atoi - Man Page

convert a string to an integer


#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)):


        || /* Glibc <= 2.19: */ _BSD_SOURCE || _SVID_SOURCE


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.


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

atoi(), atol(), atoll()Thread safetyMT-Safe locale

Conforming to

POSIX.1-2001, POSIX.1-2008, C99, SVr4, 4.3BSD. C89 and POSIX.1-1996 include the functions atoi() and atol() only.


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


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), strtod(3), strtol(3), strtoul(3)


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Referenced By

atof(3), form_field_validation.3x(3), nbdkit-plugin(3), perlintern(1), strtod(3), strtoi.3bsd(3), strtol(3), strtonum.3bsd(3), strtou.3bsd(3), strtoul(3).

The man pages atol(3), atoll(3) and atoq(3) are aliases of atoi(3).

2021-03-22 GNU Linux Programmer's Manual