I was trying to implement getchar() function using read() in unistd.h.
Since system calls are pricy, I wanted to execute less read() functions as possible.
If I use "getchar", it works fine. However, "mygetchar" does not work in this case.
Can anyone point out what I have done wrong below?
#include <stdio.h>
#include <unistd.h>
#define BUF_SIZE 1024
int startIndex;
int endIndex;
int mygetchar(void){
char buffer[BUF_SIZE];
startIndex=0;
endIndex=0;
if(startIndex == endIndex){
int r;
r = read(0,buffer,BUF_SIZE);
startIndex=0;
endIndex=r;
}
return buffer[startIndex++];
}
int main(){
char c;
int i=0;
do{
c = mygetchar();
putchar(c);
i++;
}
while(c != EOF);
return 0;
}
Think carefully about your buffer. What happens to the buffer when the function call ends? It goes away.
This means that for 1023 out of 1024 calls, your buffer is unitialized and your offsets are pointing into nonsensical data.
Basically you need a global variable for the buffer too:
static char buf[BUF_SIZE];
static size_t bufCur = 0;
static size_t bufEnd = 0;
int mygetchar(void)
{
// ...
}
(Note that the static is pretty much pointless when your code is all in one file. If you were to pull your mygetchar into a header and implementation file though, you would want to use a static global so as to keep it from being linkable from outside of the same compilation unit.)
(Fun fact: the 0s for bufCur and bufEnd actually can be left implicit. For clarity, I would put them, but the standard dictates that they are to be zero-initialized.
As Jonathan Leffler pointed out, unless you plan on using the buffer elsewhere (and I don't know where that would be), there's no need for a global. You can just use a static variable inside of the function:
void mygetchar(void)
{
static buf[BUF_SIZE];
static size_t bufCur = 0;
static size_t bufEnd = 0;
// ...
}
Related
So I suck with functions and need to debug this. Im pretty sure the function ToPigLating does its job well at converting. However I just need help calling the function ToPigLatin inside of my main function. But when I try doing that I just get a bunch of error codes.
#include <stdlib.h>
#include <string.h>
#define LEN 32
char* ToPigLatin(char* word[LEN]){
char word[LEN];
char translation [LEN];
char temp [LEN];
int i, j;
while ((scanf ("%s", word)) != '\0') {
strcpy (translation, word);
//just pretend I have all the work to convert it in here.
} // while
}
int main(){
printf("Enter 5 words: ");
scanf("%s", word);
ToPigLatin();
}```
Roughly, variables only exist within the function they're declared in. The word in ToPigLatin exists only within ToPigLatin. It is not available in main. This lets us write functions without worrying about all the rest of the code.
You need to declare a different variable in main, it can also be called word, to store the input and then pass that into ToPigLatin.
Let's illustrate with something simpler, a function which doubles its input.
int times_two(int number) {
return number * 2;
}
We need to give times_two a number.
int main() {
// This is different from "number" in times_two.
int number = 42;
// We have to pass its value into time_two.
int doubled = times_two(number);
printf("%d doubled is %d\n", number, doubled);
}
Your case is a bit more complicated because you're working with input and memory allocation and arrays. I'd suggest just focusing on arrays and function calls for now. No scanf. No strcpy.
For example, here's a function to print an array of words.
#include <stdio.h>
// Arrays in C don't store their size, the size must be given.
void printWords(const char *words[], size_t num_words) {
for( int i = 0; i < num_words; i++ ) {
printf("word[%d] is %s.\n", i, words[i]);
}
}
int main(){
// This "words" variable is distinct from the one in printWords.
const char *words[] = {"up", "down", "left", "right"};
// It must be passed into printWords along with its size.
printWords(words, 4);
}
ToPigLatingToPigLating function expects to have a parameter like ToPigLating("MyParameter");
Hello there icecolddash.
First things first, there are some concepts missing. In main section:
scanf("%s", word);
You're probably trying to read a string format and store in word variable.
In this case, you should have it on your declaration scope. After some adjustment, it will look like this:
int main(){
char word[LEN];
As you defined LEN with 32 bytes maximum, your program will not be allowed to read bigger strings.
You're also using standard input and output funcitions as printf, and so you should ever include stdio.h, thats the header which cointains those prototypes already declared, avoiding 'implicit declaration' compiling warnings.
Next issue is how you're declaring your translation function, so we have to think about it:
char* ToPigLatin(char* word[LEN])
In this case, what you wrote:
ToPigLatin is a funcion that returns a char pointer, which means you want your function to probably return a string. If it makes sense to you, no problem at all. Although we got some real problem with the parameter char* word[LEN].
Declaring your variable like this, assume that you're passing an array of strings as a parameter. If I got it right, you want to read all five words in main section and translate each one of them.
In this case I suggest some changes in main function, for example :
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LEN 32
#define MAX_WORDS 5
char *globalname = "translated";
char* ToPigLatin(char* word){
char *translation = NULL;
//Translation work here.
if ( !strcmp(word, "icecolddash") ){
return NULL;
}
translation = globalname;
return translation;
}
int main(){
char word[LEN];
char *translatedword;
int i;
printf("Enter 5 words: \n");
for ( i=0; i < MAX_WORDS; i++ ){
fgets(word, sizeof(word), stdin);
strtok(word, "\n"); // Just in case you're using a keyboard as input.
translatedword = ToPigLatin(word);
if ( translatedword != NULL ){
//Do something with your translation
//I'll just print it out as an example
printf("%s\n", translatedword);
continue;
}
// Generic couldn't translate message
printf("Sorry, I know nothing about %s\n", word);
}
return 0;
}
The above code translate every word in a fixed word "translated".
In case of reading the exact input "icecolddash", the program will output a generic error message, simulating some problem on translation process.
I hope this help you out with your studies.
There are a few things that I see.
#include <stdio.h>
#include <stdlib.h>
char* ToPigLatin(char* word){
printf(word);
return word;
}
int main(){
printf("Enter 5 words: ");
// declare the variable read into by scanf
char * word = NULL;
scanf("%s", word);
//Pass the variable into the function
ToPigLatin(word);
// Make sure you return an int from main()
return 0;
}
I left some comments in the code with some specific details.
However, the main thing that I would like to call out is the style of the way you're writing your code. Always try to write small, testable chunks and build your way up slowly. Try to get your code to compile. ALWAYS. If you can't run your code, you can't test it to figure out what you need to do.
As for the char ** comment you left on lewis's post, here is some reading you may find useful in building up your intuition:
https://www.tutorialspoint.com/what-does-dereferencing-a-pointer-mean-in-c-cplusplus
Happy coding!
I'm trying to read text from a file and print it in the terminal while using dynamic memory(?), but as soon as I use calloc the code crashes. I'm new to C so I don't know what's wrong
#include <stdio.h>
#include <stdlib.h>
void filecheck(FILE*);
int main(void){
int i=0;
char** text=(char**)calloc(50,sizeof(char*));
for(i=0;i<50;i++) text[i]=(char*)calloc(50,sizeof(char));
FILE *file = fopen("F1.txt","r");
filecheck(file);
while(fscanf(file,"%s", text[i])!=EOF){
printf("%s\n",text[i]);
i++;
}
free(text);
return 0;
}
void filecheck(FILE*file){
if(file==NULL){
printf("Problem");
exit(0);
}
}
The problem is that you don't set i to 0 before you use it in the 2nd while loop. This cause the segmentation fault when you access text out of bounds. I addressed that issue below by using the same type of for loop that you used to initialize the array in the first place.
Bonus items:
Reformatted code for readability (to me) with spaces and moved * next to variable instead of next to type.
Introduced a couple of defines to replace your magic 50 numbers.
Moved filecheck() before main() so you don't need the declaration.
filecheck() now return a status code. This allows main() to free memory on failure which was technically a memory leak (even if the OS does this for you).
Check return value of calloc.
Use a status variable to hold exit code. This allows for clean-up to be shared in both normal and failure case.
Used variable instead of type as argument to sizeof.
Declare the variable as part of each for loop instead of reusing a variable. Reuse is not wrong, btw, but I think it's a good practice even if you use the same variable name.
fgets() instead of fscanf(). fscanf() is subject to buffer overflow when reading strings. Note: fscanf() reads a sequence of non-white-space characters, while fgets() read a line including the '\n'. Removed the the '\n' in the subsequent printf().
Only read at most ARR_LEN strings.
fclose() file descriptor (even if OS would do this for you).
Free the memory you allocate for text[i]. It is technically a memory leak if you don't (even if the OS frees it for you).
#include <stdio.h>
#include <stdlib.h>
#define ARR_LEN 50
#define STR_LEN 50
int filecheck(FILE *file) {
if(!file) {
printf("Problem");
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
int main(void) {
int status = EXIT_FAILURE;
char **text = calloc(ARR_LEN, sizeof(*text));
if(!text)
goto out;
for(int i=0; i < ARR_LEN; i++) {
text[i] = calloc(STR_LEN, sizeof(**text));
if(!text[i])
goto out;
}
FILE *file = fopen("F1.txt","r");
if(filecheck(file) != EXIT_SUCCESS)
goto out;
for(int i=0; (i < ARR_LEN) && fgets(text[i], STR_SIZE, file); i++)
printf("%s",text[i]);
status = EXIT_SUCCESS;
out:
if(file) fclose(file);
for(int i=0; i<ARR_LEN; i++)
free(text[i]);
free(text);
return status;
}
Other error (except i not set to 0) is that the function fscanf returns a number of scanned arguments. So you should use:
while (fscanf(file,"%s", text[i])!=1) {
...
}
Moreover, individual text[i] are never freed and leak.
I have a question pertaining to the extern char **environ. I'm trying to make a C program that counts the size of the environ list, copies it to an array of strings (array of array of chars), and then sorts it alphabetically with a bubble sort. It will print in name=value or value=name order depending on the format value.
I tried using strncpy to get the strings from environ to my new array, but the string values come out empty. I suspect I'm trying to use environ in a way I can't, so I'm looking for help. I've tried to look online for help, but this particular program is very limited. I cannot use system(), yet the only help I've found online tells me to make a program to make this system call. (This does not help).
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
extern char **environ;
int main(int argc, char *argv[])
{
char **env = environ;
int i = 0;
int j = 0;
printf("Hello world!\n");
int listSZ = 0;
char temp[1024];
while(env[listSZ])
{
listSZ++;
}
printf("DEBUG: LIST SIZE = %d\n", listSZ);
char **list = malloc(listSZ * sizeof(char**));
char **sorted = malloc(listSZ * sizeof(char**));
for(i = 0; i < listSZ; i++)
{
list[i] = malloc(sizeof(env[i]) * sizeof(char)); // set the 2D Array strings to size 80, for good measure
sorted[i] = malloc(sizeof(env[i]) * sizeof(char));
}
while(env[i])
{
strncpy(list[i], env[i], sizeof(env[i]));
i++;
} // copy is empty???
for(i = 0; i < listSZ - 1; i++)
{
for(j = 0; j < sizeof(list[i]); j++)
{
if(list[i][j] > list[i+1][j])
{
strcpy(temp, list[i]);
strcpy(list[i], list[i+1]);
strcpy(list[i+1], temp);
j = sizeof(list[i]); // end loop, we resolved this specific entry
}
// else continue
}
}
This is my code, help is greatly appreciated. Why is this such a hard to find topic? Is it the lack of necessity?
EDIT: Pasted wrong code, this was a separate .c file on the same topic, but I started fresh on another file.
In a unix environment, the environment is a third parameter to main.
Try this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int main(int argc, char *argv[], char **envp)
{
while (*envp) {
printf("%s\n", *envp);
*envp++;
}
}
There are multiple problems with your code, including:
Allocating the 'wrong' size for list and sorted (you multiply by sizeof(char **), but should be multiplying by sizeof(char *) because you're allocating an array of char *. This bug won't actually hurt you this time. Using sizeof(*list) avoids the problem.
Allocating the wrong size for the elements in list and sorted. You need to use strlen(env[i]) + 1 for the size, remembering to allow for the null that terminates the string.
You don't check the memory allocations.
Your string copying loop is using strncpy() and shouldn't (actually, you should seldom use strncpy()), not least because it is only copying 4 or 8 bytes of each environment variable (depending on whether you're on a 32-bit or 64-bit system), and it is not ensuring that they're null terminated strings (just one of the many reasons for not using strncpy().
Your outer loop of your 'sorting' code is OK; your inner loop is 100% bogus because you should be using the length of one or the other string, not the size of the pointer, and your comparisons are on single characters, but you're then using strcpy() where you simply need to move pointers around.
You allocate but don't use sorted.
You don't print the sorted environment to demonstrate that it is sorted.
Your code is missing the final }.
Here is some simple code that uses the standard C library qsort() function to do the sorting, and simulates POSIX strdup()
under the name dup_str() — you could use strdup() if you have POSIX available to you.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
extern char **environ;
/* Can also be spelled strdup() and provided by the system */
static char *dup_str(const char *str)
{
size_t len = strlen(str) + 1;
char *dup = malloc(len);
if (dup != NULL)
memmove(dup, str, len);
return dup;
}
static int cmp_str(const void *v1, const void *v2)
{
const char *s1 = *(const char **)v1;
const char *s2 = *(const char **)v2;
return strcmp(s1, s2);
}
int main(void)
{
char **env = environ;
int listSZ;
for (listSZ = 0; env[listSZ] != NULL; listSZ++)
;
printf("DEBUG: Number of environment variables = %d\n", listSZ);
char **list = malloc(listSZ * sizeof(*list));
if (list == NULL)
{
fprintf(stderr, "Memory allocation failed!\n");
exit(EXIT_FAILURE);
}
for (int i = 0; i < listSZ; i++)
{
if ((list[i] = dup_str(env[i])) == NULL)
{
fprintf(stderr, "Memory allocation failed!\n");
exit(EXIT_FAILURE);
}
}
qsort(list, listSZ, sizeof(list[0]), cmp_str);
for (int i = 0; i < listSZ; i++)
printf("%2d: %s\n", i, list[i]);
return 0;
}
Other people pointed out that you can get at the environment via a third argument to main(), using the prototype int main(int argc, char **argv, char **envp). Note that Microsoft explicitly supports this. They're correct, but you can also get at the environment via environ, even in functions other than main(). The variable environ is unique amongst the global variables defined by POSIX in not being declared in any header file, so you must write the declaration yourself.
Note that the memory allocation is error checked and the error reported on standard error, not standard output.
Clearly, if you like writing and debugging sort algorithms, you can avoid using qsort(). Note that string comparisons need to be done using strcmp(), but you can't use strcmp() directly with qsort() when you're sorting an array of pointers because the argument types are wrong.
Part of the output for me was:
DEBUG: Number of environment variables = 51
0: Apple_PubSub_Socket_Render=/private/tmp/com.apple.launchd.tQHOVHUgys/Render
1: BASH_ENV=/Users/jleffler/.bashrc
2: CDPATH=:/Users/jleffler:/Users/jleffler/src:/Users/jleffler/src/perl:/Users/jleffler/src/sqltools:/Users/jleffler/lib:/Users/jleffler/doc:/Users/jleffler/work:/Users/jleffler/soq/src
3: CLICOLOR=1
4: DBDATE=Y4MD-
…
47: VISUAL=vim
48: XPC_FLAGS=0x0
49: XPC_SERVICE_NAME=0
50: _=./pe17
If you want to sort the values instead of the names, you have to do some harder work. You'd need to define what output you wish to see. There are multiple ways of handling that sort.
To get the environment variables, you need to declare main like this:
int main(int argc, char **argv, char **env);
The third parameter is the NULL-terminated list of environment variables. See:
#include <stdio.h>
int main(int argc, char **argv, char **environ)
{
for(size_t i = 0; env[i]; ++i)
puts(environ[i]);
return 0;
}
The output of this is:
LD_LIBRARY_PATH=/home/shaoran/opt/node-v6.9.4-linux-x64/lib:
LS_COLORS=rs=0:di=01;34:ln=01;36:m
...
Note also that sizeof(environ[i]) in your code does not get you the length of
the string, it gets you the size of a pointer, so
strncpy(list[i], environ[i], sizeof(environ[i]));
is wrong. Also the whole point of strncpy is to limit based on the destination,
not on the source, otherwise if the source is larger than the destination, you
will still overflow the buffer. The correct call would be
strncpy(list[i], environ[i], 80);
list[i][79] = 0;
Bare in mind that strncpy might not write the '\0'-terminating byte if the
destination is not large enough, so you have to make sure to terminate the
string. Also note that 79 characters might be too short for storing env variables. For example, my LS_COLORS variable
is huge, at least 1500 characters long. You might want to do your list[i] = malloc calls based based on strlen(environ[i])+1.
Another thing: your swapping
strcpy(temp, list[i]);
strcpy(list[i], list[i+1]);
strcpy(list[i+1], temp);
j = sizeof(list[i]);
works only if all list[i] point to memory of the same size. Since the list[i] are pointers, the cheaper way of swapping would be by
swapping the pointers instead:
char *tmp = list[i];
list[i] = list[i+1];
list[i+1] = tmp;
This is more efficient, is a O(1) operation and you don't have to worry if the
memory spaces are not of the same size.
What I don't get is, what do you intend with j = sizeof(list[i])? Not only
that sizeof(list[i]) returns you the size of a pointer (which will be constant
for all list[i]), why are you messing with the running variable j inside the
block? If you want to leave the loop, the do break. And you are looking for
strlen(list[i]): this will give you the length of the string.
I'm learning the concept of prototyping in C, however I'm struggling with the correct syntax. I'm writing a function to strip all non-alphbetic characters from a c-string
#include <stdio.h>
#include <string.h>
char[30] clean(char[30] );
int main()
{
char word[30] = "hello";
char cleanWord[30];
cleanWord = clean(word);
return 0;
}
char[30] clean(char word[30])
{
char cleanWord[30];
int i;
for(i=0;i<strlen(word);i++)
if ( isalpha(word[i]) )
cleanWord[i]=word[i];
cleanWord[i]='\0';
return cleanWord;
}
How do I correctly prototype the function? What are the other syntax errors that are preventing my program from compiling?
Your problem is not with function prototyping (aka forward declaration). You just can't return an array from a function in C. Nor can you assign to an array variable. You need to make a couple of changes to get things working. One option:
change char cleanWord[30] in main to be char * cleanWord.
change the signature of clean to char *clean(char word[30])
use malloc to allocate a destnation buffer inside clean
return a pointer to that new buffer
free the buffer in main
And another:
change the signature of clean to void clean(char word[30], char cleanWord[30])
operate on the passed-in pointer rather than a local array in clean
change the call in main to be clean(word, cleanWord).
As Carl Norum said, you can't return an array. Instead, what you tend to do is supply the output:
void clean( const char word[30], char cleanWord[30] )
{
}
And you should remove the locally-scoped array from that function.
You will find that the function does not work correctly, because you only have one iterator i. That means if a character is not an alpha, you will skip over a position in the output array. You will need a second iterator that is incremented only when you add a character to cleanWord.
A couple of notes (was a bit late with writing up an answer, seems I've been beaten to them by the others )
C cannot return local (stack) objects, if you want to return an array from a function you have to malloc it
Even if you declare an array argument as (char arr[30]), (char* arr) is just as valid as arrays decay to pointers when passed as arguments to functions. Also, you won't be able to get the size correctly of such arrays by using sizeof. Even though it's 30, on my machine it returns 4 for word in clean, which is the size of the pointer for it.
You are missing an include, isalpha is part of ctype.h
I've updated your code, hopefully I've guessed your intentions correctly:
#include <stdlib.h> /* for malloc and free */
#include <string.h> /* for strlen */
#include <ctype.h> /* for isalpha */
#include <stdio.h> /* for printf */
/* Function declaration */
char* clean(char word[30]);
/* your 'main()' would now look like this: */
int main()
{
char word[30] = "hel1lo1";
char* cleanWord;
cleanWord = clean(word);
printf("%s\n", cleanWord);
free(cleanWord);
return 0;
}
/* Function definition */
char* clean(char word[30])
{
char* cleanWord = malloc(30); /* allocating dynamically an array of 30 chars,
* no need to use sizeof here as char is
* guaranteed to be 1 by the standard
*/
unsigned int i, j = 0; /* let's fix the problem with non-alpha chars already pointed out */
for (i = 0; i < (strlen(word)); i++)
if (isalpha(word[i]))
cleanWord[j++] = word[i];
cleanWord[j] = '\0';
return cleanWord;
/* return a pointer to the malloc`ed array, don't forget to free it after you're done with it */
}
The OpenGroup POSIX.1-2001 defines strerror_r, as does The Linux Standard Base Core Specification 3.1. But I can find no reference to the maximum size that could be reasonably expected for an error message. I expected some define somewhere that I could put in my code but there is none that I can find.
The code must be thread safe. Which is why strerror_r is used and not strerror.
Does any one know the symbol I can use? I should I create my own?
Example
int result = gethostname(p_buffy, size_buffy);
int errsv = errno;
if (result < 0)
{
char buf[256];
char const * str = strerror_r(errsv, buf, 256);
syslog(LOG_ERR,
"gethostname failed; errno=%d(%s), buf='%s'",
errsv,
str,
p_buffy);
return errsv;
}
From the documents:
The Open Group Base Specifications Issue 6:
ERRORS
The strerror_r() function may fail if:
[ERANGE] Insufficient storage was supplied via strerrbuf and buflen to
contain the generated message string.
From the source:
glibc-2.7/glibc-2.7/string/strerror.c:41:
char *
strerror (errnum)
int errnum;
{
...
buf = malloc (1024);
Having a sufficiently large static limit is probably good enough for all situations.
If you really need to get the entire error message, you can use the GNU version of strerror_r, or you can use the standard version
and poll it with successively larger buffers until you get what you need. For example,
you may use something like the code below.
#include <errno.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
/* Call strerror_r and get the full error message. Allocate memory for the
* entire string with malloc. Return string. Caller must free string.
* If malloc fails, return NULL.
*/
char *all_strerror(int n)
{
char *s;
size_t size;
size = 1024;
s = malloc(size);
if (s == NULL)
return NULL;
while (strerror_r(n, s, size) == -1 && errno == ERANGE) {
size *= 2;
s = realloc(s, size);
if (s == NULL)
return NULL;
}
return s;
}
int main(int argc, char **argv)
{
for (int i = 1; i < argc; ++i) {
int n = atoi(argv[i]);
char *s = all_strerror(n);
printf("[%d]: %s\n", n, s);
free(s);
}
return 0;
}
I wouldn't worry about it - a buffer size of 256 is far more than sufficient, and 1024 is overkill. You could use strerror() instead of strerror_r(), and then optionally strdup() the result if you need to store the error string. This isn't thread-safe, though. If you really need to use strerror_r() instead of strerror() for thread safety, just use a size of 256. In glibc-2.7, the longest error message string is 50 characters ("Invalid or incomplete multibyte or wide character"). I wouldn't expect future error messages to be significantly longer (in the worst case, a few bytes longer).
This program (run online (as C++) here):
#include <stdio.h>
#include <errno.h>
#include <string.h>
int main(){
const int limit = 5;
int unknowns = 0;
int maxlen = 0;
int i=0; char* s = strerror(i);
while(1){
if (maxlen<strlen(s)) maxlen = strlen(s);
if (/*BEGINS WITH "Unknown "*/ 0==strncmp("Unknown ", s , sizeof("Unknown ")-1) )
unknowns++;
printf("%.3d\t%s\n", i, s);
i++; s=strerror(i);
if ( limit == unknowns ) break;
}
printf("Max: %d\n", maxlen);
return 0;
}
lists and prints all the errors on the system and keeps track of the maximum length. By the looks of it, the length does not exceed 49 characters (pure strlen's without the final \0) so with some leeway, 64–100 should be more than enough.
I got curious if the whole buffer size negotiation couldn't simply be avoided by returning structs and whether there was a fundamental reason for not returning structs. So I benchmarked:
#define _POSIX_C_SOURCE 200112L //or else the GNU version of strerror_r gets used
#include <stdio.h>
#include <errno.h>
#include <stdlib.h>
#include <string.h>
typedef struct { char data[64]; } error_str_t;
error_str_t strerror_reent(int errn) __attribute__((const));
error_str_t strerror_reent(int errn){
error_str_t ret;
strerror_r(errn, ret.data, sizeof(ret));
return ret;
}
int main(int argc, char** argv){
int reps = atoi(argv[1]);
char buf[64];
volatile int errn = 1;
for(int i=0; i<reps; i++){
#ifdef VAL
error_str_t err = strerror_reent(errn);
#else
strerror_r(errn, buf, 64);
#endif
}
return 0;
}
and the performance difference between the two at -O2 is minimal:
gcc -O2 : The VAL version is slower by about 5%
g++ -O2 -x c++ : The VAL version is faster by about 1% than the standard version compiled as C++ and by about 4% faster than the standard version compiled as C (surprisingly, even the slower C++ version beats the faster C version by about 3%).
In any case, I think it's extremely weird that strerror is even allowed to be thread unsafe. Those returned strings should be pointers to string literals. (Please enlighten me, but I can't think of a case where they should be synthesized at runtime). And string literals are by definition read only and access to read only data is always thread safe.
Nobody has provided a definitive answer yet, so I looked into this further and there's a better function for the job, perror(3), as you will probably want to display this error somewhere, which is what I'd recommend you use unless your requirements really require you not to.
That's not a full answer, but the reason to use it is because it uses proper size buffer suitable for any locale. It internally uses strerror_r(3), these two functions conform to POSIX standard and are widely available, therefore in my eyes they're authoritative source of truth in this matter.
excerpt from glibc implementation:
static void
perror_internal (FILE *fp, const char *s, int errnum)
{
char buf[1024];
const char *colon;
const char *errstring;
if (s == NULL || *s == '\0')
s = colon = "";
else
colon = ": ";
errstring = __strerror_r (errnum, buf, sizeof buf);
(void) __fxprintf (fp, "%s%s%s\n", s, colon, errstring);
}
From this I can infer, that at this moment in time, and given stability of such things, in forseeable future, you will never go wrong with a buffer size of 1024 chars.