How can I check if file is text (ASCII) or binary in C

Question

I need to write C code that checks to see if a file is text(ASCII) or Binary

Could someone help? Thanks

*Precisely* define what it means for a file to be "text" and what it means for a file to be "binary". Then test according to your definition. (If a file is empty, what is it? If it contains all alphabetic characters but a single null on the end, what is it?) Also, if you need to write C code, why tag the question C++? — David Schwartz, Jan 31 '13 at 03:36

score 5 · Answer 1 · answered Jan 31 '13 at 03:42

Typical method is to read the first several hundred bytes and look for ASCII NUL.

If the file contains NUL, it is definitely a binary file. Most binary files do contain NUL bytes, but text files should never contain NUL bytes.

#include <string.h>
bool is_binary(const void *data, size_t len)
{
    return memchr(data, '\0', len) != NULL;
}

Be warned that this is a heuristic. In other words, it will be wrong sometimes.

score 3 · Answer 2 · answered Jan 31 '13 at 03:41

3

Read all characters and see if all of them are ASCII, that is, with codes from 0 to 127 inclusive.

Some tools determine whether a file is a text file or a binary file by just checking whether or not it has any byte with code 0.

Clearly, if you apply both of these methods, you will get different results for some files, so, you have to define what it is exactly that you're looking for.

answered Jan 31 '13 at 03:41

Alexey Frunze

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Under this definition, various image formats might be "text" if no pixel intensity ever exceeds 127... – R.. GitHub STOP HELPING ICE Jan 31 '13 at 06:06
@R..GitHubSTOPHELPINGICE it's a very failsafe answer if he changes it to **1 to 127** and not *0 to 127* – A P Jo Sep 11 '20 at 15:39

score 0 · Answer 3 · answered Sep 29 '20 at 22:12

You can use libmagic. The code below will show you roughly the way the "file" command does it. (The code below is quick and dirty -- it probably needs to be cleaned up.)

#include <string.h>
#include <magic.h>
#include <stdio.h>


//------------------------------------------------------------------------------
struct magic_set * prep_magic(int flags)
{
struct magic_set *magic = magic_open(flags);
const char *errstring;
int action = 0;
const char *magicfile = NULL;

if (magicfile == NULL)
    magicfile = magic_getpath(magicfile, action);

if (magic == NULL)
    {
        printf("Can't create magic");
        return NULL;
    }
if (magic_load(magic, magicfile) == -1)
    {
        printf("%s", magic_error(magic));
        magic_close(magic);
        return NULL;
    }
if ((errstring = magic_error(magic)) != NULL)
        printf("%s\n", errstring);

return magic;
/* END FUNCTION prep_magic */ }
//------------------------------------------------------------------------------
int main(int argc, char **argv)
{
int flags = 0;
struct magic_set *msetptr = NULL;

const char *testfile = (char *)"/etc/motd";


msetptr = prep_magic(flags);
if( msetptr == NULL )
    printf("no mset ptr\n");

const char *typer;
typer = magic_file( msetptr, testfile );
printf("typer = %s\n", typer );

return 0;
/* END PROGRAM */ }

How can I check if file is text (ASCII) or binary in C

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