errno

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

#include <stdio.h>
#include <math.h>
#include <errno.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;
    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();
}

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

References