What Type of Data Type Does the Atoi () Function Return?


Heather Bennett

What Type of Data Type Does the Atoi () Function Return?

The atoi() function is a commonly used function in C programming that converts a string representation of an integer to its equivalent integer value. This function is defined in the stdlib.h header file and returns an integer value.

Return Type of the atoi() Function

The atoi() function returns an integer value, specifically of type int. The return type is important to consider when using this function, as it determines how you should handle the returned value.

Using atoi() to Convert a String to an Integer

To convert a string to its corresponding integer value using the atoi() function, you need to pass the string as an argument. The function will then parse and convert the string into an integer representation.


  • The atoi() function stops converting when it encounters the first non-numeric character or whitespace in the input string.
  • If no valid conversion can be performed, such as when the input string contains non-numeric characters from the start, then 0 will be returned.
  • If the converted value exceeds the range of representable values for type int, then undefined behavior occurs. It’s important to handle potential overflow situations appropriately.

An Example:

To illustrate how the atoi() function works, let’s consider an example:

#include <stdio.h>
#include <stdlib.h>

int main() {
   char str[] = "123";
   int num = atoi(str);
   printf("The converted integer is: %d\n", num);
   return 0;

In this example, the string "123" is passed as an argument to the atoi() function. The function will convert it to the corresponding integer value 123, which is then stored in the variable num. Finally, we print out the converted integer using the printf() function.

The output of this code will be:

The converted integer is: 123


The atoi() function in C is a useful tool for converting string representations of integers into their corresponding integer values. It returns an integer value of type int. However, it’s important to handle potential cases of non-numeric characters or overflow situations that may occur during conversion.

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