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ATOI(3) Library Functions Manual ATOI(3)

atoi
convert ASCII string to integer

#include <stdlib.h>
int
atoi(const char *nptr);

The atoi() function converts the initial portion of the string pointed to by nptr to integer representation.
It is equivalent to:
(int)strtol(nptr, (char **)NULL, 10);

atof(3), atol(3), strtod(3), strtol(3), strtonum(3), strtoul(3)

The atoi() function conforms to ANSI X3.159-1989 (“ANSI C89”).

An atoi() function first appeared in Version 1 AT&T UNIX.

atoi does no overflow checking, handles unsigned numbers poorly, and handles strings containing trailing extra characters (like “123abc”) poorly. Careful use of strtol(3) and strtoul(3) can alleviate these problems, but strtonum(3) can be used to convert numbers from strings much more safely and easily.
September 10, 2015 OpenBSD-current