Oct 13, 2023
C Library Sleep System Calls: Delays program execution for a specified time

C Library Sleep System Calls

The sleep function suspends the execution of a thread temporarily. Other CPU operations will function normally, but the execution of the calling thread will be delayed by the period of time specified in the argument.

The return value of the sleep function is zero if the requested time has elapsed. Otherwise, it returns the unslept quantity (the desired time minus the time slept) in seconds.


The sleep system call pauses the execution of a program or thread for the number of seconds specified by the argument. The program will resume executing when the requested time period is complete or when a signal is delivered.

The return value of sleep is an unsigned integer, typically 0 or the amount of time slept in seconds, if any. This value is calculated as (requested time minus the actual time slept). The unsigned value of sleep is consistent across all POSIX-compliant systems.

The sleep function is non-interruptible, which distinguishes it from the wait and sleepex system calls. This function should not be mixed with other interruptible functions. Mixing these system calls can result in unpredictable behavior. It is best to use a different system-provided interruptible multithreaded programming model, such as those provided by the nanosleep or msleep system functions. Using the sleep system call may cause problems on some systems where scheduling delays make the actual time paused slightly longer or shorter than the requested time.


The sleep function pauses the execution of the program for a specified amount of time, in seconds. It suspends the thread until the specified time elapses or the program receives a signal. The time lapsed is returned in the form of an unsigned integer.

If you’re writing a program that uses input or output functions, there may be times when you want to delay execution for a short period of time. This is when the sleep function comes in handy.

Sleep is a non-interruptible system call and is not guaranteed to return at exactly the requested time. It is up to the operating system scheduler to decide when to resume execution of a given thread, and it can take longer than needed. Therefore, you should use the wait or nanosleep functions instead of sleep. These functions can be interrupted by external signals and will return a value that is less than the desired time. They will also update a structure pointer to display the remaining elapsed time.


The sleep function delays program execution for a specified number of seconds. It takes a single unsigned int parameter, indicating the number of seconds to wait. However, due to processor delays, the actual time spent might be different from the requested one.

The c library sleep function is a system call that suspends the execution of the calling process, task or thread until either the number of real-time seconds indicated by its argument elapses or it receives a signal whose action is to invoke a signal-catching function or terminate the process. The suspension time may be longer than the requested one because of system planned activities and scheduling delays. The return value is 0 if the requested period elapsed and the return value is the “unslept” amount (requested time minus the time actually slept) if it returns because of delivery of a signal. This routine is useful for imposing a delay in loops, for example.


The sleep function suspends the implementation of a program for a set number of seconds. The other operations of the CPU continue to function.

The return value is the difference between the requested time and the actual time slept in seconds. This is an unsigned integer. However, scheduling delays may cause the actual time to be returned from sleep to be significantly later than expected.

t is the time as reported by the localtime() or gmtime() functions. A ValueError is raised if any field of t is not within the range supported by your system clock.

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