22    
23    return fp;
24}
25
26// Safe string to number conversion
27long safe_strtol(const char *str, int *error) {
28    char *endptr;
29    errno = 0;  // Clear errno
30    
31    long val = strtol(str, &endptr, 10);
32    
33    // Check for errors
34    if (errno == ERANGE) {
35        *error = 1;
36        fprintf(stderr, "Error: Number out of range: %s\n", str);
37        return 0;
38    }
39    
40    if (endptr == str) {
41        *error = 2;
42        fprintf(stderr, "Error: No digits found in: %s\n", str);
43        return 0;
44    }
45    
46    if (*endptr != '\0') {
47        *error = 3;
48        fprintf(stderr, "Warning: Extra characters after number: %s\n", endptr);
49    }
50    
51    *error = 0;
52    return val;
53}
54
55int main() {
56    // Test safe_fopen
57    FILE *fp = safe_fopen("test.txt", "r");
58    if (fp) {
59        printf("File opened successfully\n");
60        fclose(fp);
61    }
62    
63    // Test safe_strtol
64    int error;
65    const char *tests[] = {"42", "9999999999999999999", "abc", "123abc"};
66    
67    printf("\n--- String to Long Conversion ---\n");
68    for (int i = 0; i < 4; i++) {
69        long val = safe_strtol(tests[i], &error);
70        if (error == 0) {
71            printf("'%s' -> %ld\n", tests[i], val);
72        }
73    }
74    
75    return 0;
76}
77
78/* Output:
79Error: Cannot open 'test.txt' (mode 'r'): No such file or directory
80
81--- String to Long Conversion ---
82'42' -> 42
83Error: Number out of range: 9999999999999999999
84Error: No digits found in: abc
85Warning: Extra characters after number: abc
86'123abc' -> 123
87*/

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code rarely checks errno properly, leading to silent failures that are hard to debug.

08Frequently Asked Questions

Q:Why should I check errno immediately after a function call?

A: Because errno can be overwritten by the next function call, even if it succeeds! Save it to a local variable immediately: int err = errno;

Q:What's the difference between perror() and strerror()?

A: perror() prints directly to stderr with your prefix. strerror() returns a string you can format yourself, store, or log. Use perror for quick debugging, strerror for custom error handling.

Q:Why should I set errno = 0 before calling a function?

A: Some functions don't modify errno on success. If errno already had a non-zero value from an earlier error, you might mistake it for a new error. Clear it first to be safe.

Q:Is errno thread-safe?

A: Yes, in modern C (C11+) and POSIX systems! Each thread has its own errno. However,strerror() may not be thread-safe — usestrerror_r() in multi-threaded code.

08Quick Reference

Error Checking Pattern

main.c

errno = 0;           // Clear
result = function(); // Call
if (result == ERROR) {
    int err = errno;  // Save
    handle_error(err);
}

Functions

perror(msg) - Print to stderr
strerror(errno) - Get error string

← Previous

What You Will Learn

?Why Use errno.h?

01Introduction to errno.h

What is errno.h?

Key Components

02The errno Variable

03Common Error Codes

04perror() - Print Error Message

05strerror() - Get Error String

06Error Handling Best Practices

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code rarely checks errno properly, leading to silent failures that are hard to debug.

08Frequently Asked Questions

Q:Why should I check errno immediately after a function call?

Q:What's the difference between perror() and strerror()?

Q:Why should I set errno = 0 before calling a function?

Q:Is errno thread-safe?

08Quick Reference

Error Checking Pattern

Functions

stddef.h Library

assert.h Library

Test Your Knowledge

Related Tutorials

C stddef.h Library Reference

C assert.h Library Reference

C stdint.h Library Reference

Have Feedback?

What You Will Learn

?Why Use errno.h?

01Introduction to errno.h

What is errno.h?

Key Components

02The errno Variable

03Common Error Codes

04perror() - Print Error Message

05strerror() - Get Error String

06Error Handling Best Practices

!Code Pitfalls: Common Mistakes & What to Watch For

Copied code rarely checks errno properly, leading to silent failures that are hard to debug.

08Frequently Asked Questions

Q:Why should I check errno immediately after a function call?

Q:What's the difference between perror() and strerror()?

Q:Why should I set errno = 0 before calling a function?

Q:Is errno thread-safe?

08Quick Reference

Error Checking Pattern

Functions

stddef.h Library

assert.h Library

Test Your Knowledge

Related Tutorials

C stddef.h Library Reference

C assert.h Library Reference

C stdint.h Library Reference

Have Feedback?