---
name: "atol"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/atoi"
---

# atol(3)

convert a string to an integer

Part of [atoi(3)](/atoi/3.md).

## NAME

atoi, atol, atoll - convert a string to an integer

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

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

**int atoi(const char \****nptr***);** **long atol(const char \****nptr***);** **long long atoll(const char \****nptr***);**

Feature Test Macro Requirements for glibc (see [**feature\_test\_macros**(7)](/feature_test_macros/7.md)):

**atoll**():

```
    _ISOC99_SOURCE
        || /* glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE
```

## DESCRIPTION

The **atoi**() function converts the initial portion of the string pointed to by *nptr* to *int*. The behavior is the same as

```
strtol(nptr, NULL, 10);
```

except that **atoi**() does not detect errors.

The **atol**() and **atoll**() functions behave the same as **atoi**(), except that they convert the initial portion of the string to their return type of *long* or *long long*.

## RETURN VALUE

The converted value or 0 on error.

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **atoi**(), **atol**(), **atoll**() | Thread safety | MT-Safe locale |

## VERSIONS

POSIX.1 leaves the return value of **atoi**() on error unspecified. On glibc, musl libc, and uClibc, 0 is returned on error.

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C99, POSIX.1-2001, SVr4, 4.3BSD.

C89 and POSIX.1-1996 include the functions **atoi**() and **atol**() only.

## BUGS

*errno* is not set on error so there is no way to distinguish between 0 as an error and as the converted value. No checks for overflow or underflow are done. Only base-10 input can be converted. It is recommended to instead use the **strtol**() and **strtoul**() family of functions in new programs.

## SEE ALSO

[**atof**(3)](/atof/3.md), [**strtod**(3)](/strtod/3.md), [**strtol**(3)](/strtol/3.md), [**strtoul**(3)](/strtoul/3.md)