Home > Articles > Operating Systems, Server > Linux/UNIX/Open Source

This chapter is from the book

This chapter is from the book

3.13 fcntl Function

The fcntl function can change the properties of a file that is already open.

    #include <sys/types.h>
    #include <unistd.h>
    #include <fcntl.h>

    int fcntl(int filedes, int 
cmd, ... /* int arg */ ); Returns: depends on cmd if OK
(see following), -1 on error

In the examples we show in this section, the third argument is always an integer, corresponding to the comment in the function prototype just shown. But when we describe record locking in Section 12.3, the third argument becomes a pointer to a structure.

The fcntl function is used for five different purposes:

  • duplicate an existing descriptor (cmd = F_DUPFD),

  • get/set file descriptor flags (cmd = F_GETFD or F_SETFD),

  • get/set file status flags (cmd = F_GETFL or F_SETFL),

  • get/set asynchronous I/O ownership (cmd = F_GETOWN or F_SETOWN),

  • get/set record locks (cmd = F_GETLK, F_SETLK, or F_SETLKW).

We'll now describe the first seven of these 10 cmd values. (We'll wait until Section 12.3 to describe the last three, which deal with record locking.) Refer to Figure 3.2 since we'll be referring to both the file descriptor flags associated with each file descriptor in the process table entry and the file status flags associated with each file table entry.

F_DUPFD Duplicate the file descriptor filedes. The new file descriptor is returned as the value of the function. It is the lowest numbered descriptor that is not already open, that is greater than or equal to the third argument (taken as an integer). The new descriptor shares the same file table entry as filedes. (Refer to Figure 3.4.) But the new descriptor has its own set of file descriptor flags and its FD_CLOEXEC file descriptor flag is cleared. (This means that the descriptor is left open across an exec, which we discuss in Chapter 8.)
F_GETFD Return the file descriptor flags for filedes as the value of the function. Currently only one file descriptor flag is defined: the FD_CLOEXEC flag.

Set the file descriptor flags for filedes. The new flag value is set from the third argument (taken as an integer).

Be aware that many existing programs that deal with the file descriptor flags don't use the constant FD_CLOEXEC. Instead the programs set the flag to either 0 (don't close-on-exec, the default) or 1 (do close-on-exec).

F_GETFL Return the file status flags for filedes as the value of the function. We described the file status flags when we described the open function. They are listed in Figure 3.5.
  Unfortunately, the three access mode flags (O_RDONLY, O_WRONLY, and O_RDWR) are not separate bits that can be tested. (As we mentioned earlier, these three often have the values 0,1, and 2, respectively, for historical reasons; also these three values are mutually exclusive—a file can have only one of the three enabled.) Therefore we must first use the O_ACCMODE mask to obtain the access mode bits and then compare the result against any of the three values.

03fig05.gifFigure 3.5. File status flags for fcntl.

F_SETFL Set the file status flags to the value of the third argument (taken as an integer). The only flags that can be changed are O_APPEND, O_NONBLOCK, O_SYNC, and O_ASYNC.
F_GETOWN Get the process ID or process group ID currently receiving the SIGIO and SIGURG signals. We describe these 4.3+BSD asynchronous I/O signals in Section 12.6.2.
F_SETOWN Set the process ID or process group ID to receive the SIGIO and SIGURG signals. A positive arg specifies a process ID. A negative arg implies a process group ID equal to the absolute value of arg.

The return value from fcntl depends on the command. All commands return -1 on an error or some other value if OK. The following four commands have special return values: F_DUPFD, F_GETFD, F_GETFL, and F_GETOWN. The first returns the new file descriptor, the next two return the corresponding flags, and the final one returns a positive process ID or a negative process group ID.


Program 3.4 takes a single command-line argument that specifies a file descriptor and prints a description of the file flags for that descriptor.

Program 3.4 Print file flags for specified descriptor.

#include    <sys/types.h>
#include    <fcntl.h>
#include    "ourhdr.h"

main(int argc, char *argv[])

    int     accmode, val;

    if (argc != 2)
        err_quit("usage: a.out <descriptor#>");

    if ( (val = fcntl(atoi(argv[1]), F_GETFL, 0)) < 0)
        err_sys("fcntl error for fd %d", atoi(argv[1]));

    accmode = val & O_ACCMODE;
    if      (accmode == O_RDONLY)   printf("read only");
    else if (accmode == O_WRONLY)   printf("write only");
    else if (accmode == O_RDWR)     printf("read write");
    else err_dump("unknown access mode");

    if (val & O_APPEND)         printf(", append");
    if (val & O_NONBLOCK)       printf(", nonblocking");
#if !defined(_POSIX_SOURCE) && defined(O_SYNC)
    if (val & O_SYNC)           printf(", synchronous writes");

Notice that we use the feature test macro _POSIX_SOURCE and conditionally compile the file access flags that are not part of POSIX.1. The following script shows the operation of the program, when invoked from a KornShell.

    $ a.out 0 < /dev/tty
   read only
   $ a.out 1 > temp.foo
   $ cat temp.foo
   write only
   $ a.out 2 2>>temp.foo
   write only, append
   $ a.out 5 5<>temp.foo
   read write

The KornShell clause 5<>temp.foo opens the file temp.foo for reading and writing on file descriptor 5.


When we modify either the file descriptor flags or the file status flags we must be careful to fetch the existing flag value, modify it as desired, and then set the new flag value. We can't just do an F_SETFD or an F_SETFL, as this could turn off flag bits that were previously set.

Program 3.5 shows a function that sets one or more of the file status flags for a descriptor.

Program 3.5 Turn on one or more of the file status flags for a descriptor.

#include    <fcntl.h>
#include    "ourhdr.h"

set_fl(int fd, int flags) /* flags are file status flags to turn on */
    int     val;

    if ( (val = fcntl(fd, F_GETFL, 0)) < 0)
        err_sys("fcntl F_GETFL error");

    val |= flags;       /* turn on flags */

    if (fcntl(fd, F_SETFL, val) < 0)
        err_sys("fcntl F_SETFL error");

If we change the middle statement to

    val &= ~flags;      /* turn flags off */

we have a function named clr_fl that we'll use in some later examples. This statement logically ANDs the 1's-complement of flags with the current val.

If we call set_fl from Program 3.3 by adding the line

    set_fl(STDOUT_FILENO, O_SYNC);

at the beginning of the program, we'll turn on the synchronous-write flag. This causes each write to wait for the data to be written to disk before returning. Normally in Unix, a write only queues the data for writing, and the actual I/O operation can take place sometime later. A database system is a likely candidate for using O_SYNC, so that it knows on return from a write that the data is actually on the disk, in case of a system crash.

We expect the O_SYNC flag to increase the clock time when the program runs. To test this we can run Program 3.3, copying a 1.5 Mbyte file from one file on disk to another and compare this with a version that does the same thing with the O_SYNC flag set. The results are in Figure 3.6.

03fig06.gifFigure 3.6. Timing results using synchronous writes (O_SYNC).

The three rows in Figure 3.6 were all measured with a BUFFSIZE of 8192. The results in Figure 3.1 were measured reading a disk file and writing to /dev/null, so there was no disk output. The second row in Figure 3.6 corresponds to reading a disk file and writing to another disk file. This is why the first and second rows in Figure 3.6 are different. The system time increases when we write to a disk file because the kernel now copies the data from our process and queues the data to for writing by the disk driver. The clock time increases also when we write to a disk file. When we enable synchronous writes, the system time increases slightly and the clock time increases by a factor of 6.

With this example we see the need for fcntl. Our program operates on a descriptor (standard output), never knowing name of the file that was opened by the shell on that descriptor. We can't set the O_SYNC flag when the file is opened, since the shell opened the file, fcntl allows us to modify the properties of a descriptor, knowing only the descriptor for the open file. We'll see another need for fcntl when we describe nonblocking pipes (Section 14.2), since all we have with a pipe is a descriptor.

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020