c Programming/C Reference/stdlib.h/strtol

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strtol is a function in the C programming language that converts a string into a long integer. strtol stands for string to long. It is included in the C standard library header file stdlib.h. Its prototype is as follows:

long strtol(const char *restrict str, char **restrict end, int base);

The str argument points to a string, represented by an array of characters, containing the character representation of a signed integer value. The string must be null-terminated. The base argument specifies the number base to use, from 2 to 36. If the number base is greater than 10, alphabetic characters ('A' up to 'Z') are used as digits in the representation. After the conversion, the value pointed to by end is set to point to the character following the last valid numeric character in the string and the converted integer value is returned. If the string does not contain a valid numerical sequence, zero (0) is returned and the global variable errno is set to EINVAL.

There also exist similar variants of these function named strtoul, strtoll and strtoull which parses and returns an integer of the type unsigned long, long long, and unsigned long long respectively.

Standards conformance[edit]

The strtol function is part of the ISO standard C library (C89, 1989), and the strtoll function was added as part of the C99 library (1999). It was added to the standard C library as a more well-behaved replacement for the existing atoi function.

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