Getting garbled output from fgets() in C

Question

#include <stdio.h>

int main(int argc, char *argv[]) {
  FILE *fp = fopen("sss", "w+");
  char buf[100];
  fputs("hello, world", fp);
  fflush(fp);
  fgets(buf, 100, fp);
  fputs(buf, stdout);
  fclose(fp);
  return 0;
}

Can someone teach me what's wrong with my code? Below is my test, but I didn't get what I expected:

% clang test.c
% ./a.out
��h��%  
% cat test.c
hello, world%

My assumption is that this is a problem of character encoding, but when I use fgets to read the text from an existing file, it works fine. All my files (and code) are written in Emacs, I don't know what causes this garbled output

should close and reopen the file in read mode.. and add proper error handling — amdixon, Jul 15 '15 at 10:37
@amdixon - "w+" means "**Open for reading and writing**. The file is created if it does not exist, otherwise it is truncated. The stream is positioned at the beginning of the file." per http://linux.die.net/man/3/fopen — Andrew Henle, Jul 15 '15 at 10:39
@BLUEPIXY - `fseek()` works too. http://linux.die.net/man/3/fseek — Andrew Henle, Jul 15 '15 at 10:40

P.P · Accepted Answer · 2015-07-15T10:44:22.360

2

fp is at the end after you write. Move it to initial position before using fseek reading again:

fflush(fp);
fseek(fp, 0, SEEK_SET);
fgets(buf, 100, fp);

Other option is to fclose it and fopen it again. It depends on what you want to do.

edited Jul 15 '15 at 10:44

answered Jul 15 '15 at 10:40

P.P

117,907
20
175
238

And clean the buffer before reading, and maybe after creating. – Alex Kroll Jul 15 '15 at 10:44
@AlexKroll What you do mean by "clean"? `fflush` would flush any buffered output. – P.P Jul 15 '15 at 10:46
I mean `memset(buf, 0, 100)`. – Alex Kroll Jul 15 '15 at 10:48
`memset` is needless. Just checking the return of `fgets` should be enough. OP should error checking for all library functions. – P.P Jul 15 '15 at 10:50
It's the problem of stream position. I use fseek and succeed, thanks – tian tong Jul 15 '15 at 10:53
Agree. Just for sure but not necessarily. – Alex Kroll Jul 15 '15 at 10:53

Getting garbled output from fgets() in C

1 Answers1