❌ <errno.h> — Error Number Definitions

Defines the global variable errno and symbolic error constants. Set by system calls and library functions on error. Use with perror() or strerror().

Include: #include <errno.h> | Compile: gcc -Wall -o prog prog.c

📋 Functions & Macros

Name	Prototype / Value	Description
errno	extern int errno	Global error number — set by failing system calls
EPERM	1	Operation not permitted
ENOENT	2	No such file or directory
EINTR	4	Interrupted system call
EIO	5	I/O error
EBADF	9	Bad file descriptor
ENOMEM	12	Out of memory
EACCES	13	Permission denied
EEXIST	17	File exists
EINVAL	22	Invalid argument
ERANGE	34	Result too large

💡 Example Program

Copy, save as demo.c, and compile with the command shown at the bottom.

#include <stdio.h>
#include <errno.h>
#include <string.h>

int main(void) {
    FILE *fp = fopen("/no/such/file.txt", "r");
    if (fp == NULL) {
        perror("fopen");
        printf("errno=%d : %s\n", errno, strerror(errno));
    }

    errno = 0;
    long val = strtol("99999999999999999", NULL, 10);
    if (errno == ERANGE)
        printf("Overflow! val=%ld errno=%d (%s)\n",
               val, errno, strerror(errno));
    return 0;
}
/* Compile: gcc -Wall -o errno_demo errno_demo.c */

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