do while 0 what is it good for

Question

Possible Duplicate    Why are there sometimes meaningless do while and if else statements in C C   macros        I ve been seeing that expression for over 10 years now  I ve been trying to think what it s good for  Since I see it mostly in  defines  I assume it s good for inner scope variable declaration and for using breaks  instead of gotos    Is it good for anything else  Do you use it

User · Answer

It is interesting to note the following situation where the do {} while (0) loop won't work for you:

If you want a function-like macro that returns a value, then you will need a statement expression: ({stmt; stmt;}) instead of do {} while(0):


#include <stdio.h>

#define log_to_string1(str, fmt, arg...) \
    do { \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    } while (0)

#define log_to_string2(str, fmt, arg...) \
    ({ \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    })

int main() {
        char buf[1000];
        int n = 0;

        log_to_string1(buf, "%s\n", "No assignment, OK");

        n += log_to_string1(buf + n, "%s\n", "NOT OK: gcc: error: expected expression before 'do'");

        n += log_to_string2(buf + n, "%s\n", "This fixes it");
        n += log_to_string2(buf + n, "%s\n", "Assignment worked!");
        printf("%s", buf);
        return 0;
}

User · Answer

It s the only construct in C that you can use to  define a multistatement operation  put a semicolon after  and still use within an if statement  An example might help    define FOO x  foo x   bar x   if  condition      FOO x   else    syntax error here            Even using braces doesn t help    define FOO x    foo x   bar x       Using this in an if statement would require that you omit the semicolon  which is counterintuitive   if  condition      FOO x  else           If you define FOO like this    define FOO x  do   foo x   bar x     while  0    then the following is syntactically correct   if  condition      FOO x   else

User · Answer

It is interesting to note the following situation where the do {} while (0) loop won't work for you:

If you want a function-like macro that returns a value, then you will need a statement expression: ({stmt; stmt;}) instead of do {} while(0):


#include <stdio.h>

#define log_to_string1(str, fmt, arg...) \
    do { \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    } while (0)

#define log_to_string2(str, fmt, arg...) \
    ({ \
        sprintf(str, "%s: " fmt, "myprog", ##arg); \
    })

int main() {
        char buf[1000];
        int n = 0;

        log_to_string1(buf, "%s\n", "No assignment, OK");

        n += log_to_string1(buf + n, "%s\n", "NOT OK: gcc: error: expected expression before 'do'");

        n += log_to_string2(buf + n, "%s\n", "This fixes it");
        n += log_to_string2(buf + n, "%s\n", "Assignment worked!");
        printf("%s", buf);
        return 0;
}

User · Answer

It is a way to simplify error checking and avoid deep nested if s   For example   do        do something   if  error        break           do something else   if  error        break           etc     while  0

User · Answer

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function  Suppose you have    define FOO n    foo n  bar n    and you do   void foobar int n      if  n       FOO n       then this expands to   void foobar int n      if  n       foo n  bar n       Notice that the second call bar n  is not part of the if statement anymore   Wrap both into do     while 0   and you can also use the macro in an if statement

User · Answer

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function  Suppose you have    define FOO n    foo n  bar n    and you do   void foobar int n      if  n       FOO n       then this expands to   void foobar int n      if  n       foo n  bar n       Notice that the second call bar n  is not part of the if statement anymore   Wrap both into do     while 0   and you can also use the macro in an if statement

User · Answer

It s the only construct in C that you can use to  define a multistatement operation  put a semicolon after  and still use within an if statement  An example might help    define FOO x  foo x   bar x   if  condition      FOO x   else    syntax error here            Even using braces doesn t help    define FOO x    foo x   bar x       Using this in an if statement would require that you omit the semicolon  which is counterintuitive   if  condition      FOO x  else           If you define FOO like this    define FOO x  do   foo x   bar x     while  0    then the following is syntactically correct   if  condition      FOO x   else

User · Answer

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function  Suppose you have    define FOO n    foo n  bar n    and you do   void foobar int n      if  n       FOO n       then this expands to   void foobar int n      if  n       foo n  bar n       Notice that the second call bar n  is not part of the if statement anymore   Wrap both into do     while 0   and you can also use the macro in an if statement

User · Answer

It is a way to simplify error checking and avoid deep nested if s   For example   do        do something   if  error        break           do something else   if  error        break           etc     while  0

User · Answer

It is a way to simplify error checking and avoid deep nested if s   For example   do        do something   if  error        break           do something else   if  error        break           etc     while  0

User · Answer

Generically  do while is good for any sort of loop construct where one must execute the loop at least once   It is possible to emulate this sort of looping through either a straight while or even a for loop  but often the result is a little less elegant   I ll admit that specific applications of this pattern are fairly rare  but they do exist   One which springs to mind is a menu-based console application   do       char c   read input         process input c     while  c     Q

User · Answer

It s the only construct in C that you can use to  define a multistatement operation  put a semicolon after  and still use within an if statement  An example might help    define FOO x  foo x   bar x   if  condition      FOO x   else    syntax error here            Even using braces doesn t help    define FOO x    foo x   bar x       Using this in an if statement would require that you omit the semicolon  which is counterintuitive   if  condition      FOO x  else           If you define FOO like this    define FOO x  do   foo x   bar x     while  0    then the following is syntactically correct   if  condition      FOO x   else

User · Answer

Generically  do while is good for any sort of loop construct where one must execute the loop at least once   It is possible to emulate this sort of looping through either a straight while or even a for loop  but often the result is a little less elegant   I ll admit that specific applications of this pattern are fairly rare  but they do exist   One which springs to mind is a menu-based console application   do       char c   read input         process input c     while  c     Q

User · Answer

It s the only construct in C that you can use to  define a multistatement operation  put a semicolon after  and still use within an if statement  An example might help    define FOO x  foo x   bar x   if  condition      FOO x   else    syntax error here            Even using braces doesn t help    define FOO x    foo x   bar x       Using this in an if statement would require that you omit the semicolon  which is counterintuitive   if  condition      FOO x  else           If you define FOO like this    define FOO x  do   foo x   bar x     while  0    then the following is syntactically correct   if  condition      FOO x   else

User · Answer

It helps to group multiple statements into a single one so that a function-like macro can actually be used as a function  Suppose you have    define FOO n    foo n  bar n    and you do   void foobar int n      if  n       FOO n       then this expands to   void foobar int n      if  n       foo n  bar n       Notice that the second call bar n  is not part of the if statement anymore   Wrap both into do     while 0   and you can also use the macro in an if statement

User · Answer

It is a way to simplify error checking and avoid deep nested if s   For example   do        do something   if  error        break           do something else   if  error        break           etc     while  0

User · Answer

Generically  do while is good for any sort of loop construct where one must execute the loop at least once   It is possible to emulate this sort of looping through either a straight while or even a for loop  but often the result is a little less elegant   I ll admit that specific applications of this pattern are fairly rare  but they do exist   One which springs to mind is a menu-based console application   do       char c   read input         process input c     while  c     Q

User · Answer

Generically  do while is good for any sort of loop construct where one must execute the loop at least once   It is possible to emulate this sort of looping through either a straight while or even a for loop  but often the result is a little less elegant   I ll admit that specific applications of this pattern are fairly rare  but they do exist   One which springs to mind is a menu-based console application   do       char c   read input         process input c     while  c     Q

[c] do { ... } while (0) — what is it good for?

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