---
title: 5.3 clock()
description: "## Overview The `clock()` function returns the **processor time (CPU time)** consumed by the current program. Unlike `time()` and `gettimeofday()`, which return the current calendar time, `clock()` measures the amount of CPU time used by the program since it started…"
url: "https://edcret.in/docs/linux-os/system-calls/time-system-calls/c-clock-function/"
updated: "2026-07-27"
category: Linux OS / System Calls / Time System Calls
---

# 5.3 clock()

## Overview

The `clock()` function returns the **processor time (CPU time)** consumed by the current program.

Unlike `time()` and `gettimeofday()`, which return the current calendar time, `clock()` measures the amount of CPU time used by the program since it started executing.

It is commonly used to measure the execution time of algorithms and compare program performance.

## Header File

```
#include <time.h>
```

## Man Pages

```
man 3 clock
```

## Function Prototype

```
clock_t clock(void);
```

## Parameters

This function does not accept any parameters.

## Return Value

On success, `clock()` returns the processor time consumed by the program as a value of type `clock_t`.

On failure, it returns:

```
(clock_t)-1
```

## Data Types

### `clock_t`

`clock_t` is a standard C data type used to represent processor time.

The value returned by `clock()` is expressed in **clock ticks**, not seconds.

To convert clock ticks to seconds, divide the returned value by `CLOCKS_PER_SEC`.

### `CLOCKS_PER_SEC`

`CLOCKS_PER_SEC` is a macro that specifies the number of clock ticks per second.

It is defined in `<time.h>`.

## Examples

### Example 1 – Measure CPU Execution Time

#### main.c

```
#include <stdio.h>
#include <time.h>

int main(void)
{
    clock_t start, end;
    double cpu_time;

    start = clock();

    for (volatile long i = 0; i < 100000000; i++);

    end = clock();

    cpu_time = (double)(end - start) / CLOCKS_PER_SEC;

    printf("CPU Time = %.6f seconds\n", cpu_time);

    return 0;
}
```

#### Output

```
CPU Time = 0.218000 seconds
```

> The output will vary depending on your processor and compiler optimization.

#### Explanation

```
start = clock();
```

Stores the processor time before executing the loop.

```
for (volatile long i = 0; i < 100000000; i++);
```

Consumes processor time by repeatedly executing the loop.

The `volatile` keyword prevents the compiler from optimizing away the loop.

```
end = clock();
```

Stores the processor time after the loop completes.

```
cpu_time = (double)(end - start) / CLOCKS_PER_SEC;
```

Calculates the processor time consumed by the loop in seconds.

```
printf("CPU Time = %.6f seconds\n", cpu_time);
```

Displays the CPU execution time.

---

### Example 2 – Measure Function Execution Time

#### main.c

```
#include <stdio.h>
#include <time.h>

void calculate(void)
{
    for (volatile long i = 0; i < 50000000; i++);
}

int main(void)
{
    clock_t start, end;

    start = clock();

    calculate();

    end = clock();

    printf("Execution Time = %.6f seconds\n",
           (double)(end - start) / CLOCKS_PER_SEC);

    return 0;
}
```

#### Output

```
Execution Time = 0.108000 seconds
```

> The output will vary depending on your system.

#### Explanation

```
start = clock();
```

Records the processor time before calling `calculate()`.

```
calculate();
```

Executes the function whose CPU execution time is to be measured.

```
end = clock();
```

Records the processor time after the function returns.

```
(double)(end - start) / CLOCKS_PER_SEC
```

Computes the total processor time consumed by the function.

## Notes

- `clock()` measures **CPU time**, not calendar time.
- The returned value is expressed in **clock ticks**.
- Divide the returned value by `CLOCKS_PER_SEC` to obtain the execution time in seconds.
- Time spent while the program is sleeping or waiting for user input is generally **not included** in the CPU time.
- `clock()` is a **C library function**, not a Linux system call.

## Related Functions

- `time()`
- `gettimeofday()`
- `clock_gettime()`
- `difftime()`