OpenBSD manual page server

NAME

fgets — get a line from a stream

SYNOPSIS

#include <stdio.h>

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

DESCRIPTION

The fgets() 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.

RETURN VALUES

Upon successful completion, fgets() returns a pointer to the string. If end-of-file or an error occurs before any characters are read, it returns NULL. The fgets() function does 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.

ERRORS

[EBADF]

The given stream is not a readable stream.

[EINVAL]

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).

SEE ALSO

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

STANDARDS

The function fgets() conforms to ANSI X3.159-1989 (“ANSI C89”).

HISTORY

The function fgets() first appeared in Version 7 AT&T UNIX.

CAVEATS

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';