Errno
errno is a C library macro identifying a modifiable integer error indicator associated with the current thread or execution context.
Modern C, Third Edition
errno is a C library-provided modifiable object or macro designating an integer error indicator associated with the current thread or execution context.
- How it is used
- Supports C library error detection, diagnosis, recovery, and cleanup around operations that may fail.
- How deep it goes
- Explains error-state use, resetting, preservation, and portability constraints, but defers detailed thread semantics and implementation internals.
- Examples
- Uses puts with EOF and perror, puts_safe with reset state, and fprintnumbers with saved errno and cleanup.
- What you can do after
- Diagnose library failures, preserve error state during cleanup, and reset errno after handled recovery.
Modern C
errno is a C standard library macro identifying the thread-local integer error indicator set by library functions.
- How it is used
- Supports C library error checking by recording specific failure conditions alongside function return values.
- How deep it goes
- Explains errno's role, perror's dependence on it, state resetting, and assignments in a strtoul implementation.
- Examples
- Uses puts with EOF, perror, and exit, then implements strtoul overflow and invalid-base handling.
- What you can do after
- Diagnose library failures with return-value checks, perror, and errno values such as ERANGE and EINVAL.
Rust in Action: Systems programming concepts and techniques
errno is a C and POSIX process-global error indicator that system calls may set to identify a failure condition.
- How it is used
- Uses a global error code to signal failures from system calls such as read() and close().
- How deep it goes
- Explains return-value checking, symbolic failure codes, and global-state drawbacks without developing POSIX internals.
- Examples
- Uses read() and close(), with EINTR and EIO illustrating interruption and input/output failure.
- What you can do after
- Diagnose system-call failures by checking a global error value after an operation returns.
Read how Rust in Action: Systems programming concepts and techniques explains errno →
Related concepts
- perrorused by · perror is a C standard library function that writes a diagnostic message describing the error indicated by errno to the standard error stream.
- closeused by · Close is an operation that terminates access to an associated resource or interface and releases or finalizes related resources or pending work.
- failurealternative to · A failure is an unsuccessful outcome or condition in which an operation, process, or system does not achieve an expected result.
- exceptionused by · An exception is a runtime signal or object representing an abnormal condition that interrupts ordinary program flow and may be handled or propagated.
- strtoulused by · strtoul is a C standard library function that converts the initial portion of a string to an unsigned long value using a specified or inferred integer base.
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