ATOI(3) Linux Programmer's Manual ATOI(3)

atoi, atol, atoll - 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 versions <= 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.

The converted value.

For an explanation of the terms used in this section, see attributes(7).
Interface Attribute Value
atoi (), atol (), atoll () Thread safety MT-Safe locale

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

atof(3), strtod(3), strtol(3), strtoul(3)
2016-03-15 GNU