man atoi:

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



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

SYNOPSIS
#include <stdlib.h>

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

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

atoll():
_BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

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. atoq() is an obsolete name for atoll().


takze atoi ti to nereklne, ale strtol ano
man strtol:
STRTOL(3) Linux Programmer's Manual STRTOL(3)



NAME
strtol, strtoll, strtoq - convert a string to a long integer


NAME
strtol, strtoll, strtoq - convert a string to a long integer

SYNOPSIS
#include <stdlib.h>

long int strtol(const char *nptr, char **endptr, int base);

long long int strtoll(const char *nptr, char **endptr, int base);

Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

strtoll():
_XOPEN_SOURCE >= 600 || _BSD_SOURCE || _SVID_SOURCE || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
or cc -std=c99

DESCRIPTION
The strtol() function converts the initial part of the string in nptr to a long integer value according to the given base, which
must be between 2 and 36 inclusive, or be the special value 0.

The string may begin with an arbitrary amount of white space (as determined by isspace(3)) followed by a single optional '+' or '-'
sign. If base is zero or 16, the string may then include a "0x" prefix, and the number will be read in base 16; otherwise, a zero
base is taken as 10 (decimal) unless the next character is '0', in which case it is taken as 8 (octal).

The remainder of the string is converted to a long int value in the obvious manner, stopping at the first character which is not a
valid digit in the given base. (In bases above 10, the letter 'A' in either uppercase or lowercase represents 10, 'B' represents
11, and so forth, with 'Z' representing 35.)

If endptr is not NULL, strtol() stores the address of the first invalid character in *endptr. If there were no digits at all, str‐
tol() stores the original value of nptr in *endptr (and returns 0). In particular, if *nptr is not '\0' but **endptr is '\0' on
return, the entire string is valid.

The strtoll() function works just like the strtol() function but returns a long long integer value.

RETURN VALUE
The strtol() function returns the result of the conversion, unless the value would underflow or overflow. If an underflow occurs,
strtol() returns LONG_MIN. If an overflow occurs, strtol() returns LONG_MAX. In both cases, errno is set to ERANGE. Precisely
the same holds for strtoll() (with LLONG_MIN and LLONG_MAX instead of LONG_MIN and LONG_MAX).