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FGETS(3) Library Functions Manual FGETS(3)

fgets, gets — get a line from a stream

#include <stdio.h>

char *
fgets(char *str, int size, FILE *stream);

char *
gets(char *str);

The () function reads at most size-1 characters from the given stream and stores them in the string str. Reading stops when a newline character is found, at end-of-file, or on error. The newline, if any, is retained. The string will be NUL-terminated if fgets() succeeds; otherwise the contents of str are undefined.

The () function is equivalent to fgets() with an infinite size and a stream of , except that the newline character (if any) is not stored in the string. It is the caller's responsibility to ensure that the input line, if any, is sufficiently short to fit in the string.

Upon successful completion, fgets() and gets() return a pointer to the string. If end-of-file or an error occurs before any characters are read, they return NULL. The fgets() and gets() functions do not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred. Whether fgets() can possibly fail with a size argument of 1 is implementation-dependent. On OpenBSD, fgets() will never return NULL when size is 1.

[]
The given stream is not a readable stream.
[]
The given size is less than or equal to 0.

The function fgets() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), read(2), or malloc(3).

The function gets() may also fail and set errno for any of the errors specified for the routine getchar(3).

feof(3), ferror(3), fgetln(3)

The functions fgets() and gets() conform to ANSI X3.159-1989 (“ANSI C89”).

The functions fgets() and gets() first appeared in Version 7 AT&T UNIX.

The following bit of code illustrates a case where the programmer assumes a string is too long if it does not contain a newline:

char buf[1024], *p;

while (fgets(buf, sizeof(buf), fp) != NULL) {
	if ((p = strchr(buf, '\n')) == NULL) {
		fprintf(stderr, "input line too long.\n");
		exit(1);
	}
	*p = '\0';
	printf("%s\n", buf);
}

While the error would be true if a line > 1023 characters were read, it would be false in two other cases:

  1. If the last line in a file does not contain a newline, the string returned by fgets() will not contain a newline either. Thus strchr() will return NULL and the program will terminate, even if the line was valid.
  2. All C string functions, including strchr(), correctly assume the end of the string is represented by a NUL (‘\0’) character. If the first character of a line returned by fgets() were NUL, strchr() would immediately return without considering the rest of the returned text which may indeed include a newline.

Consider using fgetln(3) instead when dealing with untrusted input.

It is erroneous to assume that fgets() never returns an empty string when successful. If a line starts with the NUL character, fgets will store the NUL and continue reading until it encounters a newline or end-of-file. This will result in an empty string being returned. The following bit of code illustrates a case where the programmer assumes the string cannot be zero length.

char buf[1024];

if (fgets(buf, sizeof(buf), fp) != NULL) {
	/* WRONG */
	if (buf[strlen(buf) - 1] == '\n')
		buf[strlen(buf) - 1] = '\0';
}

If strlen() returns 0, the index into the buffer becomes -1. One way to concisely and correctly trim a newline is shown below.

char buf[1024];

if (fgets(buf, sizeof(buf), fp) != NULL)
	buf[strcspn(buf, "\n")] = '\0';

Since it is usually impossible to ensure that the next input line is less than some arbitrary length, and because overflowing the input buffer is almost invariably a security violation, programs should use gets(). The gets() function exists purely to conform to ANSI X3.159-1989 (“ANSI C89”).

July 17, 2013 OpenBSD-5.4