fread is not reading other file formats - c

Question

I am fairly new to C still, but the program below compiles just fine, (using gcc) and it even works when using text files, but I when I use other file formats, i.e. png, I get nothing. The console spits out ?PNG and nothing else. I don't want the image to print as an image, obviously the program does nothing like that, but I would like the data from the png file to be printed. Why is the program not fread-ing properly? Is is because fread refuses any file other than text?
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
FILE *fp;
int main() {
char buffer[1000];
fp=fopen("FILE IN QUESTION HERE", "rb");
if(fp==NULL) {
perror("An error occured while opening the file...");
exit(1);
}
fread(buffer, 1000, 1, fp);
printf("%s\n", buffer);
fclose(fp);
return 0;
}

Answer 1

%s in printf() is for printing null-terminated string, not binary data and PNG header contains a signature to prevent the data from being transfered as text by mistake.
(Actually there are no 0x00 in the PNG signature and printf() stopped at the 0x00 contained in the size of IHDR chunk)
Use fwrite() to output binary data, or print the bytes one-by-one via putchar().
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
int main(void) {
FILE* fp; /* avoid using gloval variables unless it is necessary */
char buffer[1000] = {0}; /* initialize to avoid undefined behavior */
fp=fopen("FILE IN QUESTION HERE", "rb");
if(fp==NULL) {
perror("An error occured while opening the file...");
exit(1);
}
fread(buffer, 1000, 1, fp);
fwrite(buffer, 1000, 1, stdout); /* use fwrite instead of printf */
fclose(fp);
return 0;
}

Answer 2

fread is not reading other file formats
Code does not check the result of fread(). That is the way to determine if fread() is working.
char buffer[1000];
// fread(buffer, 1000, 1, fp);
size_t sz = fread(buffer, 1000, 1, fp);
if (sz == 0) puts("Did not read an entire block");
fread() returns the number of blocks read. With OP's case, code is attempting to read one 1000 byte block. Recommend reading 1000 blocks, each of 1 char rather than 1 block of a 1000 char. Further, avoid magic numbers.
for (;;) {
size_t sz = fread(buffer, sizeof buffer[0], sizeof buffer, fp);
if (sz == 0) break;
// Somehow print the buffer.
print_it(buffer, sz);
}
OP call to printf() expects a pointer to a string. A C string is an array of characters up to and including the terminating null character. buffer may/may not contain a null character and useful data after a null character.
// Does not work for OP
// printf("%s\n", buffer);
The data of a .png file is mostly binary and will have little textual meaning. A sample print function of mixed binary data and text follows. Most output will appears meaningless until one learns the .png file format. Untested code.
int print_it(const unsigned char *x, size_t sz) {
char buf[5];
unsigned column = 0;
while (sz > 0) {
sz--;
if (isgraph(*x) && *x != `(`) {
sprintf(buf, "%c", *x);
} else {
sprintf(buf, "(%02X)", *x);
}
column += strlen(buf);
if (column > 80) {
column = 0;
fputc('\n', stdout);
}
fputs(buf, stdout);
}
if (column > 0) fputc('\n', stdout);
}