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Full Discussion: fgets problems
Top Forums Programming fgets problems Post 302397047 by CRGreathouse on Saturday 20th of February 2010 02:29:33 PM
Old 02-20-2010
Quote:
Originally Posted by Corona688
Memory is memory. It doesn't have to track the contents of the pari block to notice clobbering happening outside it.
Sure -- but that's not the problem (and it won't be the problem). Every time the library uses the stack it ensures that there's room for what it's creating. Also, the stack is very large -- at least 20 MB and sometimes as big as 1 GB, with its pointer avma moving by [only] several bytes per call.

The real problem with memory management is leaving garbage on the stack (another possibility is returning corrupted objects). The usual technique for is

Code:
GEN genericfunction(----) {
	pari_sp ltop = avma;	// Keep a pointer to the top of the stack's current location
	GEN r;	// Return value (assigned below)

	// ...
	
	r = gerepileupto(ltop, r);	// copy r to the top of the stack and move avma just above it
	return r;
}

which ensures that you're not leaving garbage on the stack. But this is somewhat more complicated by returning complicated objects like vectors. For example:
Code:
GEN vector = cgetg(4, t_VEC); // Make a vector
gel(vector, 1) = gen_2;
int i = 2;
for(; i < 4; i++)
  gel(vector, i) = addii(gel(vector, i-1), mulii(gel(vector, i-1), gel(vector, i-1))); // v[i] = v[i-1] + v[i-1]^2

Memory diagram:

(vector with pointers to its members A, B, and C)(A)(garbage)(B)(garbage)(C)
 

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FGETS(3)						   BSD Library Functions Manual 						  FGETS(3)

NAME
fgets, gets -- get a line from a stream LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <stdio.h> char * fgets(char * restrict str, int size, FILE * restrict stream); char * gets(char *str); DESCRIPTION
The fgets() function reads at most one less than the number of characters specified by size from the given stream and stores them in the string str. Reading stops when a newline character is found, at end-of-file or error. The newline, if any, is retained. If any characters are read and there is no error, a '' character is appended to end the string. The gets() function is equivalent to fgets() with an infinite size and a stream of stdin, except that the newline character (if any) is not stored in the string. It is the caller's responsibility to ensure that the input line, if any, is sufficiently short to fit in the string. RETURN VALUES
Upon successful completion, fgets() and gets() return a pointer to the string. If end-of-file occurs before any characters are read, they return NULL and the buffer contents remain unchanged. If an error occurs, they return NULL and the buffer contents are indeterminate. The fgets() and gets() functions do not distinguish between end-of-file and error, and callers must use feof(3) and ferror(3) to determine which occurred. ERRORS
[EBADF] The given stream is not a readable stream. The function fgets() may also fail and set errno for any of the errors specified for the routines fflush(3), fstat(2), read(2), or malloc(3). The function gets() may also fail and set errno for any of the errors specified for the routine getchar(3). SEE ALSO
feof(3), ferror(3), fgetln(3), fgetws(3), getline(3) STANDARDS
The functions fgets() and gets() conform to ISO/IEC 9899:1999 (``ISO C99''). SECURITY CONSIDERATIONS
The gets() function cannot be used securely. Because of its lack of bounds checking, and the inability for the calling program to reliably determine the length of the next incoming line, the use of this function enables malicious users to arbitrarily change a running program's functionality through a buffer overflow attack. It is strongly suggested that the fgets() function be used in all cases. BSD
May 5, 2012 BSD
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