# C Programming/C Reference/stdlib.h/atoi

atoi is a function in the C programming language that converts a string into an integer numerical representation. atoi stands for ASCII to integer. It is included in the C standard library header file stdlib.h. Its prototype is as follows:

int atoi(const char *str);


The str argument is a string, represented by an array of characters, containing the characters of a signed integer number. The string must be null-terminated. When atoi encounters a string with no numerical sequence, it returns zero (0).

There are several variants of the atoi function, atol, atof and atoll , which are used to convert a string into a long, double, or long long type, respectively. The atoll was formerly known as atoq and was included into C99.

## Deficiencies

It is impossible to tell whether the string holds valid sequence of digits that represents the number 0 or invalid number as the function returns 0 in both cases. The newer function strtol does not have this deficiency.

atoi is neither thread-safe, nor async-cancel safe on some operating systems.[1]

Also, atoi only converts base ten ascii values (this may also be a benefit depending on perspective). strtol and other functions support alternate bases such as hexadecimal and octal.

## Standards conformance

The atoi, atof, and atol functions are a part of the ISO standard C library (C89), while the atoll function is added by C99.

However, because of the ambiguity in returning 0 and lack of thread-safety and async-cancel safety on some operating systems, atoi is considered to be deprecated by strtol.[1]

## References

The Version 7 Unix Manual Pages © 1979 by Bell Telephone Laboratories, Incorporated.

The Version 1 Unix Manual page for atoi written by Ken Thompson (November 1971).