FCNTL(2)
NAME
fcntl − file control
USAGE
#include <fcntl.h>
res = fcntl(fd, cmd, arg)
int res;
int fd, cmd, arg;
DESCRIPTION
Fcntl provides some control over descriptors. The argument fd is a descriptor that cmd operates on as follows:
F_DUPFD Return a new descriptor as follows:
Lowest-numbered available descriptor greater than or equal to arg.
Same object references as the original descriptor.
New descriptor shares the same file pointer if the object was a file.
Same access mode (read, write or read/write).
Same file-status flags (i.e., both file descriptors share the same file status flags).
The close-on-exec flag associated with the new file descriptor is set to remain open across execv(2) system calls.
F_GETFD Get the close-on-exec flag associated with the file descriptor fd. If the low-order bit is zero, the file will remain open across exec; otherwise, the file will close upon execution of exec.
F_SETFD Set the close-on-exec flag associated with fd to the low-order bit of arg (zero or one, as above).
F_GETFL Get descriptor status flags, as described below.
F_SETFL Set descriptor status flags.
F_GETOWN Get the process ID or process group currently receiving SIGIO and SIGURG signals; process groups are returned as negative values.
F_SETOWN Set the process or process group to receive SIGIO and SIGURG signals; you can specify process groups by supplying a negative arg; otherwise arg is interpreted as a process ID.
The flags for the F_GETFL and F_SETFL flags are as follows:
FNDELAY Non-blocking I/O; if no data is available to a read call, or if a write operation would block, the call returns -1 with the error EWOULDBLOCK.
FAPPEND Force each write to append at the end of file (corresponds to the O_APPEND flag of open(2).)
RETURN VALUE
The value returned upon successful completion depends on cmd as follows:
F_DUPFD A new file descriptor.
F_GETFD Value of flag (only the low-order bit is defined).
F_GETFL Value of flags.
F_GETOWN Value of file descriptor owner.
other Value other than −1.
Otherwise, a value of −1 is returned and errno is set to indicate the error.
ERRORS
Fcntl will fail if one or more of the following are true:
[EBADF] Fd is not a valid open file descriptor.
[EMFILE] Cmd is F_DUPFD and the maximum allowed number of file descriptors are currently open.
[EINVAL] Cmd is F_DUPFD and arg is negative or greater than the maximum allowable number (see getdtablesize(2)).