Behaviour of uninitialised char array

Question

I have written a program to practice using fread. I am passing a command to popen and reading the output using fread and storing into an uninitialised char array. Finally I am printing the contents of the array as well as writing it into another file. The print (output in the stdout) works fine but when I check the contents of the file, it has so many junk things after the actual output. Any reason why it works well in stdout? Here is my code:

#include <stdio.h>

void fread_s(char *res, int res_size,char *cmd)
{
    char *r;
    int size;
    FILE *fp=popen(cmd,"r");
    if(fp==NULL)
            return;
    size=fread(res,res_size,1,fp);
    pclose(fp);
}

int main()
{
    char cmd[500];
    fread_s(cmd,sizeof(cmd),"ls -lh i*");
    puts(cmd);
    FILE *fp=fopen("opfile","a+");
    if(fp==NULL)
            return;
    fwrite(cmd,sizeof(cmd),1,fp);
}

@md5 fgets() wouldn't work if ls output has newlines in it. — ebyrob
– ebyrob, Commented Sep 2, 2016 at 17:23

R Sahu · Accepted Answer · 2016-09-02 17:17:42Z

4

You are seeing the unwanted side effects of a char array that is not null terminated. Change:

size=fread(res,res_size,1,fp);

to

size=fread(res,res_size,1,fp);
res[size] = '\0';

Also, make sure that res_size is one less than sizeof(cmd). Otherwise, you take the chance that size is sizeof(cmd) and hence,

res[size] = '\0';

will modify the array using an out of bounds index.

Change

fread_s(cmd,sizeof(cmd),"ls -lh i*");

to

fread_s(cmd,sizeof(cmd)-1,"ls -lh i*");

or change

size=fread(res,res_size,1,fp);

to

size=fread(res,res_size-1,1,fp);

answered Sep 2, 2016 at 17:17

R Sahu

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1 Comment

ebyrob Over a year ago

size=fread(res,res_size-1,1,fp); seems correct, it would make fread_s() a pretty inconsistent function to expect: fread_s(cmd,sizeof(cmd)-1,"ls -lh i*"); meaning it would overwrite the end of the size passed in by 1 char.

chux · Accepted Answer · 2016-09-02 18:08:18Z

Code has size=fread(res,res_size,1,fp), but does not use size, which indicates the number of elements read.

The following works even if the data read contains embedded null characters.

// Return the number of elements read
size_t fread_s(char *res, int res_size,char *cmd) {
  char *r;
  size_t size;
  FILE *fp = popen(cmd,"r");
  if (fp == NULL) {
    return 0;
  }
  size = fread(res,res_size,1,fp);
  pclose(fp);
  return size;
}

Only write valid number of elements.

int main(void) {
    char cmd[500];
    size_t size = fread_s(cmd,sizeof(cmd),"ls -lh i*");

    // puts(cmd);
    fwrite(cmd, size, 1, stdout);

    FILE *fp=fopen("opfile","a+");
    if(fp==NULL) {
            return -1; // Add value to return
    }
    fwrite(cmd, size, 1, fp);
    fclose(fp); // add fclose
}

Community · Accepted Answer · 2017-05-23 12:08:35Z

1

When you write in standard output, you write a string using puts. The string is not null terminated, but there's a strong probability that puts encournters a null char in the junk chars. Some junk may be printed, but not all, since a zero can be encoutered.

(this question looks very much like that one, just saw that: C Language: popen() with fread()?)

When you write to your file, you will the whole 500 chars, including junk chars all the way (500 chars)

To get the same behaviour (but still some probable junk):

fwrite(cmd,strlen(cmd),1,fp);

or

fputs(cmd,fp);

but you have to null terminate your string after reading the output of popen.

edited May 23, 2017 at 12:08

CommunityBot

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answered Sep 2, 2016 at 16:59

Jean-François Fabre♦

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4 Comments

P.P Over a year ago

The null termination ensure that no junk chars are printed afterwards. where is it null terminated? The suggestions won't because cmd isn't null terminated either.

ebyrob Over a year ago

In this case cmd is not null-terminated since fread() doesn't null terminate the binary bytes written.

Jean-François Fabre Over a year ago

Okay, you're right. But if there's a null char just after puts or not that far, output will stop. Some junk may be visible or not depending how far the zero is.

Jean-François Fabre Over a year ago

I have updated my answer. That could explain the junk not being displayed, but that's mere luck.

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