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

errno is a preprocessor macro (but see note below) 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.

Note: Until C11, the C standards had contradictory requirements, in that they said that errno is a macro but also that "it is unspecified whether errno is a macro or an identifier declared with external linkage". C11 fixes this, requiring that it be defined as a macro (see also WG14 N1338).

Example

Run this code

#include <errno.h>
#include <math.h>
#include <stdio.h>
 
void show_errno(void)
{
    const char *err_info = "unknown error";
    switch (errno)
    {
        case EDOM:
            err_info = "domain error";
            break;
        case EILSEQ:
            err_info = "illegal sequence";
            break;
        case ERANGE:
            err_info = "pole or range error";
            break;
        case 0:
            err_info = "no error";
    }
    fputs(err_info, stdout);
    puts(" occurred");
}
 
int main(void)
{
    fputs("MATH_ERRNO is ", stdout);
    puts(math_errhandling & MATH_ERRNO ? "set" : "not set");
 
    errno = 0;
    (void)(1.0 / 0.0);
    show_errno();
 
    errno = 0;
    (void)acos(+1.1);
    show_errno();
 
    errno = 0;
    (void)log(0.0);
    show_errno();
 
    errno = 0;
    (void)sin(0.0);
    show_errno();
}

Possible output:

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

References

• C23 standard (ISO/IEC 9899:2024):

• 7.5 Errors <errno.h> (p: TBD)

• K.3.1.3 Use of errno (p: TBD)

• K.3.2 Errors <errno.h> (p: TBD)

• C17 standard (ISO/IEC 9899:2018):

• 7.5 Errors <errno.h> (p: TBD)

• K.3.1.3 Use of errno (p: TBD)

• K.3.2 Errors <errno.h> (p: TBD)

• 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)
C++ documentation for errno