fgetln man page
fgetln — get a line from a stream
fgetln(FILE *stream, size_t *len);
The fgetln() function returns a pointer to the next line from the stream referenced by stream. This line is not a C string as it does not end with a terminating NUL character. The length of the line, including the final newline, is stored in the memory location to which len points and is guaranteed to be greater than 0 upon successful completion. (Note, however, that if the line is the last in a file that does not end in a newline, the returned text will not contain a newline.)
Upon successful completion a pointer is returned; this pointer becomes invalid after the next I/O operation on stream (whether successful or not) or as soon as the stream is closed. Otherwise, NULL is returned. The fgetln() function does not distinguish between end-of-file and error; the routines feof(3) and ferror(3) must be used to determine which occurred. If an error occurs, the global variable errno is set to indicate the error. The end-of-file condition is remembered, even on a terminal, and all subsequent attempts to read will return NULL until the condition is cleared with clearerr(3).
The text to which the returned pointer points may be modified, provided that no changes are made beyond the returned size. These changes are lost as soon as the pointer becomes invalid.
The argument stream is not a stream open for reading.
The fgetln() function may also fail and set errno for any of the errors specified for the routines fflush(3), malloc(3), read(2), stat(2), or realloc(3).
ferror(3), fgets(3), fgetwln(3), fopen(3), putc(3)
The fgetln() function first appeared in 4.4BSD.