Check XML file against DTD file using expat in C - c

I need to build an application the should parse an XML file using EXPAT and also check it against a DTD file. The problem is how do I do this, I checked the documentation, and couldn't find my way around.
My code so far:
int main(int argc, char *argv[])
{
XML_Parser p = XML_ParserCreate(NULL);
if (!p)
{
fprintf(stderr, "Couldn't allocate memory for parser\n");
exit(-1);
}
FILE* xmlFile;
xmlFile = fopen("test.xml", "r");
XML_SetElementHandler(p, start, end);
XML_SetParamEntityParsing(p, XML_PARAM_ENTITY_PARSING_ALWAYS);
for (;;)
{
int done;
int len;
len = (int) fread(Buff, 1, BUFFSIZE, xmlFile);
if (ferror(xmlFile))
{
fprintf(stderr, "Read error\n");
exit(-1);
}
done = feof(xmlFile);
if (XML_Parse(p, Buff, len, done) == XML_STATUS_ERROR)
{
fprintf(stderr, "Parse error at line %" XML_FMT_INT_MOD "u:\n%s\n",
XML_GetCurrentLineNumber(p),
XML_ErrorString(XML_GetErrorCode(p)));
exit(-1);
}
if (done)
break;
}
XML_ParserFree(p);
return 0;
}
I guess that the should be a call to the XML_SetExternalEntityRefHandler function, but I fail to understand how to use it.
Thank you in advance.

Related

Unable to read from file from argv

I am trying to read from a file, and here is my code, but as I run my code nothing shows up. Have I used the getline() function incorrectly? I can not understand my problem.
const char *READ = "r";
/**
* main - Entry point of my program
*
* Return: On success, it returns 0. On
* error it returns -1
*/
int main(int ac, char **av)
{
FILE *fpointer;
char *lineptr = NULL;
size_t *n = 0;
int line_number = 1;
if (ac != 2)
{
fprintf(stderr, "USAGE: monty file\n");
exit(EXIT_FAILURE);
}
fpointer = fopen(av[1], READ);
if (fpointer == NULL)
{
fprintf(stderr, "Error: Can't open file %s\n", av[1]);
exit(EXIT_FAILURE);
}
while (getline(&lineptr, n, fpointer) != -1)
{
printf("Line %d: %s\n", line_number, lineptr);
line_number++;
}
return (0);
}
getline(&lineptr, n, fpointer) returns -1. You did not explicitly check this and print an error message.
Checking errno it's because of EINVAL: invalid argument. Also good to check errno.
Reason is that n is NULL, while a pointer to an existing size_t is required.
BTW, indenting with 8 spaces is rather uncommon; I'd stay with 4 space. (Also, never use TAB characters.)
It's advisable to stick with extremely common argc and argv.
Nice you put {s on a further empty line; I like that style.
You'd get this:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
const char *READ = "r";
int main(int argc, char **argv)
{
FILE *fpointer;
char *lineptr = NULL;
size_t n;
int line_number = 1;
if (argc != 2)
{
fprintf(stderr, "USAGE: monty file\n");
exit(EXIT_FAILURE);
}
fpointer = fopen(argv[1], READ);
if (fpointer == NULL)
{
fprintf(stderr, "Error: Can't open file '%s'.\n", argv[1]);
exit(EXIT_FAILURE);
}
if (getline(&lineptr, &n, fpointer) == -1)
{
printf("Failed to read file '%s': %s.\n", argv[1], strerror(errno));
exit(EXIT_FAILURE);
}
do
{
printf("Line %4d: %s\n", line_number, lineptr);
line_number++;
}
while (getline(&lineptr, &n, fpointer) != -1);
return (0);
}
Declaration of getline:
ssize_t getline(char **lineptr, size_t *n, FILE *stream);
As an output parameter, the type of n is size_t *. It points to a space for writing by getline.
But in your code, n points to 0, which is NOT a vaild addr to write in.

C programming, copying from one file to another using command line arguments

This is my code
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
if (argc < 4) {
printf("Missing arguments\n");
return -1;
}
// Check if buffer is valid before reading anything
int bufferSize = atoi(argv[3]);
if (!bufferSize || bufferSize < 1) {
printf("Invalid buffer size\n");
return -1;
}
printf("*** Copying from '%s' to '%s' (Buffer size: %dB) ***\n",
argv[1], argv[2], bufferSize);
// READ SOURCE FILE
FILE *inputFile = fopen(argv[1], "r");
if (!inputFile) {
printf("Error opening source file\n");
return -1;
}
// READ DESTINATION FILE
FILE *outputFile = fopen(argv[2], "w");
if (!outputFile) {
printf("Error opening destination file\n");
return -1;
}
int buffer[bufferSize];
int bytes;
do {
bytes = fread(buffer, 1, bufferSize, inputFile);
if (fwrite(buffer, 1, bytes, outputFile) != bytes) {
printf("Error writing into destination file\n");
return -1;
}
} while (bytes > 0);
fclose(inputFile);
fclose(outputFile);
return 0;
}
But when I try to exe the file it doesn't work. What could be the problem?
Here's the command line:
/Users/jurajc/Documents/Program/C/L1\ 1/C_program/c_program file.txt fileCopy.txt 512
*** Copying from 'file.txt' to 'fileCopy.txt' (Buffer size: 512B) ***
Error opening source file
The input file file.txt cannot be opened: either because it is not present in the current directory or because you do not have read access to it.
You should output more informative error messages. Note also these problems:
if (!bufferSize || bufferSize < 1) is a redundant test. if (bufferSize < 1) is sufficient.
the error messages should be output to stderr
the files should be open in binary mode to reliably copy all file types on legacy systems.
the read/write loop is incorrect: you should stop when fread returns 0 before attempting to write 0 elements to the output file.
Here is a modified version:
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
int main(int argc, char *argv[]) {
if (argc < 4) {
fprintf(stderr, "Missing arguments\n");
return -1;
}
// Check if buffer is valid before reading anything
int bufferSize = atoi(argv[3]);
if (bufferSize < 1) {
fprintf(stderr, "Invalid buffer size: %s\n", argv[3]);
return -1;
}
printf("*** Copying from '%s' to '%s' (Buffer size: %dB) ***\n",
argv[1], argv[2], bufferSize);
// READ SOURCE FILE
FILE *inputFile = fopen(argv[1], "rb");
if (!inputFile) {
fprintf(stderr, "Error opening source file %s: %s\n",
argv[1], strerror(errno));
return -1;
}
// READ DESTINATION FILE
FILE *outputFile = fopen(argv[2], "wb");
if (!outputFile) {
fprintf(stderr, "Error opening destination file %s: %s\n",
argv[2], strerror(errno));
return -1;
}
int buffer[bufferSize];
int bytes;
while ((bytes = fread(buffer, 1, bufferSize, inputFile)) != 0) {
if (fwrite(buffer, 1, bytes, outputFile) != bytes) {
fprintf(stderr, "Error writing into destination file: %s\n", strerror(errno));
return -1;
}
}
fclose(inputFile);
fclose(outputFile);
return 0;
}

Copying a file with libssh

I am trying to write a simple program (toy example) that copies a file from a remote host to the local machine.
It works when I try to copy a txt file, but not for files like mp4.
Here is my code, which is basically parts stitched together from the tutorial: https://pastebin.com/0FPrmeDx
This is where the error happens:
int scp_receive(ssh_session session, ssh_scp scp)
{
int rc;
int size, mode;
char *filename, *buffer;
rc = ssh_scp_pull_request(scp);
if (rc != SSH_SCP_REQUEST_NEWFILE)
{
fprintf(stderr, "Error receiving information about file: %s\n",
ssh_get_error(session));
return SSH_ERROR;
}
size = ssh_scp_request_get_size(scp);
filename = strdup(ssh_scp_request_get_filename(scp));
mode = ssh_scp_request_get_permissions(scp);
printf("Receiving file %s, size %d, permisssions 0%o\n",
filename, size, mode);
free(filename);
buffer = malloc(size);
if (buffer == NULL)
{
fprintf(stderr, "Memory allocation error\n");
return SSH_ERROR;
}
ssh_scp_accept_request(scp);
rc = ssh_scp_read(scp, buffer, size);
if (rc == SSH_ERROR)
{
fprintf(stderr, "Error receiving file data: %s\n",
ssh_get_error(session));
free(buffer);
return rc;
}
printf("Done!!!!!!!!!!!!!!!!!!!!!!!!!!!!!\n");
int filedesc = open("/home/user/video.mp4", O_WRONLY | O_CREAT);
if (filedesc < 0) {
return -1;
}
write(filedesc, buffer, size);
free(buffer);
close(filedesc);
rc = ssh_scp_pull_request(scp);
if (rc != SSH_SCP_REQUEST_EOF)
{
fprintf(stderr, "Unexpected request: %s\n",
ssh_get_error(session));
return SSH_ERROR;
}
return SSH_OK;
}
Error is fired with the code:
rc = ssh_scp_pull_request(scp);
if (rc != SSH_SCP_REQUEST_EOF)
{
fprintf(stderr, "Unexpected request: %s\n",
ssh_get_error(session));
return SSH_ERROR;
}
This is the error that I get:
Unexpected request: ssh_scp_pull_request called under invalid state
I tried to figure it out, but I couldn't make any progress on it.
Update 1:
The size of the copied file is exactly the same as of the source file both for txt and mp4 files. However, the copied file seems to be largely empty...
When copied, the permissions are changed from -rwxr-xr-x to --wxr-----.
Update 2:
It seems that the file size plays a major role here. Very small files (10-15kb) are copied without problems. Bigger files are not copied and produce the above mentioned error...
You cannot expect, that ssh_scp_read() reads the whole data in a single call to it. You have to iterate, until no more data is left to read:
int r = 0;
while (r < size) {
int st = ssh_scp_read(scp, buffer+r, size-r);
r += st;
}
Now, a subsequent call to ssh_scp_pull_request(scp) should succeed.

Problems with C XOR executable file encryption / decryption

I'm trying to create a simple XOR crypter / decrypter in C for .exe files. I'm still pretty new in C and don't understand everything yet, especially memory stuff. So I've been following an online tutorial on how to make a simple XOR string crypter which worked fine. Now I wanted to modify it so I can en/decrypt executable files and decided to utilize the fwrite() and fread() functions. This is what I've come up with:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h> // execve function
#define XOR_KEY 0xAA // key
#define JOB_CRYPT 1 // alter flow depending on the job
#define JOB_DECRYPT 2
//////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////
void xorFile (char *infile, char *outfile) {
FILE *nFile, *eFile;
long nFileSize; // store file size of the file we want to read
char *buffer; // buffer for reading
char *eBuffer; // buffer for storing encrypted data
size_t rResult;
size_t wResult;
///// READ FILE /////
nFile = fopen(infile, "rb");
if(nFile == NULL) {
fputs("Error opening file...", stderr);
exit(EXIT_FAILURE);
}
fseek(nFile, 0, SEEK_END);
nFileSize = ftell(nFile);
rewind(nFile);
buffer = (char *) malloc(sizeof(char) * nFileSize);
if(buffer == NULL) {
fputs("Error allocating memory...", stderr);
exit(EXIT_FAILURE);
}
rResult = fread(buffer, 1, nFileSize, nFile);
if(rResult != nFileSize) {
fputs("Error reading file...", stderr);
exit(EXIT_FAILURE);
}
fclose(nFile);
printf("File size is: %ld\n", nFileSize);
printf("Buffer size is (pointer): %u\n", sizeof(buffer));
printf("Reading result: %lu\n", rResult);
////// WRITE TO FILE //////
eFile = fopen(outfile, "wb");
if(eFile == NULL) {
fputs("Error creating file...", stderr);
exit(EXIT_FAILURE);
}
eBuffer = (char *) malloc(sizeof(char) * nFileSize);
if(eBuffer == NULL) {
fputs("Error allocating memory (2)...", stderr);
exit(EXIT_FAILURE);
}
// encrypt byte by byte and save to buffer
printf("Proceeding with encryption!\n");
for(int i = 0; buffer[i] != EOF; i++) {
eBuffer[i] = buffer[i] ^ XOR_KEY;
}
printf("Proceeding with fwrite()!\n");
wResult = fwrite(eBuffer, 1, nFileSize, eFile);
fclose(eFile);
printf("eBuffer size is (pointer)%u\n", sizeof(eBuffer));
printf("Writing result: %lu\n", wResult);
free(buffer); // free buffers in heap
free(eBuffer);
}
//////////////////////////////////////////////////////////////////
//////////////////////////////////////////////////////////////////
int main(int argc, char *argv[]) {
// checking if all parameters were given
if(argc < 4) {
fprintf(stderr, "Usage: %s [CRYPT | DECRYPT] [IN-FILE] [OUT-FILE]\n", argv[0]);
exit(EXIT_FAILURE);
}
int job;
// DOLOCIMO JOB
if(strcmp(argv[1], "CRYPT") == 0) {
job = JOB_CRYPT;
} else if (strcmp(argv[1], "DECRYPT") == 0) {
job = JOB_DECRYPT;
} else {
fprintf(stderr, "Please select [CRYPT | DECRYPT]!");
exit(EXIT_FAILURE);
}
// CRYPT/DECRYPT OUR FILE
xorFile(argv[2], argv[3]);
if(job == JOB_DECRYPT) {
char *args[] = {argv[3], NULL};
int errExec = execve(args[0], args, NULL);
if(errExec == -1) {
perror("Error executing file...");
exit(EXIT_FAILURE);
}
}
return 0;
}
I'm sorry for the ugly looking code but I first wanted to make it work and I'll refine it later.
Anyways, when I run it in command prompt, the encryption works fine, it generates an encrypted file, but when I run the decrpytion job, the program
crashes during the decryption process. Here's a picture of what happens so you can imagine it better.
Since I have less than 10 reputation, I'm not allowed to embedd pictures.
Here is a link to Imgur.
What's going wrong here? Am I creating a buffer overflow when I'm decrypting it?
Thank you!
Here's the problem:
for(int i = 0; buffer[i] != EOF; i++) {
eBuffer[i] = buffer[i] ^ XOR_KEY;
}
Binary files can contain bytes with any value. So the EOF value is valid and does not designate the end of the file. This means that if the file contains a byte with this value, the loop will quit early and you won't XOR all the bytes. If the file does not contain this byte, the loop will run past the end of the allocated memory which invokes undefined behavior which in this case manifests in a crash.
You know how many bytes you need to processes, so use that as your loop control:
for(int i = 0; i < nFileSize; i++) {
A few other minor corrections:
buffer = (char *) malloc(sizeof(char) * nFileSize);
if(buffer == NULL) {
fputs("Error allocating memory...", stderr);
exit(EXIT_FAILURE);
}
Don't cast the return value of malloc. Also, sizeof(char) is 1 by definition, so you can leave that out.
Also, if a system or library function fails, you should use perror to print the error message. This will print additional information regarding why the function failed.
buffer = malloc(nFileSize);
if(buffer == NULL) {
perror("Error allocating memory...");
exit(EXIT_FAILURE);
}

I am trying to print a txt file and it doesn't work in C homework

I'm writing code that's supposed to verify that a .txt file is a certain format.
I wrote my code as I saw in a tutorial and in the website
and for some reason my program doesn't even print my file.
Can you tell me what I'm doing wrong?
The code will do something far more complex, but I'm still trying to work on my basics.
Here's my code so far:
int main(int argc, char *argv[]) {
/* argv[0] = name of my running file
* argv[1] = the first file that i receive
*/
define MAXBUFLEN 4096
char source[MAXBUFLEN + 1];
int badReturnValue = 1;
char *error = "Error! trying to open the file ";
if (argc != 2) {
printf("please supply a file \n");
return badReturnValue;
}
char *fileName = argv[1];
FILE *fp = fopen(argv[1], "r"); /* "r" = open for reading */
if (fp != NULL) {
size_t newLen = fread(&source, sizeof(char), MAXBUFLEN, fp);
if (ferror(fp) != 0) {
printf("%s %s", error, fileName);
return badReturnValue;
}
int symbol;
while ((symbol = getc(fp)) != EOF) {
putchar(symbol);
}
printf("finish");
fclose(fp);
}
else {
printf("%s %s", error, fileName);
return badReturnValue;
}
}
I think you need a bit more explanations:
#include <stdio.h>
#include <stdlib.h>
#include <errno.h>
#include <string.h>
// there might be a macro BUFLEN defined in stdio
// which size is optimized for reading in chunks.
// Test if avaiable otherwise define it
#ifndef BUFLEN
# define BUFLEN 4096
#endif
int main(int argc, char *argv[])
{
char source[BUFLEN];
char *filename;
FILE *fp;
size_t fpread, written;
char c;
int ret_fclose;
if (argc != 2) {
fprintf(stderr, "Usage: %s filename\n", argv[0]);
exit(EXIT_FAILURE);
}
// reset errno, just in case
errno = 0;
// work on copy
filename = malloc(strlen(argv[1]) + 1);
if (filename == NULL) {
fprintf(stderr, "Allocating %zu bytes failed\n", strlen(argv[1]) + 1);
exit(EXIT_FAILURE);
}
filename = strcpy(filename, argv[1]);
// try to open the file at 'filename'
fp = fopen(filename, "r");
if (fp == NULL) {
fprintf(stderr, "Opening file \"%s\" filename failed\n", filename);
// errno might got set to something usable, check and print
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
// You have two options here. One is to read in chunks of MAXBUFLEN
while ((fpread = fread(&source, 1, BUFLEN, fp)) > 0) {
// Do something with the stuff we read into "source"
// we do nothing with it here, we just write to stdout
written = fwrite(&source, 1, fpread, stdout);
// you can use 'written' for error check when writing to an actual file
// but it is unlikely (but not impossible!) with stdout
// test if we wrote what we read
if ((fpread - written) != 0) {
fprintf(stderr, "We did not write what we read. Diff: %d\n",
(int) (fpread - written));
}
}
// fread() does not distinguish between EOF and error, we have to check by hand
if (feof(fp)) {
// we have read all, exit
puts("\n\n\tfinish\n");
// No, wait, we want to do it again in a different way, so: no exit
// exit(EXIT_SUCCESS);
} else {
// some error may have occured, check
if (ferror(fp)) {
fprintf(stderr, "Something bad happend while reading \"%s\"\n", filename);
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
}
// the other way is to read it byte by byte
// reset the filepointers/errors et al.
rewind(fp);
// rewind() should have reseted errno, but better be safe than sorry
errno = 0;
printf("\n\n\tread and print \"%s\" again\n\n\n\n", filename);
// read one byte and print it until end of file
while ((c = fgetc(fp)) != EOF) {
// just print. Gathering them into "source" is left as an exercise
fputc(c, stdout);
}
// clean up
errno = 0;
ret_fclose = fclose(fp);
// even fclose() might fail
if (ret_fclose == EOF) {
fprintf(stderr, "Something bad happend while closing \"%s\"\n", filename);
if (errno != 0) {
fprintf(stderr, "Error: %s\n", strerror(errno));
}
exit(EXIT_FAILURE);
}
// The macros EXIT_FAILURE and EXIT_SUCCESS are set to the correct values for
// the OS to tell it if we had an eror or not.
// Using exit() is noot necessary here but there exits teh function atexit()
// that runs a given function (e.g: clean up, safe content etc.) when called
exit(EXIT_SUCCESS);
}
You read from the file twice but only print once.
If the file is to small the first reading will read all of the contents, and the second reading will not produce anything so you don't print anything.
I believe you have to reset the pointer after using fread.
Try fseek(fp, SEEK_SET, 0) to reset the pointer to the beginning of the file. Then print the file.

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