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Julie Johnson's questions

Martin Hope
Julie Johnson
Asked: 2025-04-09 21:56:30 +0800 CST
  • 11

Estou trabalhando em uma tarefa de programação de sistema operacional para entender pipes. texto A tarefa envolve o uso de expressões regulares para extrair informações da saída do strace e tempos de execução de chamadas de sistema estatísticos.

No entanto, estou enfrentando problemas com o cálculo incorreto dos tempos. Suspeito que a função parse_strace_line não esteja analisando corretamente as linhas de saída do strace, especialmente a parte do carimbo de data/hora (por exemplo, <0,000123>), resultando em valores de tempo incorretos ou ausentes nas estatísticas. A função pode não estar lidando corretamente com o formato da linha, com casos extremos como linhas incompletas, ou com o padrão regex, que pode não capturar com precisão o componente de tempo da saída do strace.

Meu ambiente é o Ubuntu rodando no WSL2.

Aqui está o código:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>
#include <regex.h>

#define MAX_SYSCALLS 1024  // Maximum distinct syscalls we can track
#define TOP_N 5            // Number of top results to display

/* Syscall tracking structure
 * name: syscall name (e.g., "open", "read")
 * time: accumulated time spent in this syscall (seconds)
 */
typedef struct {
    char name[64];
    double time;
} syscall_stat;

/* Main statistics container
 * stats: array of individual syscall metrics
 * count: number of unique syscalls tracked
 * total_time: sum of all syscall times for percentage calculation
 */
typedef struct {
    syscall_stat stats[MAX_SYSCALLS];
    int count;
    double total_time;
} syscall_stats;

/* Parse a line of strace output with timing information (-T option)
 * line: input string from strace output
 * syscall_name: output parameter for extracted syscall name
 * time: output parameter for extracted duration
 * Returns: 1 if successful, 0 if line doesn't contain timing info
 * 
 * Strace line format examples:
 * openat(AT_FDCWD, "/lib/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 <0.000014>
 * read(0, "hello\n", 1024)               = 6 <0.000010>
 */
int parse_strace_line(char* line, char* syscall_name, double* time) {
    // Filter lines without timing information
    if (strstr(line, " = ") == NULL || strstr(line, "<") == NULL) {
        return 0;
    }

    // Regular expression pattern breakdown:
    // ^(\w+)       Capture group 1: syscall name at line start
    // \(.*?\)      Non-greedy match of arguments between parentheses
    // \s+=\s+.*?   Match return value portion
    // <(\d+\.?\d*)> Capture group 2: time value between <>
    regex_t regex;
    regcomp(&regex, "^(\\w+)\\(.*?\\)\\s+=\\s+.*?<(\\d+\\.?\\d*)>$", REG_EXTENDED);
    regmatch_t matches[3];  // Array for regex capture groups

    if (regexec(&regex, line, 3, matches, 0) != 0) {
        regfree(&regex);
        return 0;
    }

    // Extract syscall name from first capture group
    int name_len = matches[1].rm_eo - matches[1].rm_so;
    strncpy(syscall_name, line + matches[1].rm_so, name_len);
    syscall_name[name_len] = '\0';

    // Convert time string to double
    char* time_str = line + matches[2].rm_so;
    *time = atof(time_str);

    regfree(&regex);
    return 1;
}

/* Update statistics with new syscall data
 * stats: pointer to statistics container
 * name: syscall name to add/update
 * time: duration to accumulate
 * 
 * If the syscall already exists, we sum the time.
 * If new, we add it to the list (until MAX_SYSCALLS).
 */
void add_syscall(syscall_stats *stats, const char *name, double time) {
    // Safety check against array overflow
    if (stats->count >= MAX_SYSCALLS) {
        fprintf(stderr, "Warning: Reached maximum tracked syscalls (%d)\n", MAX_SYSCALLS);
        return;
    }

    // Check existing entries for duplicate syscall
    for (int i = 0; i < stats->count; i++) {
        if (strcmp(stats->stats[i].name, name) == 0) {
            stats->stats[i].time += time;
            stats->total_time += time;
            return;
        }
    }

    // Add new entry if space available
    if (stats->count < MAX_SYSCALLS) {
        strncpy(stats->stats[stats->count].name, name, sizeof(stats->stats[0].name));
        stats->stats[stats->count].time = time;
        stats->total_time += time;
        stats->count++;
    }
}

/* Display top N syscalls by accumulated time
 * stats: statistics container with all data
 * n: number of top results to show
 * 
 * Uses bubble sort for simplicity with small datasets.
 * Prints percentages relative to total tracked time.
 * Flushes output with nulls for clean pipe termination.
 */
void print_top_syscalls(syscall_stats *stats, int n) {
    if (stats->count == 0 || stats->total_time == 0) {
        fprintf(stderr, "Error: No valid syscall data to display\n");
        return;
    }

    // Simple O(n²) sort acceptable for MAX_SYSCALLS=1024
    for (int i = 0; i < stats->count - 1; i++) {
        for (int j = i+1; j < stats->count; j++) {
            // Swap if later entry has higher time
            if (stats->stats[j].time > stats->stats[i].time) {
                syscall_stat temp = stats->stats[i];
                stats->stats[i] = stats->stats[j];
                stats->stats[j] = temp;
            }
        }
    }

    // Determine number of results to display
    n = stats->count < n ? stats->count : n;
    for (int i = 0; i < n; i++) {
        double ratio = (stats->stats[i].time / stats->total_time) * 100;
        printf("%s (%d%%)\n", stats->stats[i].name, (int)ratio);
    }

    /* Null-terminate output for clean pipe communication
     * Some programs may read fixed buffer sizes - this ensures
     * we flush the pipe completely and avoid hanging readers */
    for (int i = 0; i < 80; i++) putchar('\0');
    fflush(stdout);
}

/* Main execution flow:
 * 1. Validate command line arguments
 * 2. Set up parent/child process communication
 * 3. Child: execute strace with command
 * 4. Parent: process and analyze strace output
 */
int main(int argc, char *argv[]) {
    if (argc < 2) {
        fprintf(stderr, "Usage: %s COMMAND [ARGS...]\n", argv[0]);
        fprintf(stderr, "Example: %s ls -l\n", argv[0]);
        return 1;
    }

    // Create pipe for inter-process communication
    int pipefd[2];
    if (pipe(pipefd) == -1) {
        perror("Failed to create pipe");
        return 1;
    }

    pid_t pid = fork();
    if (pid == -1) {
        perror("Process fork failed");
        return 1;
    }

    if (pid == 0) { // Child process - execute strace
        // Redirect stdout to pipe write end
        close(pipefd[0]);
        dup2(pipefd[1], STDOUT_FILENO);
        close(pipefd[1]);

        // Build strace argument array:
        // strace -T [user command] [args...]
        char* strace_argv[argc + 2];
        strace_argv[0] = "/usr/bin/strace";
        strace_argv[1] = "-T";  // Enable timing reports
        for (int i = 1; i < argc; i++) {
            strace_argv[i+1] = argv[i];
        }
        strace_argv[argc+1] = NULL;  // NULL-terminate array

        // Execute strace with clean environment
        execve("/usr/bin/strace", strace_argv, (char *[]){ "PATH=/bin:/usr/bin", NULL });
        perror("Failed to execute strace");
        return 1;
    } else { // Parent process - analyze output
        close(pipefd[1]);  // Close unused write end
        syscall_stats stats = {0};  // Initialize all fields to zero

        char buffer[1024];
        char* line_start = buffer;
        ssize_t bytes_read;

        // Read output in chunks until EOF
        while ((bytes_read = read(pipefd[0], 
               line_start, 
               sizeof(buffer) - (line_start - buffer))) > 0) {
            // Null-terminate the received data
            buffer[bytes_read + (line_start - buffer)] = '\0';
            line_start = buffer;

            // Process complete lines (delimited by newlines)
            char* line_end;
            while ((line_end = strchr(line_start, '\n')) != NULL) {
                *line_end = '\0';  // Temporarily terminate at newline

                char syscall_name[64];
                double time;
                if (parse_strace_line(line_start, syscall_name, &time)) {
                    add_syscall(&stats, syscall_name, time);
                }

                line_start = line_end + 1;  // Move to next line
            }

            // Handle partial line remaining in buffer
            if (line_start != buffer) {
                size_t remaining = buffer + sizeof(buffer) - line_start;
                // Move partial line to buffer start for next read
                memmove(buffer, line_start, remaining);
                line_start = buffer + remaining;
            }
        }

        // Cleanup and display results
        close(pipefd[0]);
        wait(NULL);  // Wait for strace to terminate completely
        print_top_syscalls(&stats, TOP_N);
    }

    return 0;
}

Esta é a linha de comando para teste:

root@DimLights:/mnt/f/WSL/Data/OperatingSystems/2025/os2025/sperf# gcc -g -o sperf sperf.c -lm -lrt
root@DimLights:/mnt/f/WSL/Data/OperatingSystems/2025/os2025/sperf# ./sperf echo "Hello WSL"

execve("/bin/echo", ["echo", "Hello WSL"], 0x7ffe6258ee20 /* 1 var */) = 0 <0.000363>
brk(NULL)                               = 0x55d4dee1c000 <0.000104>
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f483b684000 <0.000137>
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory) <0.000115>
openat(AT_FDCWD, "/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3 <0.000133>
fstat(3, {st_mode=S_IFREG|0644, st_size=36191, ...}) = 0 <0.000115>
mmap(NULL, 36191, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f483b67b000 <0.000097>
close(3)                                = 0 <0.000079>
openat(AT_FDCWD, "/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3 <0.000108>
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0\220\243\2\0\0\0\0\0"..., 832) = 832 <0.000094>
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784 <0.000101>
fstat(3, {st_mode=S_IFREG|0755, st_size=2125328, ...}) = 0 <0.000096>
pread64(3, "\6\0\0\0\4\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0@\0\0\0\0\0\0\0"..., 784, 64) = 784 <0.000088>
mmap(NULL, 2170256, PROT_READ, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f483b469000 <0.000107>
mmap(0x7f483b491000, 1605632, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x28000) = 0x7f483b491000 <0.000117>
mmap(0x7f483b619000, 323584, PROT_READ, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1b0000) = 0x7f483b619000 <0.000099>
mmap(0x7f483b668000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1fe000) = 0x7f483b668000 <0.000102>
mmap(0x7f483b66e000, 52624, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f483b66e000 <0.000103>
close(3)                                = 0 <0.000104>
mmap(NULL, 12288, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f483b466000 <0.000085>
arch_prctl(ARCH_SET_FS, 0x7f483b466740) = 0 <0.000049>
set_tid_address(0x7f483b466a10)         = 1543 <0.000319>
set_robust_list(0x7f483b466a20, 24)     = 0 <0.000098>
rseq(0x7f483b467060, 0x20, 0, 0x53053053) = 0 <0.000123>
mprotect(0x7f483b668000, 16384, PROT_READ) = 0 <0.000096>
mprotect(0x55d4de99a000, 4096, PROT_READ) = 0 <0.000093>
mprotect(0x7f483b6bc000, 8192, PROT_READ) = 0 <0.000103>
prlimit64(0, RLIMIT_STACK, NULL, {rlim_cur=8192*1024, rlim_max=RLIM64_INFINITY}) = 0 <0.000095>
munmap(0x7f483b67b000, 36191)           = 0 <0.000118>
getrandom("\xc0\x8d\xa3\xbc\x88\x59\xbb\xd3", 8, GRND_NONBLOCK) = 8 <0.000098>
brk(NULL)                               = 0x55d4dee1c000 <0.000113>
brk(0x55d4dee3d000)                     = 0x55d4dee3d000 <0.000097>
fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0 <0.000092>
write(1, "Hello WSL\n", 10Received from pipe: Hello WSL
)             = 10 <0.000216>
close(1)                                = 0 <0.000165>
close(2)                                = 0 <0.000092>
exit_group(0)                           = ?
+++ exited with 0 +++
Total count is zero

Aqui está o que eu tentei e o que eu esperava: usei a depuração do GDB e notei que o loop while ((line = strchr(line_start, '\n')) != NULL) foi executado apenas uma vez ao testar o comando. Não consigo entender por que a saída mostra +++ exited with 0 +++ e Total count is zero. Também pedi ajuda à IA e modifiquei partes da função parse_strace_line e o padrão de expressão regular, mas com pouco sucesso.

O que realmente aconteceu: As estatísticas permanecem vazias (a contagem total é zero), indicando que a análise ou extração de dados falhou.

O que eu esperava: O código deveria analisar corretamente os registros de data e hora das chamadas do sistema a partir da saída do strace, acumular seus tempos e calcular a proporção do tempo de cada chamada em relação ao total.

Como estudante, eu agradeceria muito se você pudesse apontar os erros no meu código e explicar como ler corretamente os tempos de chamada do sistema e convertê-los na proporção.

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