Trying to write a program in C, which I've never used before, to convert hexadecimal to binary as a string and write it to a file, The only problem is that it prepends to the output file when I want it to append.
This occurs for both fprintf() and fputs().
This is the append code:
while(fgets(line,1024,inputFile)){
lineLen = strlen(line);
binary = hexToBin(line, lineLen);
printf("Binary: %s\n", binary);
// output file does not exist
FILE *writeFile = fopen(argv[2], "a");
if (writeFile == NULL) {
fprintf(stderr, "write file not found: %s\n", strerror(errno));
exit(1);
}
printf("appending binary to file\n");
fprintf(writeFile,"%s\n",binary);
printf("appended\n");
}
This is the hexToBin function:
char* hexToBin(char hex[], size_t len){
// convert hexadecimal to binary
char *binString = (char*)malloc(sizeof(char)*4*len);
for (int i=0;i<len;i++){
char *binNum = hexToBinHelp(hex[i]);
strcat(binString,binNum);
free(binNum);
}
return binString;
}
hexToBinHelp returns the hexadecimal character as a char* of it's binary representation (binNum=[0][0][0][0] for example). It's really long so I'd rather not put it here, but can if it would help.
When input file is:
000a
a000
Output file is:
1010000000000000
0000000000001010
Thanks
Your loop is calling fopen() every iteration, and never closing or flushing this file stream. Writes are buffered, waiting for the buffer to fill or be flushed or the file to be closed; so the next time you call fopen() you get the same pointer as the first time.
You're lucky it's not crashing. :) Do not call fopen() repeatedly. Call it once outside the loop, inside use or move the pointer over, and back outside call fclose().
Related
I'm trying to read a binary file of 32 bytes in C, however I'm keep getting "segmentation fault (code dumped)" when I run my program,
it would be great if somebody can help me out by pointing where did I go wrong?.
my code is here below:
int main()
{
char *binary = "/path/to/myfiles/program1.ijvm";
FILE *fp;
char buffer[32];
// Open read-only
fp = fopen(binary, "rb");
// Read 128 bytes into buffer
fread (buffer, sizeof(char), 32, fp);
return 0;
}
It's because of the path. Make sure that "/path/to/myfiles/program1.ijvm" points to an existing file.
You should always check the return value of fopen.
\\Open read-only
fp = fopen(binary, "rb");
if(fp==NULL){
perror("problem opening the file");
exit(EXIT_FAILURE);
}
Notice also that you are reading 32 bytes in your buffer and not 128 as your comment says.
You must check the return result from fopen().
I'm assuming you are getting the segfault in the fread() call because your data file doesn't exist, or couldn't be opened, and you are trying to work on a NULL FILE structure.
See the following safe code:
#include <stdio.h>
#include <stdint.h>
#define SIZE_BUFFER 32
int main()
{
char *binary = "data.txt";
FILE *fp = NULL;
char buffer[SIZE_BUFFER];
// Open read-only
fp = fopen(binary, "rb");
// Read SIZE_BUFFER bytes into buffer
if( fp )
{
printf("Elements read %ld\n", fread (buffer, sizeof(char), SIZE_BUFFER, fp));
fclose(fp);
}
else
{
// Use perror() here to show a text description of what failed and why
perror("Unable to open file: ");
}
return 0;
}
When I execute this code it doesn't crash and will print the number of elements read if the file is opened or it will print "Unable to open file" if the file could not be opened.
As mentioned in the comments you should also close the file being exiting. Another thing you can do is the following:
FILE *fp = fopen(.....);
Instead of declaring and assigning in two separate steps.
There are two possible reasons
The fopen(3) function failed due to some reason, which means fp is NULL, and then you are trying to use the null-pointer in fread(3). This can crash. #OznOg has already given a subtle hint to look into this direction.
If the fopen call is a success (i.e. fp is non-NULL after calling fopen), the code can still crash because you are reading 32 chars into the variable binary, while binary has been initialized with only 30 chars.
I'm working on an example problem where I have to reverse the text in a text file using fseek() and ftell(). I was successful, but printing the same output to a file, I had some weird results.
The text file I input was the following:
redivider
racecar
kayak
civic
level
refer
These are all palindromes
The result in the command line works great. In the text file that I create however, I get the following:
ÿsemordnilap lla era esehTT
referr
levell
civicc
kayakk
racecarr
redivide
I am aware from the answer to this question says that this corresponds to the text file version of EOF in C. I'm just confused as to why the command line and text file outputs are different.
#include <stdio.h>
#include <stdlib.h>
/**********************************
This program is designed to read in a text file and then reverse the order
of the text.
The reversed text then gets output to a new file.
The new file is then opened and read.
**********************************/
int main()
{
//Open our files and check for NULL
FILE *fp = NULL;
fp = fopen("mainText.txt","r");
if (!fp)
return -1;
FILE *fnew = NULL;
fnew = fopen("reversedText.txt","w+");
if (!fnew)
return -2;
//Go to the end of the file so we can reverse it
int i = 1;
fseek(fp, 0, SEEK_END);
int endNum = ftell(fp);
while(i < endNum+1)
{
fseek(fp,-i,SEEK_END);
printf("%c",fgetc(fp));
fputc(fgetc(fp),fnew);
i++;
}
fclose(fp);
fclose(fnew);
fp = NULL;
fnew = NULL;
return 0;
}
No errors, I just want identical outputs.
The outputs are different because your loop reads two characters from fp per iteration.
For example, in the first iteration i is 1 and so fseek sets the current file position of fp just before the last byte:
...
These are all palindromes
^
Then printf("%c",fgetc(fp)); reads a byte (s) and prints it to the console. Having read the s, the file position is now
...
These are all palindromes
^
i.e. we're at the end of the file.
Then fputc(fgetc(fp),fnew); attempts to read another byte from fp. This fails and fgetc returns EOF (a negative value, usually -1) instead. However, your code is not prepared for this and blindly treats -1 as a character code. Converted to a byte, -1 corresponds to 255, which is the character code for ÿ in the ISO-8859-1 encoding. This byte is written to your file.
In the next iteration of the loop we seek back to the e:
...
These are all palindromes
^
Again the loop reads two characters: e is written to the console, and s is written to the file.
This continues backwards until we reach the beginning of the input file:
redivider
^
Yet again the loop reads two characters: r is written to the console, and e is written to the file.
This ends the loop. The end result is that your output file contains one character that doesn't exist (from the attempt to read past the end of the input file) and never sees the first character.
The fix is to only call fgetc once per loop:
while(i < endNum+1)
{
fseek(fp,-i,SEEK_END);
int c = fgetc(fp);
if (c == EOF) {
perror("error reading from mainText.txt");
exit(EXIT_FAILURE);
}
printf("%c", c);
fputc(c, fnew);
i++;
}
In addition to #melpomene correction about using only 1 fgetc() per loops, other issues exist.
fseek(questionable_offset)
fopen("mainText.txt","r"); opens the file in text mode and not binary mode. Thus the using fseek(various_values) as a valid offset into the file is prone to troubles. Usually not a problem in *nix systems.
I do not have a simple alternative.
ftell() return type
ftell() return long. Use long instead of int i, endNum. (Not a concern with small files)
Check return values
ftell() and fseek() can fail. Test for error returns.
I'm trying to read a txt file containing strings of 1s and 0s and print it out in the manner below. I tried my code a couple of months ago and it worked fine in reading the text file. Now when I tried it, it outputs something really strange. Also I tried changing the directory of the file to a non-existant file but it still outputs the same thing when it should've quit the program immediately. Please help!
The content of txt file:-
10000001
01110111
01111111
01111010
01111010
01110111
Expected output:-
data_in<=24'b10000001;
#10000;
Real output:-
data_in<=24'b(some weird symbol that changes everytime I recompile);
#10000;
My code:-
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main (int argc, char *argv[])
{
int i, j;
j = 0;
char words[50];
FILE *fp;
fp = fopen (argv[1], "r");
if (fp == NULL) {
printf ("Can't open file\n");
}
while (feof (fp) == 0) {
fscanf (fp, "%s", words);
printf ("data_in<=24'b%s\n", words);
printf ("#10000\n");
}
fclose (fp);
system ("PAUSE");
return 0;
}
The input argument is the following:-
"C:\Users\Beanz\Documents\MATLAB\football frame\frame1.txt"
Read each line one by one with getline(3) -if available- or with fgets (you'll then need a large enough line buffer, at least 256 bytes), then parse each line buffer appropriately, using sscanf (the %n might be useful, and you should test the scanned item count result of sscanf) or other functions (e.g. strtok, strtol, etc...)
Remember that 'feof()' is only set AFTER trying to read PAST the end of the file, not when at the end of the file.
So the final iteration through the loop will try to read/process data that contains trash or prior contents.
Always check the returned value from 'fscanf()' before trying to use the associated data.
strongly suggest
eliminate the call to feof() and use the fscanf() to control the loop
Why does fprintf give different results in the following example programs?
Example 1:
int main(){
FILE *f;
char buf[512];
char name[128] = {"filename"};
f = fopen(name, "w");
fprintf(f, "asdas\n");
fprintf(f, "asdas\n");
while(1){}
return 0;
}
If I terminate this program using CTRL+C, I get an empty file named filename.
However, using
Example 2:
int main(){
FILE *f;
char buf[512];
char name[128] = {"wpa_supplicant.conf"};
f = fopen(name,"w");
while(1){
fprintf(f, "asdas\n");
}
return 0;
}
If I terminate this program using CTRL+C, I get file named filename, and it contains many lines with the string asdas.
Why are the strings not written to the file in the first example, but they are written to the file in the second example?
In the second case, there are enough fprintf calls for the internal buffers to be flushed to disk.
With the first program, if you put a fflush(f) before the while loop, the strings will be written to the file.
#include <stdio.h>
int main(void) {
FILE *f = fopen("filename", "w");
if (!f) {
perror("Failed to open 'filename' for writing");
exit(EXIT_FAILURE);
}
fprintf(f, "asdas\n");
fprintf(f, "asdas\n");
if ( fflush(f) != 0 ) {
perror("Flushing output failed");
exit(EXIT_FAILURE);
}
while(1){}
return 0;
}
Output:
C:\...\Temp> cl file.c
Microsoft (R) C/C++ Optimizing Compiler Version 18.00.31101 for x64
...
/out:file.exe
C:\...\Temp> file
^C
C:\...\Temp> type filename
asdas
asdas
Keep in mind:
Upon successful completion, fflush() shall return 0; otherwise, it shall set the error indicator for the stream, return EOF, and set errno to indicate the error.
As mentioned in the answer by #SinanÜnür this is indeed an issue with the buffering of data in internal buffers. You need to flush manually in the first case to get that data actually written into the file.
However, FWIW, I just want to add here, you see this behavior because of the abnormal termination of the program by a signal (generated by CTRL+C).
If your program would have ended normally, (for example, by calling exit(), after a large-enough but controlled while() loop), then both the cases would have shown the same behavior, as in that scenario, all the open streams would have been flushed automatically.
The exit() function shall then flush all open streams with unwritten buffered data and close all open streams. Finally, the process shall be terminated ...
I don't know exactly why a file pointer reads an extra line from a file, specifically the last line, here is the code:
FILE *fp ;
fp = fopen ("mac_ip.txt", "r") ;
int mac;
char *ip = (char *) malloc(15);
while(!feof(fp)){
fscanf(fp,"%i",&mac);
fscanf(fp,"%s",ip);
printf("MAC: %i\n",mac);
printf("IP: %s\n",ip);
}
and the file has exactly 20 lines, but the line 20, is printed twice.
Which is the error?
Thanks in advance.
Because after reading the last two values, you still haven't hit EOF. So the loop goes on. In the next pass of the loop, fscanf actually does not read the last line for the second time like it appears, the fscanfs fail, but the printfs print out the values from the previous pass of the loop.
feof does not "know" it's at the end of file until you try to read some more. Since fscanf tells you how many items it got, you can use this simple trick:
for(;;){
if (fscanf(fp,"%i%s", &mac, ip) != 2) break;
printf("MAC: %i\n",mac);
printf("IP: %s\n",ip);
}
After you have done the two reads on the twentieth line, you have got to the end of the file but the system doesn't know this. feof will only trigger when you try to get past the end of the file, not when you are exactly on it ...
Also, you may have a line-end (CR or CR-LF) on the 20th line which it will only get past with another attempted read.
The solution is to read the line in one go (there is a specific C command for this) and then parse that to get your data. If the whole-line read fails, then you've got to the end.
Your code resembles to the following example
#include <stdio.h>
int main(void)
{
char buffer[256];
FILE * myfile;
myfile = fopen("some.txt","r");
while (!feof(myfile))
{
fgets(buffer,256,myfile);
printf("%s",buffer);
}
fclose(myfile);
return 0;
}
from
http://www.friedspace.com/feof.html
You better test for fscanf return value before printing result. I bet that in the last iteration of your loop, fscanf calls fail and you print the last returned results.
FILE *fp ;
int mac;
char ip[15];
fp = fopen ("mac_ip.txt", "r") ;
if (!fp) return;
while(1){
if (fscanf(fp,"%i",&mac) < 1) break;
if (fscanf(fp,"%s",ip) < 1) break;
printf("MAC: %i\n",mac);
printf("IP: %s\n",ip);
}
fclose (fp);
fscanf() returns the number of assignments it mad (or -1 on eof). By using the return value, you don't need the eof() function. BTW I don't think you can read a MAC address into an int. Maybe you need to read that into a string, too ?
Explanation: feof() does not do what the OP expects. feof() should only be inspected after one of the file operations failed. In most cases you don't need feof().