Saveptr

Saveptr is a term associated with the POSIX reentrant string tokenizer strtok_r. It refers to the external state that the function uses to resume tokenization between calls. The strtok_r function has the prototype char *strtok_r(char *str, const char *delim, char **saveptr). The saveptr argument is a pointer to a char* variable supplied by the caller; strtok_r writes the current parsing position into this variable so subsequent calls can continue from where the previous call left off.

Usage pattern: on the first call, provide the input string in str and a valid saveptr variable, for example by declaring char *saveptr; char *token = strtok_r(str, delim, &saveptr); On subsequent calls, pass NULL for str and reuse the same saveptr, e.g., token = strtok_r(NULL, delim, &saveptr); Each call returns the next token as a pointer into the original string, and the function inserts null terminators to end tokens in place.

Important considerations: the caller owns the saveptr and must ensure it remains valid for the duration of tokenization. The function does not allocate memory or copy tokens; tokens point into the original string, which must remain modifiable and in scope. strtok_r is designed to be thread-safe because it does not rely on internal static state; instead, each thread or concurrent context provides its own saveptr and working string.

Compared to strtok, strtok_r avoids shared state and is therefore suitable for multi-threaded programs, provided separate saveptrs are used per tokenization context.

See also: strtok, strtok_s, strsep.