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FGETLN(3) Library Functions Manual FGETLN(3)

fgetlnget a line from a stream

#include <stdio.h>

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

Using this function is error-prone in multiple ways; consider using the safer and more portable function getline(3) instead.

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 last line in the stream 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), fgetc(3), fgets(3), fgetwln(3), fopen(3), fparseln(3), getline(3)

The fgetln() function first appeared in 4.4BSD.

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))) {
		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");
December 1, 2017 OpenBSD-current