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Frequently Asked Questions (FAQS);faqs.220 Section 11. Stdio 11.1: Why doesn't the code "char c; while((c = getchar()) != EOF)..." work? A: The variable to hold getchar's return value must be an int. 11.2: Why does errno contain ENOTTY after a call to printf? A: Don't worry about it. It is only meaningful for a program to inspect the contents of errno after an error has occurred. 11.3: My program's prompts and intermediate output don't always show up on the screen, especially when I pipe the output through another program. A: It is best to use an explicit fflush(stdout) whenever output should definitely be visible. 11.4: When I read from the keyboard with scanf, it seems to hang until I type one extra line of input. A: scanf was designed for free-format input, which is seldom what you want when reading from the keyboard. 11.5: Will fflush(stdin) flush unread characters from the standard input stream? A: No. 11.6: Once I've used freopen, how can I get the original stdout back? A: It's not easy. Try avoiding freopen. 11.7: How can I recover the file name given an open file descriptor? A: This problem is, in general, insoluble. It is best to remember the names of files yourself when you open them. Section 12. Library Subroutines 12.1: I'm trying to sort an array of strings with qsort, using strcmp as the comparison function, but it's not working. A: You'll have to write a "helper" comparison function which takes two generic pointer arguments, converts them to char **, and dereferences them, yielding char *'s which can be usefully compared. 12.2: Now I'm trying to sort an array of structures with qsort. My comparison routine takes pointers to structures, but the compiler complains that the function is of the wrong type for qsort. How can I cast the function pointer to shut off the warning? A: The conversions must be in the comparison function, which must be declared as accepting "generic pointers" (void * or char *). 12.3: How can I convert numbers to strings? A: Just use sprintf. 12.4: How can I get the time of day in a C program? A: Just use the time, ctime, and/or localtime functions. 12.5: How can I convert a struct tm or a string into a time_t? A: The ANSI mktime routine converts a struct tm to a time_t. No standard routine exists to parse strings. 12.6: I need a random number generator. A: The standard C library has one: rand(). 12.7: Each time I run my program, I get the same sequence of random numbers. A: You can call srand() to seed the pseudo-random number generator with a more random initial value. 12.8: I need a random true/false value, so I'm taking rand() % 2, but it's just alternating 0, 1, 0, 1, 0... A: Try using the higher-order bits. 12.9-13: I'm trying to port this old program. Why do I get "undefined external" errors for some library routines? A: Some semistandard routines have been renamed or replaced over the years; see the full list for details. 12.14: How can I execute a command with system() and read its output into a program? A: Unix and some other systems provide a popen() routine. 12.15: How can I read a directory in a C program? A: See if you can use the opendir() and readdir() routines. Section 13. Lint 13.1: I just typed in this program, and it's acting strangely. Can you see anything wrong with it? A: Try running lint first. 13.2: How can I shut off the "warning: possible pointer alignment problem" message lint gives me for each call to malloc? A: It may be easier simply to ignore the message, perhaps in an automated way with grep -v. 13.3: Where can I get an ANSI-compatible lint? A: See the unabridged list for two commercial products. Section 14. Style 14.1: Is the code "if(!strcmp(s1, s2))" good style? A: Perhaps; perhaps not. 14.2: What's the best style for code layout in C? A: There is no one "best style," but see the full list for a few suggestions. 14.3: Where can I get the "Indian Hill Style Guide" and other coding standards? A: See the unabridged list. Section 15. Floating Point 15.1: My floating-point calculations are acting strangely and giving me different answers on different machines. A: First, make sure that you have #included <math.h>, and correctly declared other functions returning double. If the problem isn't that simple, see the full list for a brief explanation, or any good programming book for a better one. 15.2: I keep getting "undefined: _sin" compilation errors. A: Make sure you're linking against the correct math library. 15.3: Why doesn't C have an exponentiation operator? A: Try using the pow() function. 15.4: I'm having trouble with a Turbo C program which crashes and says something like "floating point formats not linked." A: Some compilers for small machines, including Turbo C, attempt to leave out floating point support if it looks like it will not be needed. The programmer must occasionally insert a dummy explicit floating-point call to force loading of floating-point support. Section 16. System Dependencies 16.1: How can I read a single character from the keyboard without waiting for a newline? A: Contrary to popular belief and many people's wishes, this is not a C-related question. How to do so is a function of the operating system in use. 16.2: How can I find out if there are characters available for reading (and if so, how many)? Alternatively, how can I do a read that will not block if there are no characters available? A: These, too, are entirely operating-system-specific. 16.3: How can I clear the screen? A: Such things depend on the output device you're using. 16.4: How do I read the mouse? A: What system are you using? 16.5: How can my program discover the complete pathname to the executable file from which it was invoked? A: argv[0] may contain all or part of the pathname. You may be able to duplicate the command language interpreter's search path logic to locate the executable. 16.6: How can a process change an environment variable in its caller? A: In general, it cannot. 16.7: How can I find out the size of a file, prior to reading it in? A: You might be able to get an estimate using stat() or fseek/ftell. 16.8: How can a file be shortened in-place without completely clearing or rewriting it? A: There are various ways to do this, but there is no truly portable solution. 16.9: How can I implement a delay, or time a user's response, with sub-second resolution? A: Unfortunately, there is no portable way. 16.10: How can I read in an object file and jump to routines in it? A: You want a dynamic linker and/or loader. Section 17. Miscellaneous 17.1: What can I safely assume about the initial values of variables which are not explicitly initialized? A: Variables with "static" duration start out as 0, as if the programmer had initialized them. Variables with "automatic" duration, and dynamically-allocated memory, start out containing garbage (with the exception of calloc). 17.2: How can I write data files which can be read on other machines with different data formats? A: The best solution is to use text files. 17.3: How can I return several values from a function? A: Either pass pointers to locations which the function can fill in, or have the function return a structure containing the desired values. 17.4: How can I call a function, given its name as a string? A: The most straightforward thing to do is maintain a correspondence table of names and function pointers. 17.5: I seem to be missing the system header file <sgtty.h>. Can someone send me a copy? A: You cannot just pick up a copy of someone else's header file and expect it to work, since the definitions within header files are frequently system-dependent. Contact your vendor. 17.6: How can I call FORTRAN (C++, BASIC, Pascal, Ada, LISP) functions from C? A: The answer is entirely dependent on the machine and the specific calling sequences of the various compilers in use. 17.7: Does anyone know of a program for converting Pascal or FORTRAN to C? A: Several public-domain programs are available, namely ptoc, p2c, and f2c. See the full list for details. 17.8: Where can I get copies of all these public-domain programs? A: See the regular postings in the comp.sources.unix and comp.sources.misc newsgroups for information. 17.9: When will the next Obfuscated C Code Contest be held How can I get a copy of the previous winning entries? A: See the full list, or send e-mail to obfuscate@toad.com . 17.10: Why don't C comments nest? Are they legal inside quoted strings? A: Nested comments would cause more harm than good. The character sequences /* and */ are not special within double-quoted strings. 17.11: How can I implement sets and/or arrays of bits? A: Use arrays of char or int, with a few macros to access the right bit at the right index. 17.12: What is the most efficient way to count the number of bits which are set in a value? A: This and many other similar bit-twiddling problems can often be sped up and streamlined using lookup tables. 17.13: How can I make this code more efficient? A: Efficiency is not important nearly as often as people tend to think it is. Most of the time, by simply paying attention to good algorithm choices, perfectly acceptable results can be achieved. 17.14: Are pointers really faster than arrays? How much do function calls slow things down? A: Precise answers to these and many similar questions depend of course on the processor and compiler in use. 17.15: This program crashes before it even runs! A: Look for very large, local arrays. (See also question 9.4.) 17.16: What does "Segmentation violation" mean? A: It generally means that your program tried to access memory it shouldn't have. 17.17: Does anyone have a C compiler test suite I can use? A: Plum Hall, among others, sells one. 17.18: Where can I get a YACC grammar for C? A: See the ANSI Standard, or the unabridged list. 17.19: How do you pronounce "char"? A: Like the English words "char," "care," or "car" (your choice). 17.20: What's a good book for learning C? A: There are far too many to list here; the full list contains a few pointers. 17.21: Where can I get extra copies of this list? A: For now, just pull it off the net; the unabridged version is normally posted on the first of each month, with an Expiration: line which should keep it around all month. It can also be found in the newsgroup news.answers . Several sites archive news.answers postings and other FAQ lists, including this one; the archie server should help you find them. Steve Summit scs@adam.mit.edu scs%adam.mit.edu@mit.edu mit-eddie!adam.mit.edu!scs This article is Copyright 1988, 1990-1992 by Steve Summit. It may be freely redistributed so long as the author's name, and this notice, are retained. Xref: bloom-picayune.mit.edu comp.lang.c:59948 news.answers:3846 Newsgroups: comp.lang.c,news.answers Path: bloom-picayune.mit.edu!adam.mit.edu!scs From: scs@adam.mit.edu (Steve Summit) Subject: comp.lang.c Changes to Answers to Frequently Asked Questions (FAQ) Message-ID: <1992Nov3.010510.14974@athena.mit.edu> Followup-To: poster Sender: news@athena.mit.edu (News system) Supersedes: <1992Oct1.210814.22950@athena.mit.edu> Nntp-Posting-Host: adam.mit.edu Reply-To: scs@adam.mit.edu X-Archive-Name: C-faq/diff Organization: none, at the moment Date: Tue, 3 Nov 1992 01:05:10 GMT Approved: news-answers-request@MIT.Edu Lines: 340 Archive-name: C-faq/diff This article contains changes between the previous revision of the comp.lang.c frequently-asked questions list (posted on October 1) and the new one. (Do _not_ worry if you have not seen the new one yet; it's coming up next.) As usual, these diffs have been edited for readability, and are not suitable for use with the patch program. The changes this month are mostly minor and/or cosmetic. The only mildly significant and/or interesting one is that I had somehow managed to add the same question twice (what had been 2.5 and 2.6); thanks to Mark Brader for pointing this out. ========== < [Last modified October 1, 1992 by scs.] --- > [Last modified November 2, 1992 by scs.] ========== pointer arguments. To generate a null pointer in a function < call context, an explicit cast is typically required: --- > call context, an explicit cast is typically required, to force > the 0 to be in a pointer context: ========== 6. The "null string," which is another name for an empty < string. The term "null string" can be confusing in C --- > string (""). The term "null string" can be confusing in ========== < (The exceptions are when the array is the operand of a sizeof() < or & operator, or is a literal string initializer for a --- > (The exceptions are when the array is the operand of a sizeof or > & operator, or is a literal string initializer for a character ========== A: Since arrays decay immediately into pointers, an array is never < actually passed to a function. Therefore, any parameter --- > actually passed to a function. As a convenience, any parameter ========== < 2.5: Why doesn't sizeof() properly report the size of an array that's < a parameter to a subroutine? < < A: sizeof() reports the size of the pointer parameter which the < subroutine really receives (see question 2.4). < < 2.6: Why doesn't sizeof(array) work when the array is a function < parameter? < < A: The compiler pretends that the array parameter was declared as a < pointer (see question 2.4), and sizeof reports the size of the < pointer. --- > 2.5: Why doesn't sizeof properly report the size of an array which is > a parameter to a function? > > A: The sizeof operator reports the size of the pointer parameter > which the function actually receives (see question 2.4). ========== no longer needed. You must also be extremely cautious when < passing dynamically-allocated arrays down to other subroutines, < if those subroutines are also to accept conventional, --- > passing dynamically-allocated arrays down to other functions, if > those functions are also to accept conventional, statically- ========== < 2.13: I passed a pointer to a subroutine which initialized it: --- > 2.12: I passed a pointer to a function which initialized it: ========== static int dummy; < ip = &i; --- > ip = &dummy; ========== < A: Did the subroutine try to initialize the pointer itself, or just --- > A: Did the function try to initialize the pointer itself, or just ========== < value. The called subroutine altered only the passed copy of < the pointer. You'll want to pass the address of the pointer < (the subroutine will end up accepting a pointer-to-a-pointer). --- > value. The called function altered only the passed copy of the > pointer. You'll want to pass the address of the pointer (the > function will end up accepting a pointer-to-a-pointer). ========== < 3.4: I have a function that returns a string, and it seems to work < fine under the debugger, but when it returns to its caller, the < returned string is garbage. --- > 3.4: I have a function that is supposed to return a string, but when > it returns to its caller, the returned string is garbage. ========== A: Under C's integral promotion rules, the multiplication is < carried out using integer arithmetic, and the result overflows < and/or is truncated before being assigned to the long int left- --- > carried out using int arithmetic, and the result may overflow > and/or be truncated before being assigned to the long int left- ========== "int func(x) float x;". Old C (and ANSI C, in the absence of < prototypes) silently promotes floats to doubles when passing < them as arguments, and arranges that doubles being passed are < coerced back to floats if the formal parameters are declared < that way. --- > prototypes, and in variable-length argument lists) "widens" > certain arguments when they are passed to functions. floats > are promoted to double, and characters and short integers are > promoted to integers. (The values are automatically coerced > back to the corresponding narrower types within the body of the > called function, if they are declared that way there.) ========== < rely on such an extension? It is usually a bad idea to try to < determine language properties, especially pertaining to the ANSI < Standard, by performing experiments with a particular compiler. --- > rely on such an extension? It is usually a bad idea to perform > experiments with a particular compiler to determine properties > of a language; the applicable standard may permit variations, or > the compiler may be wrong. ========== A: There is no good answer to this question. If the values are integers, a well-known trick using exclusive-OR could perhaps be used, but it will not work for floating-point values or < pointers, (and it will not work if the two values are the same < variable, and the "obvious" supercompressed implementation for < integral types a^=b^=a^=b is, strictly speaking, illegal due to < multiple side-effects, and...). If the macro is intended to be --- > pointers, or if the two values are the same variable (and the > "obvious" supercompressed implementation for integral types > a^=b^=a^=b is in fact illegal due to multiple side-effects, > and...). If the macro is intended to be used on values of ========== The best all-around solution is probably to forget about using a < macro, unless you don't mind passing in the type as a third --- > macro, unless you're willing to pass in the type as a third ========== < 6.5: Does sizeof() work in preprocessor #if directives? --- > 6.5: Does the sizeof operator work in preprocessor #if directives? ========== compilation, before type names have been parsed. Consider using < the predefined constants in <limits.h>, or a "configure" script, --- > the predefined constants in ANSI's <limits.h>, or a "configure" ========== < #include <stddef.h> /* for NULL, size_t */ < #include <stdlib.h> /* for malloc */ --- > #include <stdlib.h> /* for malloc, NULL, size_t */ ========== Under a pre-ANSI compiler, rewrite the function definition without a prototype ("char *vstrcat(first) char *first; {"), < include <stdio.h> rather than <stddef.h>, replace "#include < <stdlib.h>" with "extern char *malloc();", and use int instead --- > include <stdio.h> rather than <stdlib.h>, add "extern > char *malloc();", and use int instead of size_t. You may also ========== > Remember that in variable-length argument lists, function > prototypes do not supply parameter type information; therefore, > default argument promotions apply (see question 5.6), and null > pointer arguments must be typed explicitly (see question 1.2). ========== < A: Structures may have this padding, so that alignment properties < will be preserved when an array of contiguous structures is < allocated. --- > A: Structures may have this padding (as well as internal padding; > see also question 9.5), so that alignment properties will be > preserved when an array of contiguous structures is allocated. ========== < 9.10: How can I turn off structure padding, so that I can get a struct < to conform to an externally-imposed storage layout? --- > 9.10: My compiler is leaving holes in structures, which is wasting > space and preventing "binary" I/O to external data files. Can I > turn off the padding, or otherwise control the alignment of > structs? ========== < attempting to conform to some externally-imposted storage --- > attempting to conform to some externally-imposed storage layout, ========== < It is usually better to use fgets() to read a whole line, and < then use sscanf() or other string functions to pick apart the < line buffer. --- > It is usually better to use fgets to read a whole line, and > then use sscanf or other string functions to pick apart the line > buffer. If you do use sscanf, don't forget to check the return > value to make sure that the expected number of items were found. ========== < links). It is best to remember the names of open files yourself < (perhaps with a wrapper function around fopen). --- > links). It is best to remember the names of files yourself when > you open them (perhaps with a wrapper function around fopen). ========== < 12.7: Each time I run my program, I get the same sequence of random < numbers. --- > 12.7: Each time I run my program, I get the same sequence of numbers > back from rand(). ========== A: You can call srand() to seed the pseudo-random number generator < with a more random value. Popular random initial seeds are the --- > with a more random initial value. Popular random initial seeds ========== < A: Try running lint first. Many C compilers are really only half- --- > A: Try running lint first (perhaps with the -a, -c, -h, -p and/or > other options). Many C compilers are really only half- ========== A: Make sure you're linking against the correct math library. For < instance, under Unix, you usually need to add -lm after the < source and object files when compiling/linking. --- > instance, under Unix, you usually need to use the -lm option at > the end of the command line when compiling/linking. ========== A: You can #include <math.h> and use the pow() function, although < explicit multiplication is often better for small integral < exponents. --- > explicit multiplication is often better for small positive > integral exponents. ========== A: BSD systems provide ftruncate(), several others supply chsize(), and a few may provide a (possibly undocumented) fcntl option F_FREESP. Under MS-DOS, you can sometimes use < write(fd, (char *)NULL, 0). However, but there is no truly < portable solution. --- > write(fd, "x", 0). However, there is no truly portable > solution. ========== < 17.3: How can I return several values from a subroutine? --- > 17.3: How can I return several values from a function? ========== < A: Either pass pointers which the subroutine can fill in, or have < the subroutine return a structure containing the desired values. --- > A: Either pass pointers to locations which the function can fill > in, or have the function return a structure containing the ========== < 17.7: Does anyone know of a program for converting Pascal (FORTRAN, < LISP, "Old" C, ...) to C? --- > 17.7: Does anyone know of a program for converting Pascal or FORTRAN > (or LISP, Ada, AWK, "Old" C, ...) to C? ========== < A PL/M to C converter was posted to alt.sources in April, 1991. < < The following companies sell various translation tools and < services: < < Cobalt Blue < 2940 Union Ave., Suite C < San Jose, CA 95124 USA < (+1) 408 723 0474 < < Promula Development Corp. < 3620 N. High St., Suite 301 < Columbus, OH 43214 USA < (+1) 614 263 5454 < < Micro-Processor Services Inc. < 92 Stone Hurst Lane < Dix Hills, NY 11746 USA < (+1) 519 499 4461 --- > This FAQ list's maintainer also has available a list of other > commercial translation products, and some for more obscure > languages. ========== A: The contest typically runs from early March through mid-May. To obtain a current copy of the rules and other information, send e-mail with the Subject: line "send rules" to: < {apple,pyramid,sun,uunet}!hoptoad!judges or judges@toad.com --- > {apple,pyramid,sun,uunet}!hoptoad!obfuscate or > obfuscate@toad.com ========== A: Use arrays of char or int, with a few macros to access the right < bit at the right index: --- > bit at the right index (try using 8 for CHAR_BIT if you don't > have <limits.h>): ========== < #define Mask(bit) (1 << ((bit) % CHAR_BIT)) < #define Slot(bit) ((bit) / CHAR_BIT) < #define Set(ary, bit) ((ary)[Slot(bit)] |= Mask(bit)) < #define Test(ary, bit) ((ary)[Slot(bit)] & Mask(bit)) --- > #define BITMASK(bit) (1 << ((bit) % CHAR_BIT)) > #define BITSLOT(bit) ((bit) / CHAR_BIT) > #define BITSET(ary, bit) ((ary)[BITSLOT(bit)] |= BITMASK(bit)) > #define BITTEST(ary, bit) ((ary)[BITSLOT(bit)] & BITMASK(bit)) ========== > 17.19: How do you pronounce "char"? > > A: You can pronounce the C keyword "char" in at least three ways: > like the English words "char," "care," or "car;" the choice is > arbitrary. ========== > Knuth Donald E. Knuth, The Art of Computer Programming, (3 vols.), > Addison Wesley, 1981. ========== Thanks to Jamshid Afshar, Sudheer Apte, Dan Bernstein, Stan Brown, Joe Buehler, Burkhard Burow, D'Arcy J.M. Cain, Raymond Chen, Christopher Calabrese, James Davies, Norm Diamond, Ray Dunn, Stephen M. Dunn, Bjorn Engsig, Dave Gillespie, Samuel Goldstein, Alasdair Grant, Ron Guilmette, Doug Gwyn, Tony Hansen, Joe Harrington, Guy Harris, Jos Horsmeier, Blair Houghton, Kirk Johnson, Peter Klausler, Andrew Koenig, Tom Koenig, John > Lauro, Don Libes, Christopher Lott, Tim McDaniel, Evan Manning, Barry > Margolin, Mark Moraes, Darren Morby, Richard A. O'Keefe, Hans Olsson, Francois Pinard, randall@virginia, Pat Rankin, Rich Salz, Chip Salzenberg, Paul Sand, Doug Schmidt, Patricia Shanahan, Peter da Silva, Joshua Simons, Henry Spencer, Erik Talvola, Clarke Thatcher, Chris Torek, Goran Uddeborg, Wietse Venema, Ed Vielmetti, Larry Virden, Freek Wiedijk, and Dave Wolverton, who have contributed, directly or indirectly, to this article. Special thanks to Karl Heuer, and particularly to Mark Brader, who (to borrow a line from Steve Johnson) have goaded me beyond my inclination, and occasionally beyond my endurance, in relentless pursuit of a better FAQ list. ========== Steve Summit scs@adam.mit.edu scs%adam.mit.edu@mit.edu mit-eddie!adam.mit.edu!scs Xref: bloom-picayune.mit.edu comp.lang.c:61497 news.answers:4291 Newsgroups: comp.lang.c,news.answers Path: bloom-picayune.mit.edu!adam.mit.edu!scs From: scs@adam.mit.edu (Steve Summit) Subject: comp.lang.c Answers to Frequently Asked Questions (FAQ List) Message-ID: <1992Dec1.025024.25129@athena.mit.edu> Followup-To: poster Sender: news@athena.mit.edu (News system) Supersedes: <1992Nov3.010706.15151@athena.mit.edu> Nntp-Posting-Host: adam.mit.edu Reply-To: scs@adam.mit.edu X-Archive-Name: C-faq/faq Organization: none, at the moment Date: Tue, 1 Dec 1992 02:50:24 GMT X-Last-Modified: November 2, 1992 Approved: news-answers-request@MIT.Edu Expires: Sun, 3 Jan 1993 00:00:00 GMT Lines: 2879 Archive-name: C-faq/faq Newsgroup-name-archive-name: C-FAQ-list [Last modified November 2, 1992 by scs.] Certain topics come up again and again on this newsgroup. They are good questions, and the answers may not be immediately obvious, but each time they recur, much net bandwidth and reader time is wasted on repetitive responses, and on tedious corrections to the incorrect answers which are inevitably posted. This article, which is posted monthly, attempts to answer these common questions definitively and succinctly, so that net discussion can move on to more constructive topics without continual regression to first principles. No mere newsgroup article can substitute for thoughtful perusal of a full-length tutorial or language reference manual. Anyone interested enough in C to be following this newsgroup should also be interested enough to read and study one or more such manuals, preferably several times. Some vendors' compiler manuals are unfortunately inadequate; a few even perpetuate some of the myths which this article attempts to refute. Several noteworthy books on C are listed in this article's bibliography. Many of the questions and answers are cross-referenced to these books, for further study by the interested and dedicated reader (but beware of ANSI vs. ISO C Standard section numbers; see question 5.1). If you have a question about C which is not answered in this article, first try to answer it by checking a few of the referenced books, or by asking knowledgeable colleagues, before posing your question to the net at large. There are many people on the net who are happy to answer questions, but the volume of repetitive answers posted to one question, as well as the growing number of questions as the net attracts more readers, can become oppressive. If you have questions or comments prompted by this article, please reply by mail rather than following up -- this article is meant to decrease net traffic, not increase it.