Segmentation fault in a program ! Help !

Discussion in 'C' started by lionaneesh, Mar 24, 2010.

  1. lionaneesh

    lionaneesh Active Member

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    Code:
    #include<stdio.h>
    #include<string.h>
    
    int main()
    {
    
        int i,x;
        char c[100];
    
        printf("Enter input :");
        fgets(c,100,stdin);
    
        for( i = 0 ; i <= (x = (strlen(c))) ; i++)
        {
    
            if( c[i] = '\t')
            {
                c[i] = (' ');
                            printf("   ");
            }
            printf("%c",c[i]);
        }
    }
    the aim of this program is that it will replace tabs entered in the input with 4 spaces.

    need help with this program .

    this programe is giving compile error:-

    SEGMENTATION FAULT
     
  2. shabbir

    shabbir Administrator Staff Member

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    Though I am out of touch with C but as far as I can remember SEGMENTATION FAULT is not a compile time error.
     
  3. karthigayan

    karthigayan New Member

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    Your way of coding is making a easy things to difficult one.Also as shabbir said segmentation fault is not a compile time error . It is a run time error .But when I run I didn't get segmentation fault . I got a unterminated loop.


    Code:
        
    for( i = 0 ; i <= (x = (strlen(c))) ; i++)
    
    You can write like this ,

    Code:
       
     for( i = 0 ; i <= strlen(c); i++)
    
    Also you have used '=' for the comparison.You need to use '==' for that.

    Code:
    #include<stdio.h>
    #include<string.h>
    
    int main()
    {
        int i,x;
        char c[100];
    
        printf("Enter input :");
        fgets(c,100,stdin);
    
        for( i = 0 ; i <= strlen(c); i++)
        {
    
            if( c[i] == '\t')
            {
                            printf("****"); // I used '*' for the understanding , you just change it to space .
            }
            else
            {
            printf("%c",c[i]);
            }
        }
    }
    
     
  4. xpi0t0s

    xpi0t0s Mentor

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    The reason the loop was unending was that in the last iteration of the loop, i.e. when i==strlen(c), c is the terminating NULL, which due to using assign rather than compare you were overwriting with a tab character, then on the next iteration you would start to get undefined behaviour (which often leads to a segfault).

    Remember in C counting starts from 0. So a less than or equal comparison in the loop will count one MORE than you might have intended. Try it: let's count i from 0 to 5 using <=. 0, 1, 2, 3, 4 are all obvious. What about 5? Is 5 less than or equal to 5? YES. So i<=5 will still evaluate TRUE. So i will loop through the values 0,1,2,3,4,5, which is SIX iterations.

    So unless you actually meant for the count to include the terminating NULL, you should use
    Code:
    for (i=0; i<strlen(c); i++)
    
    And ALWAYS be careful about = and ==. Some languages interpret = differently according to the context, for example in BASIC the clause "IF A=5" is taken to be a comparison, but "A=5" is an assignment. But in C = *ALWAYS ALWAYS ALWAYS* means "assign", so "if (a=5)" will assign 5 to a, then evaluate 5 as a logical value where 0 is FALSE and any other value is TRUE. Since 5 is non-zero, it is TRUE, so "if (a=5)" will assign 5 to x and what comes after the if will always be performed (and if there is an else, that will never be performed). Some compilers will throw a "condition is always true" warning if they spot this, but it will have to be assignment to a constant.

    Sometimes an assignment in an if is what you really want. Take the following:
    Code:
    int i=0;
    while (c=str[i++])
        ...;
    
    This will also loop over a string. For all non-zero values str will be copied to c and the expression will evaluate TRUE. When it gets to the terminating NULL, str which is 0 will be copied to c, this will evaluate FALSE and the loop will terminate.
     

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