errno - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Family C – Error Reporting

What it is – installed errno.h defines extern int *__errno_location (void); and #define errno (*__errno_location ()): a modifiable lvalue backed by thread-local storage, so each thread has its own error code. The manual documents errno as volatile int and notes ISO C allows exactly this macro expansion (manual, Checking for Errors); code that needs the address of the object takes &errno or calls __errno_location directly. E* constants are macros in the same header, all distinct except EAGAIN == EWOULDBLOCK.

When it is set – only on failure is meaningful: the initial value at startup is 0, and the manual warns that many functions may set errno to an unspecified nonzero value on success, so testing errno != 0 is never a correct error check; test the function’s return first (manual). Correct discipline for functions whose success value can equal the failure value – the manual names sqrt and atan; strtol is the classic case (strtol.3: 0, LONG_MAX, LONG_MIN are legitimate returns, ERANGE/EINVAL disambiguate) – is errno = 0; immediately before the call and an errno check immediately after, with nothing library-side in between.

Signal interaction – a handler should save and restore errno (manual). glibc’s own cancellation-safe wrappers follow that rule.