FGETLN(3)              FreeBSD Library Functions Manual              FGETLN(3)

NAME
     fgetln - get a line from a stream

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdio.h>

     char *
     fgetln(FILE *stream, size_t *len);

DESCRIPTION
     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.
     (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.)

RETURN VALUES
     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.

ERRORS
     [EBADF]            The argument stream is not a stream open for reading.

     [ENOMEM]           The internal line buffer could not be expanded due to
                        lack of available memory, or because it would need to
                        expand beyond INT_MAX in size.

     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).

SEE ALSO
     ferror(3), fgets(3), fgetwln(3), fopen(3), getline(3), putc(3)

HISTORY
     The fgetln() function first appeared in 4.4BSD.

CAVEATS
     Since the returned buffer is not a C string (it is not NUL terminated), a
     common practice is to replace the newline character with `\0'.  However,
     if the last line in a file does not contain a newline, the returned text
     won't contain a newline either.  The following code demonstrates how to
     deal with this problem by allocating a temporary buffer:

             char *buf, *lbuf;
             size_t len;

             lbuf = NULL;
             while ((buf = fgetln(fp, &len)) != NULL) {
                     if (buf[len - 1] == '\n')
                             buf[len - 1] = '\0';
                     else {
                             /* EOF without EOL, copy and add the NUL */
                             if ((lbuf = malloc(len + 1)) == NULL)
                                     err(1, NULL);
                             memcpy(lbuf, buf, len);
                             lbuf[len] = '\0';
                             buf = lbuf;
                     }
                     printf("%s\n", buf);
             }
             free(lbuf);
             if (ferror(fp))
                     err(1, "fgetln");

FreeBSD 13.1-RELEASE-p6          June 11, 2020         FreeBSD 13.1-RELEASE-p6