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INTRO(2) System Calls Manual INTRO(2)


introintroduction to system calls and error numbers


Standard C Library (libc, -lc)


#include <errno.h>


This section provides an overview of the system calls, their error returns, and other common definitions and concepts.


Nearly all of the system calls provide an error number referenced via the external identifier errno. This identifier is defined in <sys/errno.h> as

extern int * __error();
#define errno (* __error())

The __error() function returns a pointer to a field in the thread specific structure for threads other than the initial thread. For the initial thread and non-threaded processes, __error() returns a pointer to a global errno variable that is compatible with the previous 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 .
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 corresponding to that specified by the given process ID.
An asynchronous signal (such as SIGINT or SIGQUIT) was caught by the process during the execution of an interruptible function. If the signal handler performs a normal return, the interrupted system call will seem to have returned the error condition.
5 EIO .
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 (over written) 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 .
The number of bytes used for the argument and environment list of the new process exceeded the current limit (NCARGS in <sys/param.h>).
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) 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.
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.
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.
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) system call.
18 EXDEV .
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 component of the specified pathname existed, but it was not a directory, when a directory was expected.
An attempt was made to open a directory with write mode specified.
Some invalid argument was supplied. (For example, specifying an undefined signal to a signal(3) function or a kill(2) system call).
23 ENFILE Too many open files in system.
Maximum number of open files allowable on the system has been reached and requests for an open cannot be satisfied until at least one has been closed.
24 EMFILE Too many open files.
Maximum number of file descriptors allowable in the process has been reached and requests for an open cannot be satisfied until at least one has been closed. The getdtablesize(2) system call 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.
The new process was a pure procedure (shared text) file which was open for writing by another process, or while the pure procedure file was being executed an open(2) call requested write access.
27 EFBIG File too large.
The size of a file exceeded the maximum.
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) system call was issued on a socket, pipe or FIFO.
30 EROFS .
An attempt was made to modify a file or directory on a file system that was read-only at the time.
31 EMLINK Too many links.
Maximum allowable hard links to a single file has been exceeded (limit of 32767 hard links per file).
32 EPIPE .
A write on a pipe, socket or FIFO for which there is no process to read the data.
33 EDOM .
A numerical input argument was outside the defined domain of the mathematical function.
A numerical result of the function was too large to fit in the available space (perhaps exceeded precision).
35 EAGAIN 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.
A required address was omitted from an operation on a socket.
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 ARPA 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 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 should not necessarily expect to be able to use NS addresses with ARPA 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.
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.
An 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.
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 path name lookup involved more than 32 (MAXSYMLINKS) symbolic links.
63 ENAMETOOLONG File name too long.
A component of a path name exceeded {NAME_MAX} characters, or an entire path name exceeded {PATH_MAX} characters. (See also the description of _PC_NO_TRUNC in pathconf(2).)
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.
A directory with entries other than ‘.’ and ‘..’ was supplied to a remove directory or rename call.
67 EPROCLIM Too many processes.
68 EUSERS Too many users.
The quota system ran out of table entries.
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.
An attempt was made to access an open file (on an NFS file system) 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.
72 EBADRPC RPC struct is bad.
Exchange of RPC information was unsuccessful.
73 ERPCMISMATCH RPC version wrong.
The version of RPC on the remote peer is not compatible with the local version.
74 EPROGUNAVAIL RPC prog. not avail.
The requested program is not registered on the remote host.
The requested version of the program is not available on the remote host (RPC).
76 EPROCUNAVAIL Bad procedure for program.
An RPC call was attempted for a procedure which does not exist in the remote program.
77 ENOLCK No locks available.
A system-imposed limit on the number of simultaneous file locks was reached.
Attempted a system call that is not available on this system.
79 EFTYPE Inappropriate file type or format.
The file was the wrong type for the operation, or a data file had the wrong format.
80 EAUTH .
Attempted to use an invalid authentication ticket to mount a NFS file system.
An authentication ticket must be obtained before the given NFS file system may be mounted.
82 EIDRM .
An IPC identifier was removed while the current process was waiting on it.
83 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.
A numerical result of the function was too large to be stored in the caller provided space.
85 ECANCELED Operation canceled.
The scheduled operation was canceled.
86 EILSEQ Illegal byte sequence.
While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid.
The specified extended attribute does not exist.
A function or API is being abused in a way which could only be detected at run-time.
89 EBADMSG Bad message.
A corrupted message was detected.
This error code is unused, but present for compatibility with other systems.
This error code is unused, but present for compatibility with other systems.
92 EPROTO Protocol error.
A device or socket encountered an unrecoverable protocol error.
An operation on a capability file descriptor requires greater privilege than the capability allows.
94 ECAPMODE Not permitted in capability mode.
The system call or operation is not permitted for capability mode processes.
The state protected by a robust mutex is not recoverable.
The owner of a robust mutex terminated while holding the mutex lock.


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 csh(1).
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 csh(1) and tty(4)).
Orphaned Process Group
A process group is considered to be 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.
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 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 super-user.)
A process is recognized as a process and is granted special privileges if its effective user ID is 0.
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 {NAME_MAX} characters may be used to name an ordinary file, special file, or directory.

These characters may be arbitrary eight-bit values, excluding NUL (ASCII 0) and the ‘/’ character (slash, ASCII 47).

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.

Path Name
A path name 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 path name must be less than {PATH_MAX} characters. (On some systems, this limit may be infinite.)

If a path name begins with a slash, the path search begins at the directory. Otherwise, the search begins from the current working directory. A slash by itself names the root directory. An empty pathname refers to the current 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 and 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 super-user. (Note: even the super-user 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.


intro(3), perror(3)

May 4, 2013 Debian