fgets() function in C

Question

I know everybody has told me to use fgets and not gets because of buffer overflow. However, I am a bit confused about the third parameter in fgets(). As I understand it, fgets is dependent on:

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

char* str is the ptr to where my input will be stored.

num is the max number of character to be read.

but what is FILE *stream? If I am just prompting the user to enter a string (like a sentence) should I just type "stdin" ?

And should I type FILE *stdin at the top, near main()?

Carl Norum · Accepted Answer · 2010-01-04 19:42:13Z

17

You are correct. stream is a pointer to a FILE structure, like that returned from fopen. stdin, stdout, and stderr are already defined for your program, so you can use them directly instead of opening or declaring them on your own.

For example, you can read from the standard input with:

fgets(buffer, 10, stdin);

Or from a specific file with:

FILE *f = fopen("filename.txt", "r");
fgets(buffer, 10, f);

edited Jan 4, 2010 at 19:42

answered Jan 4, 2010 at 19:36

Carl Norum

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2 Comments

ant2009 Over a year ago

I think the second parameter should be strlen(buffer) -1. Unless you buffer is a 11 bytes.

Carl Norum Over a year ago

@robUK, that would only work if the buffer weren't all zeros to begin with. As the OP mentioned, buffer overflows aren't really the concern of the question.

Brian Campbell · Accepted Answer · 2010-01-04 19:37:24Z

4

Yes, you should just use stdin. That is a predefined FILE * that reads from the standard input of your program. And it should already be defined if you have a #include <stdio.h> at the top of your file (which you'll need for fgets).

answered Jan 4, 2010 at 19:37

Brian Campbell

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user108127 · Accepted Answer · 2010-01-04 19:41:39Z

3

Broadly there are two ways you can communicate with files in C. One is using the low-level OS dependent system calls such as open(), read(), write() etc which work with file descriptors. Another one is using FILE structures which are used in C library functions like fread(), fwrite() etc including the one you mentioned above.

As it is with the UNIX philosophy, everything is a file. So even the standard input (stdin) is treated as a pointer to a FILE structure.

tl;dr Yes, you should use stdin for FILE* stream in your call to fgets()

answered Jan 4, 2010 at 19:41

user108127

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David Nehme · Accepted Answer · 2010-01-04 19:36:48Z

1

FILE is the standard C file. Yes, if you want to read from standard input, stdin is the correct symbol.

answered Jan 4, 2010 at 19:36

David Nehme

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