Why system call read() does not work when using user input

Question

It`s a file copying program.

#include <stdio.h>
#include <fcntl.h>
#include <sys/stat.h>
#include <stdlib.h>

int main()
{
    int fd1,fd2, ndata;
    char data[128];
    char *openname[1], *creatname[1]; 

    write(1, "Write open file name\n", 24); //user input

    read(0, openname, 30 );

    write(1, "Write creat file name\n", 25); //user input

    read(0, creatname,30);

    if((fd1 = open(openname, 0666))<0)
        {
            perror("cannot open the file");
            exit(1);
        }

    if((fd2 = creat(creatname,0666))<0)
        {
            perror("cannot create the file");
            exit(1);
        }

    while((ndata = read(fd1, data, 128))>0)
        {
            if((write(fd2, data, ndata))!=ndata)
                {
                    perror("cannot write the file");
                    exit(1); 
                } 
        }

    close(fd1);
    close(fd2); 

    write(1, "File copy is done.",19);

    return 0;

}

This code ain`t work. This code print the error message:

cannot open the file.

but if i change the code to this :

if((fd1 = open("copy.c", 0666))<0)

and this :

if((fd2 = creat("real.c",0666))<0)

worked well.

Why this error happend? Please answer.

`perror` would follow `cannot open the file` with a message explaining the `errno`. What is it for you? — Basile Starynkevitch, Mar 15 '14 at 10:22
sorry. that is the ENOENT : No such file or directory (POSIX.1) — mjkim, Mar 15 '14 at 10:25
you are using variable `openname` and `creatname` wrong. check by implementing break point that what you are getting in these variables. — Heena Goyal, Mar 15 '14 at 10:25

score 2 · Accepted Answer · answered Mar 15 '14 at 10:26

Your declarations of openname and creatname are incorrect. They should be:

char openname[31], creatname[31];

read() does not add a null terminator to the input, you need to add it. read() returns the number of bytes read. So it should be:

int nread = read(0, openname, sizeof openname -1);
openname[nread-1] = '\0'; // subtract 1 to overwrite the newline

score 1 · Answer 2 · answered Mar 15 '14 at 10:24

The type of openname and creatname is wrong, and gcc -Wall -g would have warned you. Declare e.g. char openname[256];

And you should use fgets(openname, sizeof(openname), stdin); to read it.

If you insist on using read, take care of the newline (if any) and add a zero terminating byte.

Learn also to use the gdb debugger.

score 1 · Answer 3 · edited May 23 '17 at 12:05

1

read is very low level. In this case, it reads 30 bytes, including your enter key and also without a terminating null-byte. So the filename won't be what you think you've entered, it will contain additional garbage (and could even make your program crash due to the missing null-termination). You want to use fgets or readline instead.

edited May 23 '17 at 12:05

Community

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answered Mar 15 '14 at 10:24

DarkDust

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score 1 · Answer 4 · answered Mar 15 '14 at 10:24

In a nutshell, by using read() to input the file names, you are making this unnecessarily hard for yourself: it does not terminate the input with NUL, is not guaranteed to read the number of characters you expect, etc.

My advice would be to stick with scanf() or fgets().

Why system call read() does not work when using user input

4 Answers4