# How to convert String to Integer in C

In this article, we are going to learn different techniques of how to convert string to integer in C language. The techniques that we are going to learn in this article are:

### How to convert string to Integer in C

• atoi() Function to Convert a String to an Integer in C
• strtol() Function to Convert a String to an Integer in C
• strtoumax() Function for Convert String to an Integer in C

### 1. atoi() Function to Convert a String to an Integer in C

In this example, we are going to make use of the atoi() function from the C standard library. This function can be used by just including the header file in your program. This function accepts a char input and returns the integer.Let us understand this function used in the below code example to convert a string to an integer number.If we have to convert integer to string

#### Syntax

```int atoi(const char *str);
```
• *str is a pointer to a string to be converted to an integer
```#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main (void)
{
int Num;
char str;
strcpy(str,"55");

Num = atoi(str);
printf("The String input = %s is equal to integer = %d\n", str, Num);

return(0);
}
```

Output

```The String input = 55 is equal to integer = 55
```

### 2. strtol() Function to Convert a String to an Integer in C

In this example, we are going to make use of the strol() function from the C Standard library. This function can be used by just including the header file in your program. This function accepts a char input and returns the number equivalent in a long integer.

#### Syntax

```long int strtol(const char *str, char **end,int base);
```

#### Parameters

• *str = is a pointer to a string to be converted to a long integer.
• **end = is a pointer to indicate where the conversion stops.
• base = is the base representation with the range of [2, 36]. For example 2 is binary, 8 is octal etc.
```#include <stdio.h>
#include <stdlib.h>
#include <string.h>

int main()
{
char str;
char *ptr;
long Num;
strcpy(str, "55");
Num = strtol(str, &ptr, 10);

printf("The String input = %s is equal to Base 10(Decimal) long integer = %ld\n", str, Num);

return 0;
}
```

Output

```he String input = 55 is equal to Base 10(Decimal) long integer = 55
```

### 3. strtoumax() Function for Convert String to an Integer in C

In this example, we are going to make use of the strtoumax() function from the C Standard library. This function can be used by just including header file in your program.This function accepts a char input and return the number equivalent in integer.

#### Syntax

```int strtoumax(const char* str, char** end, int base);
```

#### Parameters

• *str = is a pointer to a string to be converted to a long integer.
• **end = is a pointer to indicate where the conversion stops.
• base = is the base representation with the range of [2, 36]. For example 2 is binary, 8 is octal etc.

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

int main()
{
char str;
char *ptr;
int Num;
strcpy(str, "55");

Num = strtoumax(str, &ptr,10);

printf("The String input = %s is equal to integer = %d\n", str, Num);

return 0;
}
```

Output

```The String input = 55 is equal to integer = 55
```