NAME

atoi - convert a string to an int

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

LIBRARY

Standard C library (libc, -lc)

SYNOPSIS

Header File

#include <stdlib.h>

Prototype

int atoi(string s);

#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)):

atoll():

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

DESCRIPTION

This function converts a (positive or negative) integer from a string (e.g., "50") to an int (e.g., 50).

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

This function returns its input, s, as an int.

The converted value or 0 on error.

EXAMPLE

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

int main(void)
{
    printf("This is CS%i\n", atoi("50"));
}

ATTRIBUTES

For an explanation of the terms used in this section, see attributes(7).

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.