Thread: Segmentation Fault With Pointer

I'm writing a program that mirrors an image using Memory Mapped Files.
These are the parts of the code I find relevant for this particular error.


First I create the output file.

Code:

if ( (fpout = open(argv[2], O_RDWR | O_CREAT | O_TRUNC,FILE_MODE)) < 0){
		fprintf(stderr,"can't creat %s for writing\n", argv[2]);
		exit(1);
	}

Then "set" the MMF.

Code:

	if( (dst = mmap(0, size, PROT_READ, MAP_FILE | MAP_SHARED, fpout, 0)) == (caddr_t) - 1){
		fprintf(stderr, "mmap error for output file");
		exit(1);
	}

Call the function that writes the imagem header and where the error is.

Code:

if(writeImageHeader(h, dst) == -1){
		printf("Could not write to output file\n");
		return -1;
	}

Function located on the ppmtools.c file, whose header file is included at the top of the main C file.

Code:

int writeImageHeader(header* h, char* dst){
	//write header fields with newline between them
	if(sprintf(dst,"%s\n%c\n%c\n%c\n",h->type,h->width,h->height,h->depth) < 0){
		return -1;
	}
	return 0;
}

Using GDB I get:

Code:

(gdb) step
Saving header to output file
138		if(writeImageHeader(h, dst) == -1){
(gdb) step
writeImageHeader (h=0x804c170, 
    dst=0xb7f7a000 <Address 0xb7f7a000 out of bounds>) at ppmtools.c:150

Have googled for a similar error but can't find an answer and I have exhausted all options I could think of.

Any help is appreciated.
Thanks.

Are you using errno? You could perhaps get some more information about the problem then.

It's hard to tell the specific problem without seeing all your code, but it looks either you are calling writeImageHeader with an uninitialized variable or you have some memory corruption issue (buffer overflow?) that scrambles dst after you assign the mmap result to it. When it gets into writeImageHeader and calls sprintf, it tries to write to an invalid address, hence the seg fault and gdb reporting Address out of bounds.

Oh, duh. How did I miss this:

Code:

	if( (dst = mmap(0, size, PROT_READ, MAP_FILE | MAP_SHARED, fpout, 0)) == (caddr_t) - 1){

You call mmap and request the memory page be PROT_READ, i.e. read-only. sprintf is trying to write to that memory, generating your seg fault. Try:

Code:

	if( (dst = mmap(0, size, PROT_READ|PROT_WRITE, MAP_FILE | MAP_SHARED, fpout, 0)) == (caddr_t) - 1){

Yep, that did it. I just copy/pasted that part from where I was mapping the original image. Would have never guessed the error was there.

Now it is working, thank you very much!