How to change some letter in a file with the buffer and lseek

Question

i'm having some trouble using lseek and the buffer for an assigment. The objective is to read a file and change every letter 'a' to a '?'. I'm running some minor programs to understand how the functions and buffer works and i'm having some trouble.. Imagine that my file "teste" has only "abcabcabc" in it. If i do this:

int fd = open("teste", O_RDWR);
char buf[1];
int fptr = lseek(fd, 0, SEEK_SET);
fflush(stdout);
read(fd, buf, 1);
printf("%s\n", buf);

i get on my console "a", so it reads well the first letter, because i put the pointer to the beggining. But if i do a if condition before the printf, comaparing buf to 'a', like:

if(buf == 'a') printf("%s\n", buf);

It doesn't work, it doesn't print anything, so it doesn't enter the if statement.. I need to compare the buffer to letters so i can change all 'a' of the file.. How can i do this guys?

Ok, this part is already solved due to the answers bellow, but now i'm trying to read all the file and compare each charecter to 'a', making a simple printf just to see if it's working.. i wrote this:

int fd = open("teste", O_RDWR);
char buf[1];
int fptr = lseek(fd, 1, SEEK_SET);
fflush(stdout);
read(fd, buf, 1);
while(fptr > 0){
  read(fd, buf, 1);
  if(buf[0] == 'a'){
    printf("%s\n",buf);
  }
  fflush(stdout);
  fptr=lseek(fd, (off_t)(1), SEEK_CUR);
}
close(fd);

But it's now working.. It prints only one'a', and then doesn't close and don't do anything.. Its like an infinite cycle but without entering the if statement. What's wrong?

`printf("%s\n", buf);` is [UB](https://en.wikipedia.org/wiki/Undefined_behavior). `%s` wants a `NULL` terminated string. — LPs, Mar 24 '16 at 16:09
but without the if statement it gives me the right output... The problem is comparing buf with 'a'.. — César Pereira, Mar 24 '16 at 16:12
In C a string literal is between double-quotes. A single character literal is in single-quotes. Also in C a string is an array of character bytes that ends with a zero, aka `'\0'`. Your `char buf[1]` is too small to contain a zero and so your printf is likely to continue reading memory until it finds one, and that is likely to result in random-looking strings. Since your `buf` is an array `buf == 'a'` is actually something like `0x601035 == 0x61` which will never match. Your compiler should have yelled loudly at you about mismatched variable types. If it didn't, increase the warning level. — Zan Lynx, Mar 24 '16 at 16:23
When i changed %s to %c i get "w" on the console, and i don't even have any 'w' on my file O.o — César Pereira, Mar 24 '16 at 17:11
Take note that read already move fd forward of 1 char each time a char is read. You do not need lseek inside loop. — LPs, Mar 24 '16 at 17:18
that is true... I can't read all sentence. Can you write me a code of how can i do it? Read each charecter and compare each? — César Pereira, Mar 24 '16 at 17:20
I'm sure you didn't wrote into you code printf("%c\n", buf[0]); — LPs, Mar 24 '16 at 17:22
Let us [continue this discussion in chat](http://chat.stackoverflow.com/rooms/107254/discussion-between-cesar-pereira-and-lps). — César Pereira, Mar 24 '16 at 17:24

LPs · Accepted Answer · 2016-03-24T16:59:28.547

2

printf("%s\n", buf); is UB. %s wants a NULL terminated string. Use printf("%c\n", buf[0]);
if(buf == 'a') must be if(buf[0] == 'a'). You are comparing address of buf with 'a' char but you want to compare the content of first (an unique) cell of buf array.

edited Mar 24 '16 at 16:59

answered Mar 24 '16 at 16:11

LPs

16,045
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When i change the printf it prints me 'w', and i don't even have any 'w' on my file.. With your second suggestion it already works.. But now can you see what i added to the question and help me out on that stage now? @LPs – César Pereira Mar 24 '16 at 17:12
I commented the post. See there. – LPs Mar 24 '16 at 17:19

score 1 · Answer 2 · edited Mar 24 '16 at 16:19

1

buf is a pointer containing the address. So the conditon

if(buf=='a')

becomes false and doesnt goes into if block.
Try using

if(*buf=='a') OR if(buf[0]=='a')

edited Mar 24 '16 at 16:19

kvorobiev

5,012
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answered Mar 24 '16 at 16:14

Mukesh

83
6

thanks, that worked.. Can you see what i added to the question and help me out at that stage now? @MukeshKumarChaursiya – César Pereira Mar 24 '16 at 17:05
use while(*buf != '\n') and in printf use printf("%c\n",*buf); and remove dat line calling lseek in while – Mukesh Mar 24 '16 at 20:13

How to change some letter in a file with the buffer and lseek

2 Answers2