The atol function in the stdlib.h library of the C programming language is used to convert a string that represents a long integer into a long int value. The syntax of the atol function is as follows:

long int atol(const char *str);

Here, the argument str is a pointer to the null-terminated string to be converted.

The atol function scans the input string pointed to by str, and converts it to a long int value. The function stops scanning the input string when it encounters the first character that cannot be part of a long integer. The function returns the converted value as a long int value.

For example, the following code uses the atol function to convert a string representing a long integer into a long int value:

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

int main() {
   char str[] = "1234567890123456789";
   long int value;
   value = atol(str);
   printf("The converted value is: %ld\n", value);
   return 0;
}

In this example, the input string is the null-terminated string "1234567890123456789". The atol function converts this string to the long integer value 1234567890123456789, which is stored in the value variable. The program then uses the printf function to print the converted value. The output of the program is:

The converted value is: 1234567890123456789

Note that the atol function does not perform any error checking. If the input string is not a valid long integer, the function returns a result, but the result is undefined. It is the responsibility of the caller to ensure that the input string is a valid long integer before calling the atol function.