C程序分段错误

Question

I got segmentation fault for the code here.

gdb :

Program received signal SIGSEGV, Segmentation fault.
_IO_fgets (buf=0x601080 <rc> "", n=100, fp=0x0) at iofgets.c:50
50  iofgets.c: No such file or directory.

Code:

#include <unistd.h>
#include <stdio.h>

char rc[100];
FILE *fp;
int status;

void main() {    
    fp = popen("sudo lshw |", "grep UUID");
    if (fp == NULL)
        printf("NULL pointer");
    while (fgets(rc, 100, fp) != '\0')
        printf("%s", rc);
    status = pclose(fp);
    if (status == -1) {
        printf("pclose error");
        /* Error reported by pclose() */
    } else{
        printf("Unknown error");
    }
    //return 0;
}

null pointer I guess? i tried with solutions given , but not worked. Somehow silly mistake I guess

sorry , the shell command will b sudo dmidecode | grep UUID sudo dmidecode | grep UUID

Answer 1

This is wrong

fp = popen("sudo lshw |", "grep UUID");

maybe you mean, read popen()

fp = popen("sudo lshw | grep UUID", "r");

the call failed, but even though you checked for fp == NULL , you continued anyway causing undefined behavior, and leading to the segmentation fault, the fp == NULL check needs to abort the program, like this

#include <unistd.h>
#include <stdio.h>
#include <errno.h>
#include <string.h>

int main() {    
    char  rc[100];
    FILE *fp;
    int   status;

    fp = popen("sudo lshw | grep UUID", "r");
    if (fp == NULL)
    {
        printf("error:%d: %s", errno, strerror(errno));
        return -1;
    }
    while (fgets(rc, 100, fp) != NULL)
        printf("%s", rc);
    status = pclose(fp);
    if (status == -1) {
        printf("error:%d: %s", errno, strerror(errno));
    } else { /* this means no error happened */
        printf("success: pipe, closed.");
    }
    return 0;
}

note that main() shall return an int , and as Joachim Pileborg commented, fgets(...) == '\\0' is wrong, fgets() returns NULL on error and NULL == '\\0' is not necessarily true.

Also, pclose() returns -1 on error, if it does not return -1 then everything went as expected.