errno

C

Language

Headers

Type support

Program utilities

Variadic function support

Error handling

Dynamic memory management

Date and time utilities

Strings library

Algorithms

Numerics

Input/output support

Localization support

Atomic operations (C11)

Thread support (C11)

Technical Specifications

Error handling

Error codes

Error codes

errno

Assertions

assert

static_assert (C11)

Bounds checking

set_constraint_handler_s (C11)

abort_handler_s (C11)

ignore_handler_s (C11)

Defined in header <errno.h>
#define errno /*implementation-defined*/

errno is a preprocessor macro that expands to a thread-local (since C11) modifiable lvalue of type int. Several standard library functions indicate errors by writing positive integers to errno. Typically, the value of errno is set to one of the error codes listed in <errno.h> as macro constants beginning with the letter E followed by uppercase letters or digits.

The value of errno is 0 at program startup, and although library functions are allowed to write positive integers to errno whether or not an error occurred, library functions never store 0 in errno.

Library functions perror and strerror can be used to obtain textual descriptions of the error conditions that correspond to the current errno value.

Example

Run this code

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

void show_errno(void)
{
    if(errno==EDOM)   printf("domain error");
    if(errno==EILSEQ) printf("illegal sequence");    
    if(errno==ERANGE) printf("pole or range error");
    if(errno==0)      printf("no error");
    printf(" occurred\n");
}

int main(void)
{
    printf("MATH_ERRNO is%s\n", math_errhandling & MATH_ERRNO ? "set" : "not set");

    errno = 0;
    1.0/0.0;
    show_errno();

    errno = 0;
    acos(+1.1);
    show_errno();

    errno = 0;
    log(0.0);
    show_errno();

    errno = 0;
    sin(0.0);
    show_errno();
}

Output:

MATH_ERRNO is set
pole or range error occurred
domain error occurred
pole or range error occurred
no error occurred

References

• C11 standard (ISO/IEC 9899:2011):

• 7.5 Errors <errno.h> (p: 205)

• K.3.1.3 Use of errno (p: 584)

• K.3.2 Errors <errno.h> (p: 585)

• C99 standard (ISO/IEC 9899:1999):

• 7.5 Errors <errno.h> (p: 186)

• C89/C90 standard (ISO/IEC 9899:1990):

• 4.1.3 Errors <errno.h>

See also

E2BIG, EACCES, ..., EXDEV	macros for standard POSIX-compatible error conditions (macro constant)
perror	displays a character string corresponding of the current error to stderr (function)
strerrorstrerror_sstrerrorlen_s (C11)(C11)	returns a text version of a given error code (function)
math_errhandlingMATH_ERRNOMATH_ERREXCEPT (C99)(C99)(C99)	defines the error handling mechanism used by the common mathematical functions (macro constant)