fgets / getline / getdelim - glibc

Split from the chapter glibc: stdio and Formatted I/O into the per-function layout; see the method page and the glibc ledger for coverage status.

Signature – char *fgets(char *s, int count, FILE *stream); ssize_t getline(char **lineptr, size_t *n, FILE *stream); ssize_t getdelim(char **lineptr, size_t *n, int delim, FILE *stream);

Semantics – fgets reads up to count - 1 bytes, always stopping after (and including) a newline, and NUL-terminates; returns s, or NULL on EOF-with-nothing-read or error. It cannot detect embedded NUL bytes, which is why the manual recommends getline for untrusted or user-edited input (manual, Line-Oriented Input). getline reads an entire line into a malloc-managed buffer: on entry *lineptr may be a malloc’d buffer of *n bytes or NULL with *n == 0; the call reallocs as needed and stores the new pointer and size back. It returns the byte count including the newline (so embedded NULs are measurable), or -1 on EOF/error. getdelim generalizes the delimiter; getline is getdelim with '\n' (manual shows the one-line implementation).

Allocation rules – the buffer stays owned by the caller and stays allocated even when the first call fails; glibc NUL-terminates a buffer it allocated itself so -1 returns are still printable (manual). POSIX leaves *lineptr undefined on -1; free the buffer exactly once when done regardless.

Return and errors – -1 with errno from the underlying read (EAGAIN, EINTR, EIO, …).

References – man3/getline.3; glibc manual Line-Oriented Input.