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errno

From cppreference.com
< c‎ | error
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.

[edit] 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();
}

Possible output:

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

[edit] 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>

[edit] See also

macros for standard POSIX-compatible error conditions
(macro constant) [edit]
displays a character string corresponding of the current error to stderr
(function) [edit]
returns a text version of a given error code
(function) [edit]
defines the error handling mechanism used by the common mathematical functions
(macro constant) [edit]