INTRO(2) | System Calls Manual | INTRO(2)

# NAME

`intro`, `errno` — introduction to system calls and error numbers

# SYNOPSIS

`#include <errno.h>`

# DESCRIPTION

The manual pages in section 2 provide an overview of the system calls, their error returns, and other common definitions and concepts.

Programs may be restricted to a subset of system calls with pledge(2).

# DIAGNOSTICS

Nearly all of the system calls provide an error number via the identifier errno, which expands to an addressable location of type int. The address of errno in each thread is guaranteed to be unique for the lifetime of the thread. Applications must use errno as defined in `<errno.h>` and not attempt to use a custom definition.

When a system call detects an error, it returns an integer value indicating failure (usually -1) and sets the variable errno accordingly. (This allows interpretation of the failure on receiving a -1 and to take action accordingly.) Successful calls never set errno; once set, it remains until another error occurs. It should only be examined after an error. Note that a number of system calls overload the meanings of these error numbers, and that the meanings must be interpreted according to the type and circumstances of the call.

The following is a complete list of the errors and their names as given in `<sys/errno.h>`.

`0` Undefined error: 0.
  Not used.
`1 EPERM` Operation not permitted.
  An attempt was made to perform an operation limited to processes with appropriate privileges or to the owner of a file or other resources.
`2 ENOENT` No such file or directory.
  A component of a specified pathname did not exist, or the pathname was an empty string.
`3 ESRCH` No such process.
  No process could be found which corresponds to the given process ID.
`4 EINTR` Interrupted system call.
  An asynchronous signal (such as `SIGINT` or `SIGQUIT`) was caught by the thread during the execution of an interruptible function. If the signal handler performs a normal return, the interrupted function call will seem to have returned the error condition.
`5 EIO` Input/output error.
  Some physical input or output error occurred. This error will not be reported until a subsequent operation on the same file descriptor and may be lost (overwritten) by any subsequent errors.
`6 ENXIO` Device not configured.
  Input or output on a special file referred to a device that did not exist, or made a request beyond the limits of the device. This error may also occur when, for example, a tape drive is not online or no disk pack is loaded on a drive.
`7 E2BIG` Argument list too long.
  The number of bytes used for the argument and environment list of the new process exceeded the limit `ARG_MAX`.
`8 ENOEXEC` Exec format error.
  A request was made to execute a file that, although it has the appropriate permissions, was not in the format required for an executable file.
`9 EBADF` Bad file descriptor.
  A file descriptor argument was out of range, referred to no open file, or a read (write) request was made to a file that was only open for writing (reading).
`10 ECHILD` No child processes.
  A wait(2), waitid(2), or waitpid(2) function was executed by a process that had no existing or unwaited-for child processes.
`11 EDEADLK` Resource deadlock avoided.
  An attempt was made to lock a system resource that would have resulted in a deadlock situation.
`12 ENOMEM` Cannot allocate memory.
  The new process image required more memory than was allowed by the hardware or by system-imposed memory management constraints. A lack of swap space is normally temporary; however, a lack of core is not. Soft limits may be increased to their corresponding hard limits.
`13 EACCES` Permission denied.
  An attempt was made to access a file in a way forbidden by its file access permissions.
`14 EFAULT` Bad address.
  The system detected an invalid address in attempting to use an argument of a call.
`15 ENOTBLK` Block device required.
  A block device operation was attempted on a non-block device or file.
`16 EBUSY` Device busy.
  An attempt to use a system resource which was in use at the time in a manner which would have conflicted with the request.
`17 EEXIST` File exists.
  An existing file was mentioned in an inappropriate context, for instance, as the new link name in a link(2) function.
`18 EXDEV` Cross-device link.
  A hard link to a file on another file system was attempted.
`19 ENODEV` Operation not supported by device.
  An attempt was made to apply an inappropriate function to a device, for example, trying to read a write-only device such as a printer.
`20 ENOTDIR` Not a directory.
  A file descriptor or a component of the specified pathname existed, but it was not a directory, when a directory was expected.
`21 EISDIR` Is a directory.
  An attempt was made to open a directory with write mode specified.
`22 EINVAL` Invalid argument.
  Some invalid argument was supplied. (For example, specifying an undefined signal to a signal(3) or kill(2) function).
`23 ENFILE` Too many open files in system.
  Maximum number of file descriptors allowable on the system has been reached and a request for an open cannot be satisfied until at least one has been closed. The sysctl(2) variable kern.maxfiles contains the current limit.
`24 EMFILE` Too many open files.
  The maximum number of file descriptors allowable for this process has been reached and a request for an open cannot be satisfied until at least one has been closed. getdtablesize(3) will obtain the current limit.
`25 ENOTTY` Inappropriate ioctl for device.
  A control function (see ioctl(2)) was attempted for a file or special device for which the operation was inappropriate.
`26 ETXTBSY` Text file busy.
  An attempt was made either to execute a pure procedure (shared text) file which was open for writing by another process, or to open with write access a pure procedure file that is currently being executed.
`27 EFBIG` File too large.
  The size of a file exceeded the maximum. (The system-wide maximum file size is 2**63 bytes. Each file system may impose a lower limit for files contained within it.)
`28 ENOSPC` No space left on device.
  A write(2) to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because no more disk blocks were available on the file system, or the allocation of an inode for a newly created file failed because no more inodes were available on the file system.
`29 ESPIPE` Illegal seek.
  An lseek(2) function was issued on a socket, pipe or FIFO.
`30 EROFS` Read-only file system.
  An attempt was made to modify a file or create a directory on a file system that was read-only at the time.
`31 EMLINK` Too many links.
  The maximum allowable number of hard links to a single file has been exceeded (see pathconf(2) for how to obtain this value).
`32 EPIPE` Broken pipe.
  A write on a pipe, socket or FIFO for which there is no process to read the data.
`33 EDOM` Numerical argument out of domain.
  A numerical input argument was outside the defined domain of the mathematical function.
`34 ERANGE` Result too large.
  A result of the function was too large to fit in the available space (perhaps exceeded precision).
`35 EAGAIN` = `EWOULDBLOCK` Resource temporarily unavailable.
  This is a temporary condition and later calls to the same routine may complete normally.
`36 EINPROGRESS` Operation now in progress.
  An operation that takes a long time to complete (such as a connect(2)) was attempted on a non-blocking object (see fcntl(2)).
`37 EALREADY` Operation already in progress.
  An operation was attempted on a non-blocking object that already had an operation in progress.
`38 ENOTSOCK` Socket operation on non-socket.
  Self-explanatory.
`39 EDESTADDRREQ` Destination address required.
  A required address was omitted from an operation on a socket.
`40 EMSGSIZE` Message too long.
  A message sent on a socket was larger than the internal message buffer or some other network limit.
`41 EPROTOTYPE` Protocol wrong type for socket.
  A protocol was specified that does not support the semantics of the socket type requested. For example, you cannot use the Internet UDP protocol with type `SOCK_STREAM`.
`42 ENOPROTOOPT` Protocol not available.
  A bad option or level was specified in a getsockopt(2) or setsockopt(2) call.
`43 EPROTONOSUPPORT` Protocol not supported.
  The protocol has not been configured into the system or no implementation for it exists.
`44 ESOCKTNOSUPPORT` Socket type not supported.
  The support for the socket type has not been configured into the system or no implementation for it exists.
`45 EOPNOTSUPP` Operation not supported.
  The attempted operation is not supported for the type of object referenced. Usually this occurs when a file descriptor refers to a file or socket that cannot support this operation, for example, trying to accept a connection on a datagram socket.
`46 EPFNOSUPPORT` Protocol family not supported.
  The protocol family has not been configured into the system or no implementation for it exists.
`47 EAFNOSUPPORT` Address family not supported by protocol family.
  An address incompatible with the requested protocol was used. For example, you shouldn't necessarily expect to be able to use NS addresses with Internet protocols.
`48 EADDRINUSE` Address already in use.
  Only one usage of each address is normally permitted.
`49 EADDRNOTAVAIL` Can't assign requested address.
  Normally results from an attempt to create a socket with an address not on this machine.
`50 ENETDOWN` Network is down.
  A socket operation encountered a dead network.
`51 ENETUNREACH` Network is unreachable.
  A socket operation was attempted to an unreachable network.
`52 ENETRESET` Network dropped connection on reset.
  The host you were connected to crashed and rebooted.
`53 ECONNABORTED` Software caused connection abort.
  A connection abort was caused internal to your host machine.
`54 ECONNRESET` Connection reset by peer.
  A connection was forcibly closed by a peer. This normally results from a loss of the connection on the remote socket due to a timeout or a reboot.
`55 ENOBUFS` No buffer space available.
  An operation on a socket or pipe was not performed because the system lacked sufficient buffer space or because a queue was full.
`56 EISCONN` Socket is already connected.
  A connect(2) request was made on an already connected socket; or, a sendto(2) or sendmsg(2) request on a connected socket specified a destination when already connected.
`57 ENOTCONN` Socket is not connected.
  A request to send or receive data was disallowed because the socket was not connected and (when sending on a datagram socket) no address was supplied.
`58 ESHUTDOWN` Can't send after socket shutdown.
  A request to send data was disallowed because the socket had already been shut down with a previous shutdown(2) call.
`59 ETOOMANYREFS` Too many references: can't splice.
  Not used in OpenBSD.
`60 ETIMEDOUT` Operation timed out.
  A connect(2) or send(2) request failed because the connected party did not properly respond after a period of time. (The timeout period is dependent on the communication protocol.)
`61 ECONNREFUSED` Connection refused.
  No connection could be made because the target machine actively refused it. This usually results from trying to connect to a service that is inactive on the foreign host.
`62 ELOOP` Too many levels of symbolic links.
  A pathname lookup involved more than 32 (`SYMLOOP_MAX`) symbolic links.
`63 ENAMETOOLONG` File name too long.
  A component of a pathname exceeded 255 (`NAME_MAX`) characters, or an entire pathname (including the terminating NUL) exceeded 1024 (`PATH_MAX`) bytes.
`64 EHOSTDOWN` Host is down.
  A socket operation failed because the destination host was down.
`65 EHOSTUNREACH` No route to host.
  A socket operation was attempted to an unreachable host.
`66 ENOTEMPTY` Directory not empty.
  A directory with entries other than ‘`.`’ and ‘`..`’ was supplied to a remove directory or rename call.
`67 EPROCLIM` Too many processes.
  Not used in OpenBSD.
`68 EUSERS` Too many users.
  The quota system ran out of table entries.
`69 EDQUOT` Disk quota exceeded.
  A write(2) to an ordinary file, the creation of a directory or symbolic link, or the creation of a directory entry failed because the user's quota of disk blocks was exhausted, or the allocation of an inode for a newly created file failed because the user's quota of inodes was exhausted.
`70 ESTALE` Stale NFS file handle.
  An attempt was made to access an open file on an NFS filesystem which is now unavailable as referenced by the file descriptor. This may indicate the file was deleted on the NFS server or some other catastrophic event occurred.
`71 EREMOTE` Too many levels of remote in path.
  Not used in OpenBSD.
`72 EBADRPC` RPC struct is bad.
  Exchange of rpc(3) information was unsuccessful.
`73 ERPCMISMATCH` RPC version wrong.
  The version of rpc(3) on the remote peer is not compatible with the local version.
`74 EPROGUNAVAIL` RPC program not available.
  The requested rpc(3) program is not registered on the remote host.
`75 EPROGMISMATCH` Program version wrong.
  The requested version of the rpc(3) program is not available on the remote host.
`76 EPROCUNAVAIL` Bad procedure for program.
  An rpc(3) call was attempted for a procedure which doesn't exist in the remote program.
`77 ENOLCK` No locks available.
  A system-imposed limit on the number of simultaneous file locks was reached.
`78 ENOSYS` Function not implemented.
  Attempted a system call that is not available on this system.
`79 EFTYPE` Inappropriate file type or format.
  The file contains invalid data or set to invalid modes.
`80 EAUTH` Authentication error.
  Attempted to use an invalid authentication ticket to mount a NFS filesystem.
`81 ENEEDAUTH` Need authenticator.
  An authentication ticket must be obtained before the given NFS filesystem may be mounted.
`82 EIPSEC` IPsec processing failure.
  IPsec subsystem error. Not used in OpenBSD.
`83 ENOATTR` Attribute not found.
  A UFS Extended Attribute is not found for the specified pathname.
`84 EILSEQ` Illegal byte sequence.
  An illegal sequence of bytes was used when using wide characters.
`85 ENOMEDIUM` No medium found.
  Attempted to use a removable media device with no medium present.
`86 EMEDIUMTYPE` Wrong medium type.
  Attempted to use a removable media device with incorrect or incompatible medium.
`87 EOVERFLOW` Value too large to be stored in data type.
  A numerical result of the function was too large to be stored in the caller provided space.
`88 ECANCELED` Operation canceled.
  The requested operation was canceled.
`89 EIDRM` Identifier removed.
  An IPC identifier was removed while the current thread was waiting on it.
`90 ENOMSG` No message of desired type.
  An IPC message queue does not contain a message of the desired type, or a message catalog does not contain the requested message.
`91 ENOTSUP` Not supported.
  The operation has requested an unsupported value.
`92 EBADMSG` Bad message.
  A corrupted message was detected.
`93 ENOTRECOVERABLE` State not recoverable.
  The state protected by a robust mutex is not recoverable.
`94 EOWNERDEAD` Previous owner died.
  The owner of a robust mutex terminated while holding the mutex lock.
`95 EPROTO` Protocol error.
  A device-specific protocol error occurred.

# DEFINITIONS

Process
  A process is a collection of one or more threads, plus the resources shared by those threads such as process ID, address space, user IDs and group IDs, and root directory and current working directory.
Process ID
  Each active process in the system is uniquely identified by a non-negative integer called a process ID. The range of this ID is from 0 to 99999.
Parent Process ID
  A new process is created by a currently active process; (see fork(2)). The parent process ID of a process is initially the process ID of its creator. If the creating process exits, the parent process ID of each child is set to the ID of a system process, init(8).
Process Group
  Each active process is a member of a process group that is identified by a non-negative integer called the process group ID. This is the process ID of the group leader. This grouping permits the signaling of related processes (see termios(4)) and the job control mechanisms of ksh(1) and csh(1).
Session
  A session is a set of one or more process groups. A session is created by a successful call to setsid(2), which causes the caller to become the only member of the only process group in the new session.
Session Leader
  A process that has created a new session by a successful call to setsid(2), is known as a session leader. Only a session leader may acquire a terminal as its controlling terminal (see termios(4)).
Controlling Process
  A session leader with a controlling terminal is a controlling process.
Controlling Terminal
  A terminal that is associated with a session is known as the controlling terminal for that session and its members.
Terminal Process Group ID
  A terminal may be acquired by a session leader as its controlling terminal. Once a terminal is associated with a session, any of the process groups within the session may be placed into the foreground by setting the terminal process group ID to the ID of the process group. This facility is used to arbitrate between multiple jobs contending for the same terminal; (see ksh(1), csh(1), and tty(4)).
Orphaned Process Group
  A process group is considered to be orphaned if it is not under the control of a job control shell. More precisely, a process group is orphaned when none of its members has a parent process that is in the same session as the group, but is in a different process group. Note that when a process exits, the parent process for its children is changed to be init(8), which is in a separate session. Not all members of an orphaned process group are necessarily orphaned processes (those whose creating process has exited). The process group of a session leader is orphaned by definition.
Thread
  A thread is a preemptively scheduled flow of control within a process, with its own set of register values, floating point environment, thread ID, signal mask, pending signal set, alternate signal stack, thread control block address, resource utilization, errno variable location, and values for thread-specific keys. A process initially has just one thread, a duplicate of the thread in the parent process that created this process.
Real User ID and Real Group ID
  Each user on the system is identified by a positive integer termed the real user ID.

  Each user is also a member of one or more groups. One of these groups is distinguished from others and used in implementing accounting facilities. The positive integer corresponding to this distinguished group is termed the real group ID.

  All processes have a real user ID and real group ID. These are initialized from the equivalent attributes of the process that created it.

Effective User ID, Effective Group ID, and Group Access List
  Access to system resources is governed by two values: the effective user ID, and the group access list. The first member of the group access list is also known as the effective group ID. (In POSIX.1, the group access list is known as the set of supplementary group IDs, and it is unspecified whether the effective group ID is a member of the list.)

  The effective user ID and effective group ID are initially the process's real user ID and real group ID respectively. Either may be modified through execution of a set-user-ID or set-group-ID file (possibly by one of its ancestors) (see execve(2)). By convention, the effective group ID (the first member of the group access list) is duplicated, so that the execution of a set-group-ID program does not result in the loss of the original (real) group ID.

  The group access list is a set of group IDs used only in determining resource accessibility. Access checks are performed as described below in “File Access Permissions”.

Saved Set User ID and Saved Set Group ID
  When a process executes a new file, the effective user ID is set to the owner of the file if the file is set-user-ID, and the effective group ID (first element of the group access list) is set to the group of the file if the file is set-group-ID. The effective user ID of the process is then recorded as the saved set-user-ID, and the effective group ID of the process is recorded as the saved set-group-ID. These values may be used to regain those values as the effective user or group ID after reverting to the real ID (see setuid(2)). (In POSIX.1, the saved set-user-ID and saved set-group-ID are optional, and are used in setuid and setgid, but this does not work as desired for the superuser.)
Superuser
  A process is recognized as a superuser process and is granted special privileges if its effective user ID is 0.
Special Processes
  The processes with process IDs of 0 and 1 are special. Process 0 is the scheduler. Process 1 is the initialization process init(8), and is the ancestor of every other process in the system. It is used to control the process structure.
Descriptor
  An integer assigned by the system when a file is referenced by open(2) or dup(2), or when a socket is created by pipe(2), socket(2) or socketpair(2), which uniquely identifies an access path to that file or socket from a given process or any of its children.
File Name
  Names consisting of up to 255 (`NAME_MAX`) characters may be used to name an ordinary file, special file, or directory.

  These characters may be arbitrary eight-bit values, excluding 0 (NUL) and the ASCII code for ‘`/`’ (slash).

  Note that it is generally unwise to use ‘`*`’, ‘`?`’, ‘`[`’ or ‘`]`’ as part of file names because of the special meaning attached to these characters by the shell.

  Note also that `NAME_MAX` is an upper limit fixed by the kernel, meant to be used for sizing buffers. Some filesystems may have additional restrictions. These can be queried using pathconf(2) and fpathconf(2).

Pathname
  A pathname is a NUL-terminated character string starting with an optional slash ‘`/`’, followed by zero or more directory names separated by slashes, optionally followed by a file name. The total length of a pathname must be less than 1024 (`PATH_MAX`) characters. Additional restrictions may apply, depending upon the filesystem, to be queried with pathconf(2) or fpathconf(2) if needed.

  If a pathname begins with a slash, the path search begins at the root directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. An empty pathname is invalid.

Directory
  A directory is a special type of file that contains entries that are references to other files. Directory entries are called links. By convention, a directory contains at least two links, ‘`.`’ and ‘`..`’, referred to as dot and dot-dot respectively. Dot refers to the directory itself and dot-dot refers to its parent directory.
Root Directory and Current Working Directory
  Each process has associated with it a concept of a root directory and a current working directory for the purpose of resolving path name searches. A process's root directory need not be the root directory of the root file system.
File Access Permissions
  Every file in the file system has a set of access permissions. These permissions are used in determining whether a process may perform a requested operation on the file (such as opening a file for writing). Access permissions are established at the time a file is created. They may be changed at some later time through the chmod(2) call.

  File access is broken down according to whether a file may be: read, written, or executed. Directory files use the execute permission to control if the directory may be searched.

  File access permissions are interpreted by the system as they apply to three different classes of users: the owner of the file, those users in the file's group, anyone else. Every file has an independent set of access permissions for each of these classes. When an access check is made, the system decides if permission should be granted by checking the access information applicable to the caller.

  Read, write, and execute/search permissions on a file are granted to a process if:

  The process's effective user ID is that of the superuser. (Note: even the superuser cannot execute a non-executable file.)

  The process's effective user ID matches the user ID of the owner of the file and the owner permissions allow the access.

  The process's effective user ID does not match the user ID of the owner of the file, and either the process's effective group ID matches the group ID of the file, or the group ID of the file is in the process's group access list, and the group permissions allow the access.

  Neither the effective user ID nor effective group ID and group access list of the process match the corresponding user ID and group ID of the file, but the permissions for “other users” allow access.

  Otherwise, permission is denied.

Sockets and Address Families
  A socket is an endpoint for communication between processes. Each socket has queues for sending and receiving data.

  Sockets are typed according to their communications properties. These properties include whether messages sent and received at a socket require the name of the partner, whether communication is reliable, the format used in naming message recipients, etc.

  Each instance of the system supports some collection of socket types; consult socket(2) for more information about the types available and their properties.

  Each instance of the system supports some number of sets of communications protocols. Each protocol set supports addresses of a certain format. An Address Family is the set of addresses for a specific group of protocols. Each socket has an address chosen from the address family in which the socket was created.

# SEE ALSO

pledge(2), intro(3), perror(3)

# STANDARDS

Most of the macro names that resolve to error numbers are required to be defined by. In some cases, the standard requires certain kinds of failures to be reported with certain error numbers. It some cases, it allows certain kinds of failures to optionally be reported with certain error numbers. In some cases, it is entirely silent about whether and how the system should report problems.

The following defined constants are extensions to the standard: `EAUTH`, `EBADRPC`, `EFTYPE`, `EHOSTDOWN`, `EIPSEC`, `EMEDIUMTYPE`, `ENEEDAUTH`, `ENOATTR`, `ENOMEDIUM`, `ENOTBLK`, `EPFNOSUPPORT`, `EPROCLIM`, `EPROCUNAVAIL`, `EPROGMISMATCH`, `EPROGUNAVAIL`, `EREMOTE`, `ERPCMISMATCH`, `ESHUTDOWN`, `ETOOMANYREFS`, and `EUSERS`.

# HISTORY

An `intro` manual for section 2 first appeared in Version 5 AT&T UNIX.

# BUGS

Since some error numbers are only loosely defined, and since which error numbers any given function can set often varies from system to system, programs usually need to make a fallback decision when receiving an unexpected error number, typically treating it as a hard failure, or less commonly just ignoring it. Making portable decisions based on error numbers is only possible for some specific error numbers being set by some specific functions.

July 18, 2025 | Debian