errno

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

#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();
}

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

References