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QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
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C Library Reference
S
sem_trywait()
Wait on a named or unnamed semaphore, but don't block

QNX Momentics IDE User's Guide
This User's Guide describes version 8.0 of the Integrated Development Environment (IDE) that's part of the QNX Tool Suite.
QNX Software Development Platform
QNX SDP is a cross-compiling and debugging environment, including an IDE and command-line tools, for building binary images and programs for target boards running the QNX OS 8.0.
- Quickstart Guide
- System Architecture
  The System Architecture guide accompanies the QNX OS and is intended for both application developers and end-users.
- OS Components & Operations
- Graphics
- Programming
- System Security Guide
  The QNX System Security Guide is intended for both system integrators who are responsible for the security of a QNX OS system and developers who want to create a QNX OS resource manager free from vulnerabilities.
- Utilities & Libraries
  - C Library Reference
    - What's in a Function Description?
    - Manifests
    - Character Sets
    - A
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      - scalb()
        Load the exponent of a radix-independent floating point number
      - scalbln(), scalblnf(), scalblnl()
        Load the exponent of a radix-independent floating point number
      - scalbn(), scalbnf(), scalbnl()
        Load the exponent of a radix-independent floating point number
      - scandir()
        Scan a directory
      - scanf()
        Scan formatted input from stdin
      - sched_getparam()
        Get the current scheduling parameters of a process
      - sched_get_priority_adjust()
        Calculate the allowable priority for the scheduling policy
      - sched_get_priority_max()
        Get the maximum priority for the scheduling policy
      - sched_get_priority_min()
        Get the minimum priority for the scheduling policy
      - sched_getscheduler()
        Get the current scheduling policy for a process
      - sched_param
        Structure that describes scheduling parameters
      - sched_rr_get_interval()
        Get the execution time limit of a process
      - sched_setparam()
        Change the priority of a process
      - sched_setscheduler()
        Change the priority and scheduling policy of a process
      - sched_yield()
        Yield to other ready threads at the same priority
      - SchedCtl(), SchedCtl_r()
        Control the scheduler
      - SchedGet(), SchedGet_r()
        Get the scheduling policy and parameters for a thread
      - SchedGetCpuNum()
        Get the index of the CPU that a thread is currently running on
      - SchedInfo(), SchedInfo_r()
        Get scheduler information
      - SchedSet(), SchedSet_r()
        Set the scheduling policy and parameters for a thread
      - SchedYield(), SchedYield_r()
        Yield to other threads
      - searchenv()
        Search the directories listed in an environment variable
      - seed48()
        Initialize the seed for a sequence of pseudo-random numbers
      - seekdir()
        Set the position for the next read of the directory stream
      - select(), pselect()
        Check for files that are ready for reading or writing or have an exceptional condition pending
      - select_attach()
        Attach a file descriptor to a dispatch handle
      - select_detach()
        Detach a file descriptor from a dispatch handle
      - select_query()
        Decode the last select event
      - sem_clockwait()
        Wait on a semaphore with a timeout measured against a specific clock
      - sem_close()
        Close a named semaphore
      - sem_destroy()
        Destroy an unnamed semaphore
      - sem_getvalue()
        Get the value of a named or unnamed semaphore
      - sem_init()
        Initialize an unnamed semaphore
      - sem_open()
        Create or access a named semaphore
      - sem_post()
        Increment a named or unnamed semaphore
      - sem_timedwait(), sem_timedwait_monotonic()
        Wait on a named or unnamed semaphore, with a timeout
      - sem_trywait()
        Wait on a named or unnamed semaphore, but don't block
      - sem_unlink()
        Destroy a named semaphore
      - sem_wait()
        Wait on a named or unnamed semaphore
      - set_lowest_fd()
        Specify the lowest acceptable file descriptor
      - setbuf()
        Associate a buffer with a stream
      - setbuffer()
        Assign block buffering to a stream
      - setegid()
        Set the effective group ID for a process
      - setenv()
        Create or change an environment variable
      - seteuid()
        Set the effective user ID
      - setgid()
        Set the real, effective and saved set-group IDs
      - setfsent()
        Open the filesystem table (/etc/fstab) file
      - setgrent()
        Rewind to the start of the group database file
      - setgroups()
        Set supplementary group IDs
      - setgroupspid()
        Set supplementary group IDs for a child process
      - sethostname()
        Set the name of the current host
      - SETIOV(), SETIOV_CONST()
        Fill in the fields of an iov_t structure
      - setitimer()
        Set the value of an interval timer
      - setjmp()
        Save the calling environment for longjmp()
      - setlinebuf()
        Assign line buffering to a stream
      - setlocale()
        Select a program's locale
      - setlogmask()
        Set the syslog() log priority mask for a process
      - setpgid()
        Join or create a process group
      - setpgrp()
        Set the process group
      - setprogname()
        Set the name of the current program
      - setpwent()
        Rewind the password database file
      - setregid()
        Set real and effective group IDs
      - setreuid()
        Set the real and effective user IDs for the calling process
      - setrlimit(), setrlimit64()
        Set the limit on a system resource
      - setsid()
        Create a new session
      - setsockprefix()
        Set or get the prefix for the networking stack path for the current thread
      - setspent()
        Rewind the shadow password database file
      - setstate()
        Reset the state of a pseudo-random number generator
      - settimeofday()
        Set the time and date
      - setuid()
        Set the real, effective and saved user IDs
      - setutent()
        Return to the beginning of the user-information file
      - setvbuf()
        Associate a buffer with a stream
      - shm_create_handle()
        Create a handle so a specific process can access a shared memory object
      - shm_create_subrange_handle()
        Create a handle so a specific process can access a shared memory object
      - shm_ctl(), shm_ctl_special()
        Give special attributes to a shared memory object
      - shm_delete_handle()
        Delete a handle to a shared memory object
      - shm_open()
        Open a shared memory object based on a pathname
      - shm_open_handle(), shm_open_handle_pid()
        Open a shared memory object based on a handle
      - shm_revoke()
        Revoke mappings to shared memory
      - shm_unlink()
        Remove the name of a shared memory object
      - shutdown_system(), shutdown_system_with_reason()
        Shut down the system
      - sigaction()
        Examine or specify the action associated with a signal
      - struct sigaction
        Structure that specifies how to handle a signal
      - sigaddset()
        Add a signal to a set
      - sigaltstack()
        Set and get an alternate signal stack
      - sigdelset()
        Delete a signal from a set
      - sigemptyset()
        Initialize a set to contain no signals
      - sigevent
        Structure that describes an event
      - sigfillset()
        Initialize a set to contain all signals
      - siginfo_t
        Information about a signal
      - sigismember()
        See if a given signal is in a given set
      - siglongjmp()
        Restore the environment saved by sigsetjmp(), including the signal mask
      - sigmask()
        Construct a mask for a signal number
      - signal()
        Set handling for exceptional conditions
      - SignalAction(), SignalAction_r()
        Examine and/or specify actions for signals
      - SignalKill(), SignalKill_r()
        Send a signal to a process group, process, or thread
      - SignalKillSigval(), SignalKillSigval_r()
        Send a signal to a process group, process, or thread
      - SignalProcmask(), SignalProcmask_r()
        Modify or examine a thread's signal-blocked mask
      - SignalSuspend(), SignalSuspend_r()
        Suspend a thread until a signal is received
      - SignalWaitinfo(), SignalWaitinfo_r()
        Select a pending signal
      - SignalWaitinfoMask(), SignalWaitinfoMask_r()
        Block a set of signals and then select a pending signal
      - signbit()
        Determine whether a floating-point number is negative
      - sigpending()
        Examine the set of pending, masked signals for a process
      - sigprocmask()
        Modify or examine a thread's signal-blocked mask
      - sigqueue()
        Queue a signal to a process
      - sigsetjmp()
        Save the environment, including the signal mask
      - sigsuspend()
        Replace the signal mask, and then suspend the thread
      - sigtimedwait()
        Wait for a signal or a timeout
      - sigwait()
        Wait for a pending signal
      - sigwaitinfo()
        Wait for a specified signal and return its information
      - sin(), sinf(), sinl()
        Calculate the sine of an angle
      - sincos(), sincosf(), sincosl()
        Calculate the sine and cosine of an angle
      - sinh(), sinhf(), sinhl()
        Compute the hyperbolic sine
      - sleep()
        Suspend a thread for a given length of time
      - slog2_close_log()
        Close a system logger file
      - slog2_dump_logs_to_file()
        Write a snapshot of slog2 logs to a file
      - slog2_find_buffer()
        Find the handle for a buffer with a given name
      - slog2_get_buffer_info()
        Get information about a slogger2 buffer
      - slog2_get_log_info()
        Get information about a log file
      - slog2_get_verbosity()
        Get the verbosity level of the entire buffer set
      - slog2_hash()
        Convert an input string into an obfuscated non-reversible hash string
      - slog2_obfuscate()
        Obfuscate file and directory names by replacing some characters
      - slog2_open_log()
        Open a system log for parsing
      - slog2_packet_cmp()
        Compare two slogger2 packets
      - slog2_packet_info_t
        Structure containing information about a packet
      - slog2_parse_all()
        Parse all slogger2 files
      - slog2_parse_dynamic_buffer(), slog2_parse_static_buffer()
        Parse a slogger2 buffer
      - slog2_register()
        Register and allocate a new slog2 buffer set
      - slog2_reset()
        Reset the state of slogger2 for the process
      - slog2_set_default_buffer()
        Set the default slog2 logging buffer
      - slog2_set_verbosity()
        Set the verbosity level for the entire buffer set
      - slog2c()
        Log a constant string in a slog2 buffer
      - slog2f()
        Log a printf-style string in a slog2 buffer
      - slog2fa()
        Log a printf-style string in a slog2 buffer
      - slogb()
        Send a message to the system logger
      - slogf()
        Send a message to the system logger
      - slogi()
        Send a message to the system logger
      - snprintf()
        Write formatted output to a character array, up to a given maximum number of characters
      - sopen()
        Open a file for shared access
      - sopenfd()
        Open for shared access a file associated with a given descriptor
      - spawn()
        Create and execute a new child process
      - spawnl()
        Spawn a child process, given a list of arguments
      - spawnle()
        Spawn a child process, given a list of arguments and an environment
      - spawnlp()
        Spawn a child process, given a list of arguments and a relative path
      - spawnlpe()
        Spawn a child process, given a list of arguments, an environment, and a relative path
      - spawnp()
        Create and execute a new child process, given a relative path
      - spawnv()
        Spawn a child process, given a vector of arguments
      - spawnve()
        Spawn a child process, given a vector of arguments and an environment
      - spawnvp()
        Spawn a child process, given a vector of arguments and a relative path
      - spawnvpe()
        Spawn a child process, given a vector of arguments, an environment, and a relative path
      - sprintf()
        Print formatted output into a string
      - sqrt(), sqrtf(), sqrtl()
        Calculate the nonnegative square root of a number
      - srand()
        Start a new sequence of pseudo-random integers
      - srand48()
        Initialize a sequence of pseudo-random numbers
      - srandom()
        Set the seed for a pseudo-random number generator
      - sscanf()
        Scan input from a character string
      - stat(), stat64()
        Get information about a file or directory, given a path
      - struct stat
        Data structure for information about a file or directory
      - stat_convert_form()
        Convert a stat structure from one form to another
      - static_assert()
        Display a diagnostic message at compile time
      - statvfs(), statvfs64()
        Get filesystem information, given a path
      - stderr
        The standard error stream
      - stdin
        The standard input stream
      - stdout
        The standard output stream
      - stpcpy()
        Copy a string and return a pointer to the end of the result
      - stpncpy()
        Copy a fixed-length string and return a pointer to the end of the result
      - straddstr()
        Concatenate one string on to the end of another
      - strcasecmp()
        Compare two strings, ignoring case
      - strcasestr()
        Find one string inside another, ignoring case
      - strcat()
        Concatenate two strings
      - strchr()
        Find the first occurrence of a character in a string
      - strcmp()
        Compare two strings
      - strcoll()
        Compare two strings, using the locale's collating sequence
      - strcpy()
        Copy a string
      - strcspn()
        Count the characters at the beginning of a string that aren't in a given character set
      - strdup()
        Create a duplicate of a string
      - strerror()
        Convert an error number into an error message
      - strerror_r()
        Convert an error number into an error message (reentrant)
      - strftime()
        Format a time into a string
      - strlcat(), strlcpy()
        Size-bounded string copying and concatenation
      - strlen()
        Compute the length of a string
      - strlwr()
        Convert a string to lowercase
      - strncasecmp()
        Compare two strings, ignoring case, up to a given length
      - strncat()
        Concatenate two strings, up to a maximum length
      - strncmp()
        Compare two strings, up to a given length
      - strncpy()
        Copy a string, to a maximum length
      - strndup()
        Create a duplicate of a string, up to a maximum length
      - strnlen()
        Compute the length of a string, to a maximum number of bytes
      - strpbrk()
        Find the first character in a string that's in a given character set
      - strptime()
        Convert a string into a time
      - strrchr()
        Find the last occurrence of a character in a string
      - strrev()
        Reverse a string
      - strsep()
        Separate a string into pieces marked by given delimiters
      - strsignal()
        Return the description of a signal
      - strspn()
        Count the characters at the beginning of a string that are in a given character set
      - strstr()
        Find one string inside another
      - strtod(), strtof(), strtold()
        Convert a string into a double-precision number
      - strtoimax(), strtoumax()
        Convert a string into an integer
      - strtok()
        Break a string into tokens
      - strtok_r()
        Break a string into tokens (reentrant)
      - strtol(), strtoll()
        Convert a string into a long integer
      - strtoul(), strtoull()
        Convert a string into an unsigned long integer
      - strupr()
        Convert a string to uppercase
      - strxfrm()
        Transform one string into another, to a given length
      - swab()
        Endian-swap a given number of bytes
      - swprintf()
        Print formatted wide-character output into a string
      - swscanf()
        Scan input from a wide-character string
      - symlink()
        Create a symbolic link to a path
      - symlinkat()
        Create a symbolic link to a path, at a given location
      - sync()
        Synchronize filesystem updates
      - SyncBarrierWait_r()
        Synchronize threads at the barrier
      - SyncCondvarSignal(), SyncCondvarSignal_r()
        Wake up any threads that are blocked on a synchronization object
      - SyncCondvarWait(), SyncCondvarWait_r()
        Block a thread on a synchronization object
      - SyncCtl(), SyncCtl_r()
        Perform an operation on a synchronization object
      - SyncDestroy(), SyncDestroy_r()
        Destroy a synchronization object
      - SyncMutexLock(), SyncMutexLock_r()
        Lock a mutex synchronization object
      - SyncMutexUnlock(), SyncMutexUnlock_r()
        Unlock a mutex synchronization object
      - SyncRWLock() , SyncRWLock_r()
        Lock or unlock a read-write lock
      - SyncSemPost(), SyncSemPost_r()
        Increment a semaphore
      - SyncSemWait(), SyncSemWait_r()
        Wait on a semaphore
      - SyncTypeCreate(), SyncTypeCreate_r()
        Create a synchronization object
      - SyncTypeDestroy(), SyncTypeDestroy_r()
        Destroy a synchronization object
      - sysconf()
        Return the value of a configurable system limit or option
      - syslog()
        Write a message to the slogger2 system log
      - sysmgr_confstr_set()
        Set a configuration string
      - sysmgr_reboot()
        Reboot the system
      - sysmgr_runstate(), sysmgr_runstate_dynamic()
        Change the runstate of a processor
      - sysmgr_runstate_burst()
        Ask the system to turn on dynamically offlined CPUs
      - sysmgr_sysconf_set()
        Set the value of a configurable system limit or option
      - SYSPAGE_CPU_ENTRY()
        Return a CPU-specific entry from the system page
      - SYSPAGE_ENTRY()
        Return an entry from the system page
      - _syspage_ptr
        A pointer to the system page
      - SysSrandom(), SysSrandom_r()
        Supply entropy to the kernel's PNRG, retrieve entropy collected by the kernel, or both
      - system()
        Execute a system command
    - T
    - U
    - V
    - W
    - X
    - Y
    - Z
    - Full Safety Information
    - Glossary
  - Devctl and Ioctl Commands
  - JSON Library Reference
  - PAM Reference
  - QNX Helpers Developer's Guide
    These libraries provide QNX helpers, including helpers that assist with logging, string conversion, and number and type sizes.
  - SPI Framework Technote
  - QNX Cryptography Library
    The QNX Cryptography Library (qcrypto) is a flexible cryptography framework that allows you to choose which cryptography provider responds to requests on a per-process basis.
  - QNX devcrypto Service
  - Utilities Reference
  - WPA Control Interface Reference
    This document provides a reference for the Wi-Fi Protected Access (WPA) control interface, which provides a C API for the WPA/IEEE 802.11i protocols in Wi-Fi client stations. Application developers who want to control WPA operations and get associated status and event data should read this reference.
- Migrating to QNX OS 8.0
QNX Software in the Cloud
QNX Software in the Cloud enables developers to use the QNX OS in the Amazon cloud environment.
QNX Toolkit for Visual Studio Code
This User's Guide describes the QNX^® Toolkit for Visual Studio Code. The guide introduces you to the QNX Toolkit by explaining the QNX development environment and how to build, run, and debug your QNX^® Operating System (OS) applications and systems.
QNX Hypervisor User's Guide
This User's Guide explains the QNX hypervisor architecture and provides instructions for installing and running a QNX Hypervisor system, changing system components and configuration, and using hypervisor features such as virtual devices (vdevs).
Typographical Conventions, Support, and Licensing
This section describes the typographical conventions used throughout the documentation, explains how to obtain technical support, and provides some information about licensing.

sem_trywait()

QNX SDP8.0C Library ReferenceAPIDeveloper

Wait on a named or unnamed semaphore, but don't block

Synopsis:

#include <semaphore.h>

int sem_trywait( sem_t * sem );

Arguments:

sem: A pointer to the sem_t object for the semaphore that you want to wait on.

Library:

libc

Use the -l c option to qcc to link against this library. This library is usually included automatically.

Description:

The sem_trywait() function decrements the semaphore if the semaphore's value is greater than zero; otherwise, the function simply returns.

Returns:

0: The semaphore was successfully decremented.
-1: The state of the semaphore is unchanged (errno is set).

Errors:

EAGAIN: The count wasn't greater then 0, so sem_trywait() couldn't immediately decrement it.
EINVAL: Invalid semaphore descriptor sem.
EINTR: A signal interrupted this function.

Classification:

POSIX 1003.1


Safety:
Cancellation point	No
Signal handler	Yes
Thread	Yes

Page updated: August 11, 2025